藤原 誠

Pre-mRNA splicing is a fundamental process in eukaryotic gene expression, and the mechanism of intron definition, involving the recognition of the canonical GU (5’-splice site) and AG (3’-splice site) dinucleotides by splicing factors, has been postulated for most cases of splicing initiation in plants. Splice site mutations have played crucial roles in unraveling the mechanism of pre-mRNA splicing in planta. Typically, splice site mutations abolish splicing events or activate one or more cryptic splice sites surrounding the mutated region. In this report, we investigated the splicing pattern of the EGY1 gene in an Ar-ion-induced egy1-4 allele of Arabidopsis thaliana. egy1-4 has an AG-to-AC mutation in the 3′-end of intron 3, along with 4-bp substitutions and a 5-bp deletion in adjacent exon 4. RT-PCR, cDNA cloning, and amplicon sequencing analyses of EGY1 revealed that while most wild-type EGY1 mRNAs had a single splicing pattern, egy1-4 mRNAs had multiple splicing defects. Almost half of EGY1 transcripts showed ‘intron retention’ at intron 3, while the other half exhibited activation of 3’ cryptic splice sites either upstream or downstream of the original 3’-splice site. Unexpectedly, around 8% of EGY1 transcripts in egy1-4 exhibited activation of cryptic 5′-splice sites positioned upstream of the authentic 5’-splice site of intron 3. Whole genome resequencing of egy1-4 indicated that it has no other known impactful mutations. These results may provide a rare, but real case of activation of cryptic 5’-splice sites by downstream 3’-splice site/exon mutations in planta.

Foeniculoxin is a major phytotoxin produced by Italian strains of Phomopsis foeniculi. The first total synthesis is described utilizing the ene reaction and Sonogashira cross-coupling reaction as key steps. The absolute configuration of the C6' was determined using chiral separation and advanced Mosher's method. The phytotoxicity of the synthesized compound was demonstrated via syringe-based infiltration into Chenopodium album and Arabidopsis thaliana leaves. Synthetic foeniculoxin induced various defects in A. thaliana leaf cells before lesion formation, including protein leakage into the cytoplasm from both chloroplasts and mitochondria and mitochondrial rounding and swelling. Furthermore, foeniculoxin and the antibiotic hygromycin B caused similar agglomeration of mitochondria around chloroplasts, highlighting this event as a common component in the early stages of plant cell death.

Stromules are dynamic membrane-bound tubular structures that emanate from plastids. Stromule formation is triggered in response to various stresses and during plant development, suggesting that stromules may have physiological and developmental roles in these processes. Despite the possible biological importance of stromules and their prevalence in green plants, their exact roles and formation mechanisms remain unclear. To explore these issues, we obtained Arabidopsis thaliana mutants with excess stromule formation in the leaf epidermis by microscopy-based screening. Here, we characterized one of these mutants, stromule biogenesis altered 1 (suba1). suba1 forms plastids with severely altered morphology in a variety of non-mesophyll tissues, such as leaf epidermis, hypocotyl epidermis, floral tissues, and pollen grains, but apparently normal leaf mesophyll chloroplasts. The suba1 mutation causes impaired chloroplast pigmentation and altered chloroplast ultrastructure in stomatal guard cells, as well as the aberrant accumulation of lipid droplets and their autophagic engulfment by the vacuole. The causal defective gene in suba1 is TRIGALACTOSYLDIACYLGLYCEROL5 (TGD5), which encodes a protein putatively involved in the endoplasmic reticulum (ER)-to-plastid lipid trafficking required for the ER pathway of thylakoid lipid assembly. These findings suggest that a non-mesophyll-specific mechanism maintains plastid morphology. The distinct mechanisms maintaining plastid morphology in mesophyll versus non-mesophyll plastids might be attributable, at least in part, to the differential contributions of the plastidial and ER pathways of lipid metabolism between mesophyll and non-mesophyll plastids.

In Arabidopsis thaliana, the Ethylene-dependent Gravitropism-deficient and Yellow-green 1 (EGY1) gene encodes a thylakoid membrane-localized protease involved in chloroplast development in leaf mesophyll cells. Recently, EGY1 was also found to be crucial for the maintenance of grana in mesophyll chloroplasts. To further explore the function of EGY1 in leaf tissues, we examined the phenotype of chloroplasts in the leaf epidermal guard cells and pavement cells of two 40Ar17+ irradiation-derived mutants, Ar50-33-pg1 and egy1-4. Fluorescence microscopy revealed that fully expanded leaves of both egy1 mutants showed severe chlorophyll deficiency in both epidermal cell types. Guard cells in the egy1 mutant exhibited permanent defects in chloroplast formation during leaf expansion. Labeling of plastids with CaMV35S or Protodermal Factor1 (PDF1) promoter-driven stroma-targeted fluorescent proteins revealed that egy1 guard cells contained the normal number of plastids, but with moderately reduced size, compared with wild-type guard cells. Transmission electron microscopy further revealed that the development of thylakoids was impaired in the plastids of egy1 mutant guard mother cells, guard cells, and pavement cells. Collectively, these observations demonstrate that EGY1 is involved in chloroplast formation in the leaf epidermis and is particularly critical for chloroplast differentiation in guard cells.

Argon-ion beam is an effective mutagen capable of inducing a variety of mutation types. In this study, an argon ion-induced pale green mutant of Arabidopsis thaliana was isolated and characterized. The mutant, designated Ar50-33-pg1, exhibited moderate defects of growth and greening and exhibited rapid chlorosis in photosynthetic tissues. Fluorescence microscopy confirmed that mesophyll chloroplasts underwent substantial shrinkage during the chlorotic process. Genetic and whole-genome resequencing analyses revealed that Ar50-33-pg1 contained a large 940 kb deletion in chromosome V that encompassed more than 100 annotated genes, including 41 protein-coding genes such as TYRAAt1/TyrA1, EGY1, and MBD12. One of the deleted genes, EGY1, for a thylakoid membrane-localized metalloprotease, was the major contributory gene responsible for the pale mutant phenotype. Both an egy1 mutant and F1 progeny of an Ar50-33-pg1 × egy1 cross-exhibited chlorotic phenotypes similar to those of Ar50-33-pg1. Furthermore, ultrastructural analysis of mesophyll cells revealed that Ar50-33-pg1 and egy1 initially developed wild type-like chloroplasts, but these were rapidly disassembled, resulting in thylakoid disorganization and fragmentation, as well as plastoglobule accumulation, as terminal phenotypes. Together, these data support the utility of heavy-ion mutagenesis for plant genetic analysis and highlight the importance of EGY1 in the structural maintenance of grana in mesophyll chloroplasts.

Recently, a recessive Arabidopsis thaliana mutant with abundant stromules in leaf epidermal pavement cells was visually screened and isolated. The gene responsible for this mutant phenotype was identified as PARC6, a chloroplast division site regulator gene. The mutant allele parc6-5 carried two point mutations (G62R and W700stop) at the N- and C-terminal ends of the coding sequence, respectively. Here, we further characterized parc6-5 and other parc6 mutant alleles, and showed that PARC6 plays a critical role in plastid morphogenesis in all cell types of the leaf epidermis: pavement cells, trichome cells, and guard cells. Transient expression of PARC6 transit peptide (TP) fused to the green fluorescent protein (GFP) in plant cells showed that the G62R mutation has no or little effect on the TP activity of the PARC6 N-terminal region. Then, plastid morphology was microscopically analyzed in the leaf epidermis of wild-type (WT) and parc6 mutants (parc6-1, parc6-3, parc6-4 and parc6-5) with the aid of stroma-targeted fluorescent proteins. In parc6 pavement cells, plastids often assumed aberrant grape-like morphology, similar to those in severe plastid division mutants, atminE1, and arc6. In parc6 trichome cells, plastids exhibited extreme grape-like aggregations, without the production of giant plastids (>6 mu m diameter), as a general phenotype. In parc6 guard cells, plastids exhibited a variety of abnormal phenotypes, including reduced number, enlarged size, and activated stromules, similar to those in atminE1 and arc6 guard cells. Nevertheless, unlike atminE1 and arc6, parc6 exhibited a low number of mini-chloroplasts (< 2 mu m diameter) and rarely produced chloroplast-deficient guard cells. Importantly, unlike parc6, the chloroplast division site mutant arc11 exhibited WT-like plastid phenotypes in trichome and guard cells. Finally, observation of parc6 complementation lines expressing a functional PARC6-GFP protein indicated that PARC6-GFP formed a ring-like structure in both constricting and non-constricting chloroplasts, and that PARC6 dynamically changes its configuration during the process of chloroplast division.

The existence of numerous chloroplasts in photosynthetic cells is a general feature of plants. Chloroplast biogenesis and inheritance involve two distinct mechanisms: proliferation of chloroplasts by binary fission and partitioning of chloroplasts into daughter cells during cell division. The mechanism of chloroplast number coordination in a given cell type is a fundamental question. Stomatal guard cells (GCs) in the plant shoot epidermis generally contain several to tens of chloroplasts per cell. Thus far, chloroplast number at the stomatal (GC pair) level has generally been used as a convenient marker for identifying hybrid species or estimating the ploidy level of a given plant tissue. Here, we report that Arabidopsis thaliana leaf GCs represent a useful system for investigating the unexploited aspects of chloroplast number control in plant cells. In contrast to a general notion based on analyses of leaf mesophyll chloroplasts, a small difference was detected in the GC chloroplast number among three Arabidopsis ecotypes (Columbia, Landsberg erecta, and Wassilewskija). Fluorescence microscopy often detected dividing GC chloroplasts with the FtsZ1 ring not only at the early stage of leaf expansion but also at the late stage. Compensatory chloroplast expansion, a phenomenon well documented in leaf mesophyll cells of chloroplast division mutants and transgenic plants, could take place between paired GCs in wild-type leaves. Furthermore, modest chloroplast number per GC as well as symmetric division of guard mother cells for GC formation suggests that Arabidopsis GCs would facilitate the analysis of chloroplast partitioning, based on chloroplast counting at the individual cell level.

Stromules, or stroma-filled tubules, are thin extensions of the plastid envelope membrane that are most frequently observed in undifferentiated or non-mesophyll cells. The formation of stromules is developmentally regulated and responsive to biotic and abiotic stress; however, the physiological roles and molecular mechanisms of the stromule formation remain enigmatic. Accordingly, we attempted to obtain Arabidopsis thaliana mutants with aberrant stromule biogenesis in the leaf epidermis. Here, we characterize one of the obtained mutants. Plastids in the leaf epidermis of this mutant were giant and pleomorphic, typically having one or more constrictions that indicated arrested plastid division, and usually possessed one or more extremely long stromules, which indicated the deregulation of stromule formation. Genetic mapping, whole-genome resequencing-aided exome analysis, and gene complementation identified PARC6/CDP1/ARC6H, which encodes a vascular plant-specific, chloroplast division site-positioning factor, as the causal gene for the stromule phenotype. Yeast two-hybrid assay and double mutant analysis also identified a possible interaction between PARC6 and MinD1, another known chloroplast division site-positioning factor, during the morphogenesis of leaf epidermal plastids. To the best of our knowledge, PARC6 is the only known A. thaliana chloroplast division factor whose mutations more extensively affect the morphology of plastids in non-mesophyll tissue than in mesophyll tissue. Therefore, the present study demonstrates that PARC6 plays a pivotal role in the morphology maintenance and stromule regulation of non-mesophyll plastids.

Chloroplasts, or photosynthetic plastids, multiply by binary fission, forming a homogeneous population in plant cells. In Arabidopsis thaliana, the division apparatus (or division ring) of mesophyll chloroplasts includes an inner envelope transmembrane protein ARC6, a cytoplasmic dynamin-related protein ARC5 (DRP5B), and members of the FtsZ1 and FtsZ2 families of proteins, which co-assemble in the stromal mid-plastid division ring (FtsZ ring). FtsZ ring placement is controlled by several proteins, including a stromal factor MinE (AtMinE1). During leaf mesophyll development, ARC6 and AtMinE1 are necessary for FtsZ ring formation and thus plastid division initiation, while ARC5 is essential for a later stage of plastid division. Here, we examined plastid morphology in leaf epidermal pavement cells (PCs) and stomatal guard cells (GCs) in the arc5 and arc6 mutants using stroma-targeted fluorescent proteins. The arc5 PC plastids were generally a bit larger than those of the wild type, but most had normal shapes and were division-competent, unlike mutant mesophyll chloroplasts. The arc6 PC plastids were heterogeneous in size and shape, including the formation of giant and mini-plastids, plastids with highly developed stromules, and grape-like plastid clusters, which varied on a cell-by-cell basis. Moreover, unique plastid phenotypes for stomatal GCs were observed in both mutants. The arc5 GCs rarely lacked chlorophyll-bearing plastids (chloroplasts), while they accumulated minute chlorophyll-less plastids, whereas most GCs developed wild type-like chloroplasts. The arc6 GCs produced large chloroplasts and/or chlorophyll-less plastids, as previously observed, but unexpectedly, their chloroplasts/plastids exhibited marked morphological variations. We quantitatively analyzed plastid morphology and partitioning in paired GCs from wild-type, arc5, arc6, and atminE1 plants. Collectively, our results support the notion that ARC5 is dispensable in the process of equal division of epidermal plastids, and indicate that dysfunctions in ARC5 and ARC6 differentially affect plastid replication among mesophyll cells, PCs, and GCs within a single leaf.

We isolated a cold sensitive virescent1 (csv1) mutant from a rice (Oryza sativa L.) population mutagenized by carbon ion irradiation. The mutant exhibited chlorotic leaves during the early growth stages, and produced normal green leaves as it grew. The growth of csv1 plants displayed sensitivity to low temperatures. In addition, the mutant plants that were transferred to low temperatures at the fifth leaf stage produced chlorotic leaves subsequently. Genetic and molecular analyses revealed translocation of a 13-kb genomic fragment that disrupted the causative gene (CSV1; LOC_Os05g34040). CSV1 encodes a plastid-targeted oxidoreductase-like protein conserved among land plants, green algae, and cyanobacteria. Furthermore, CSV1 transcripts were more abundant in immature than in mature leaves, and they did not markedly increase or decrease with temperature. Taken together, our results indicate that CSV1 supports chloroplast development under cold stress conditions, in both the early growth and tillering stages in rice.

Symmetric division of leaf mesophyll chloroplasts requires MinD and MinE, which work together to suppress division other than at the mid-chloroplast. arc11 is a MinD loss-of-function mutant of Arabidopsis thaliana. In arc11 plants, asymmetric chloroplast division, as well as its delay or arrest, results in extreme size polymorphism of chloroplasts in mature mesophyll cells. The current study examined chloroplast phenotypes in the epidermis of arc11 leaves. Fluorescence microscopy analysis revealed that epidermal chloroplasts in mature leaves exhibited moderate heterogeneity in size. This probably resulted from completion of many of the previous non-equatorial or multiple division events in expanding leaves. Additionally, analyses of plastids found that epidermal chloroplasts in arc11 mutants showed several phenotypes that have not previously been described.

In the klebsormidiophyceae alga Klebsormidium flaccidum, massive oil body formation was detected by light microscopy and Nile Red fluorostaining after a two month or longer culture on agar plates containing inorganic ingredients and vitamins. While the oil bodies occupied most of the cytoplasm, chloroplasts heavily reduced their volume. However, electron microscopic observations revealed that the pyrenoids and thylakoid membranes were maintained. The marked numbers of oil bodies within the cells were tinged yellowish green in colour. Centrifugation (280,000 xg) of lysates of cells accumulating oil bodies resulted in an oil body-rich fraction at the top of the suspension. With containing virtually no thylakoid membranes, this fraction was coloured yellowish green. These observations suggest that the oil bodies, if not all, contain chlorophylls or its derivatives; nevertheless, the reduced chloroplasts still remained in the cells. This suggests that oil body production in K. flaccidum may concomitantly occur with the gradual reduction of chloroplasts.原著2015 年度神奈川大学総合理学研究所共同研究助成論文

Plastids in the leaf epidermal cells of plants are regarded as immature chloroplasts that, like mesophyll chloroplasts, undergo binary fission. While mesophyll chloroplasts have generally been used to study plastid division, recent studies have suggested the presence of tissue- or plastid type-dependent regulation of plastid division. Here, we report the detailed morphology of plastids and their stromules, and the intraplastidic localization of the chloroplast division-related protein AtFtsZ1-1, in the leaf epidermis of an Arabidopsis mutant that harbors a mutation in the chloroplast division site determinant gene AtMinE1. In atminE1, the size and shape of epidermal plastids varied widely, which contrasts with the plastid phenotype observed in atminE1 mesophyll cells. In particular, atminE1 epidermal plastids occasionally displayed grape-like morphology, a novel phenotype induced by a plastid division mutation. Observation of an atminE1 transgenic line harboring an AtMinEl promoten:AtMinE1-yellow fluorescent protein fusion gene confirmed the expression and plastidic localization of AtMinE1 in the leaf epidermis. Further examination revealed that constriction of plastids and stromules mediated by the FtsZ1 ring contributed to the plastid pleomorphism in the atminE1 epidermis. These results illustrate that a single plastid division mutation can have dramatic consequences for epidermal plastid morphology, thereby implying that plastid division and morphogenesis are differentially regulated in epidermal and mesophyll plastids.

Leaf tissues of plants usually contain several types of idioblasts, defined as specialized cells whose shape and contents differ from the surrounding homogeneous cells. The spatial patterning of idioblasts, particularly of trichomes and guard cells, across the leaf epidermis has received considerable attention as it offers a useful biological model for studying the intercellular regulation of cell fate and patterning. Excretory idioblasts in the leaves of the aquatic monocotyledonous plant Egeria densa produced light blue autofluorescence when irradiated with ultraviolet light. The use of epifluorescence microscopy to detect this autofluorescence provided a simple and convenient method for detecting excretory idioblasts and allowed tracking of those cells across the leaf surfaces, enabling quantitative measurement of the clustering and spacing patterns of idioblasts at the whole leaf level. Occurrence of idioblasts was coordinated along the proximal-distal, medial-lateral, and adaxial-abaxial axes, producing a recognizable consensus spatial pattern of idioblast formation among fully expanded leaves. Idioblast clusters, which comprised up to nine cells aligned along the proximal-distal axis, showed no positional bias or regularity in idioblast-forming areas when compared with singlet idioblasts. Up to 75% of idioblasts existed as clusters on every leaf side examined. The idioblast-forming areas varied between leaves, implying phenotypic plasticity. Furthermore, in young expanding leaves, autofluorescence was occasionally detected in a single giant vesicle or else in one or more small vesicles, which eventually grew to occupy a large portion of the idioblast volume as a central vacuole. Differentiation of vacuoles by accumulating the fluorescence substance might be an integral part of idioblast differentiation. Red autofluorescence from chloroplasts was not detected in idioblasts of young expanding leaves, suggesting idioblast differentiation involves an arrest in chloroplast development at a very early stage, rather than transdifferentiation of chloroplastcontaining epidermal cells.

We characterized a white flower mutant of allotetraploid N. tabacum as a DFR-deficient mutant; one copy of DFR has a cultivar-specific frameshift, while the other was deleted by heavy-ion irradiation.In most plants, white-flowered mutants have some kind of deficiency or defect in their anthocyanin biosynthetic pathway. Nicotiana tabacum normally has pink petals, in which cyanidin is the main colored anthocyanidin. When a relevant gene in the cyanidin biosynthetic pathway is mutated, the petals show a white color. Previously, we generated white-flowered mutants of N. tabacum by heavy-ion irradiation, which is accepted as an effective mutagen. In this study, we determined which gene was responsible for the white-flowered phenotype of one of these mutants, cv. Xanthi white flower 1 (xwf1). Southern blot analysis using a DNA fragment of the dihydroflavonol 4-reductase (DFR) gene as a probe showed that the xwf1 mutant lacked signals that were present in wild-type genomic DNAs. Sequence analysis demonstrated that one copy of the DFR gene (NtDFR2) was absent from the genome of the xwf1 mutant. The other copy of the DFR gene (NtDFR1) contained a single-base deletion resulting in a frameshift mutation, which is a spontaneous mutation in cv. Xanthi. Introduction of NtDFR2 cDNA into the petal limbs of xwf1 by particle bombardment resulted in production of the pink-colored cells, whereas introduction of NtDFR1 cDNA did not. These results indicate that xwf1 is a DFR-deficient mutant. One copy of NtDFR1 harbors a spontaneous frameshift mutation, while the other copy of NtDFR2 was deleted by heavy-ion beam irradiation.

The dioecious plant Silene latifolia has heteromorphic sex chromosomes, and comparison of the positions of sex-linked genes indicates that at least three large inversions have occurred during the evolution of the Y chromosome. In this article, we describe the isolation of a new sex-linked gene from S. latifolia, which provides new information on the evolution of this plant's young sex chromosomes. By using reverse-transcription polymerase chain reaction degenerate primers based on the Arabidopsis thaliana sequence of WUSCHEL, a flower-development gene, we found two copies in S. latifolia, which we named SlWUS1 and SlWUS2. Southern blot and genetic segregation analysis showed that SlWUS1 is located on the X chromosome and SlWUS2 is autosomal. No Y-linked copy of SlWUS1 was found by either Southern blot analysis under low-stringency conditions or polymerase chain reaction with degenerate primers, so we conclude that SlWUS1 probably has no Y-linked homolog. It is unknown whether the Y chromosome lost the SlWUS1 copy by degeneration of this individual gene or whether deletion of a larger genome region was involved. Several tests lead us to conclude that dosage compensation has not evolved for this sex-linked gene. We mapped the ortholog in the nondioecious relative S. vulgaris (SvWUS1), to compare the location in a species that has no history of having sex chromosomes. SvWUS1 maps to the same linkage group as other fully X-linked genes, indicating that it was not added to the X, but was lost from the Y. Its location differs in the maps from the two species, raising the possibility that the X chromosome, as well as the Y, may have been rearranged.

Organelle dynamics in the plant male gametophyte has received attention for its importance in pollen tube growth and cytoplasmic inheritance. We recently revealed the dynamic behaviors of plastids in living Arabidopsis pollen grains and tubes, using an inherent promoter-driven FtsZ1-green fluorescent protein (GFP) fusion. Here, we further monitored the movement of pollen tube plastids with an actin1 promoter-driven, stroma-targeted yellow fluorescent protein (YFP). In elongating pollen tubes, most plastids localized to the tube shank, where they displayed either retarded and unsteady motion, or fast, directional, and long-distance movement along the tube polarity. Efficient plastid tracking further revealed a population of tip-forwarding plastids that undergo a fluctuating motion(s) before traveling backwards. The behavior of YFP-labeled plastids in pollen basically resembled that of FtsZ1-GFP-labeled plastids, thus validating the use of FtsZ1-GFP for simultaneous visualization of the stroma and the plastid-dividing FtsZ ring.

The appearance of leaf mesophyll chloroplasts in angiosperms is characterized by their uniform and static shape, which is molded by symmetric division of the preexisting organelles, involving three prokaryote-derived proteins: the division executor protein, FtsZ, and the division site positioning proteins, MinD and MinE. On the other hand, noncolored plastids in roots, where the involvement of the known chloroplast division factors in plastid morphogenesis is yet unclear, are morphologically heterogeneous and transform dynamically. This is further emphasized by the active formation of long tubular protrusions called stromules from the main body of those plastids. Molecular regulation and physiological significance of such dynamic morphology of root plastids also remain unknown. In this context, we have recently demonstrated that the mitochondrial respiratory inhibitor antimycin A induces rapid and reversible filamentation of root plastids (leucoplasts) in Arabidopsis thaliana. In contrast, the same treatment with antimycin A did not affect the morphology of amyloplasts in the columella cells at the root tip. The alternative oxidase inhibitor salicylhydroxamic acid suppresses the antimycin-induced plastid filamentation, perhaps implying an alternative oxidase-mediated interorganellar signaling between the mitochondria and the leucoplasts in the root cells. Our data may provide some clues as to how the formation of stromules is initiated.

The unicellular rhodophyte Cyanidioschyzon merolae, having a single plastid and a single mitochondrion, is suitable for the analysis of the cell cycle involving the division of organelles. In conventional methods of synchronous culture of algae, light/dark cycles have been used as signals for synchronization, and the gene expression promoted by light is not separated from the gene expression related to cell cycle progression. We previously devised a novel synchronous culture system with controlled photosynthesis, which is triggered by 6 h-light/18 h-dark cycles combined with different levels of CO2. The cells do not enter S-phase and consequently do not divide after the minimum light period without CO2 supplementation, but do divide after a light period with 1 % CO2. In this way, we can compare a dividing cycle and a non-dividing cycle. We examined changes in the expression of 74 genes throughout the cell cycle by quantitative RTPCR. The expression of genes for two cyclins (cyclin C and H) and two CDKs (CDKA and CDKD) as well as metabolic enzymes was promoted by light, whereas the expression of genes for G1/S or G2/M cyclins and CDKs as well as DNA replication enzymes and proteins related to organellar division was promoted only in the dividing cycles. These results suggested that C. merolae has a checkpoint for G1/S progression, which is regulated by nutrients within the 6 h light period.

The behaviour and multiplication of pollen plastids have remained elusive despite their crucial involvement in cytoplasmic inheritance. Here, we present live images of plastids in pollen grains and growing tubes from transgenic Arabidopsis thaliana lines expressing stroma-localised FtsZ1-green-fluorescent protein fusion in a vegetative cell-specific manner. Vegetative cells in mature pollen contained a morphologically heterogeneous population of round to ellipsoidal plastids, whilst those in late-developing (maturing) pollen included plastids that could have one or two constriction sites. Furthermore, plastids in pollen tubes exhibited remarkable tubulation, stromule (stroma-filled tubule) extension, and back-and-forth movement along the direction of tube growth. Plastid division, which involves the FtsZ1 ring, was rarely observed in mature pollen grains.

Plastids assume various morphologies depending on their developmental status, but the basis for developmentally regulated plastid morphogenesis is poorly understood. Chemical induction of alterations in plastid morphology would be a useful tool for studying this; however, no such chemicals have been identified. Here, we show that antimycin A, an effective respiratory inhibitor, can change plastid morphology rapidly and reversibly in Arabidopsis thaliana. In the root cortex, hypocotyls, cotyledon epidermis and true leaf epidermis, significant differences in mitochondrial morphology were not observed between antimycin-treated and untreated tissues. In contrast, antimycin caused extreme filamentation of plastids in the mature cortices of main roots. This phenomenon was specifically observed in the mature root cortex. Other mitochondrial respiratory inhibitors (rotenone and carbonyl cyanide m-chlorophenylhydrazone), hydrogen peroxide, S-nitroso-N-acetylpenicillamine [a nitric oxide (NO) donor] and 3-(3,4-dichlorophenyl)-1,1-dimethylurea did not mimic the phenomenon under the present study conditions. Antimycin-induced plastid filamentation was initiated within 5 min after the onset of chemical treatment and appeared to complete within 1 h. Plastid morphology was restored within 7 h after the washout of antimycin, suggesting that the filamentation was reversible. Co-applications of antimycin and cytoskeletal inhibitors (demecolcine or latrunculin B) or protein synthesis inhibitors (cycloheximide or chloramphenicol) still caused plastid filamentation. Antimycin A was also effective for plastid filamentation in the chloroplast division mutants atftsZ1-1 and atminE1. Salicylhydroxamic acid, an alternative oxidase inhibitor, was solely found to suppress the filamentation, implying the possibility that this phenomenon was partly mediated by an antimycin-activated alternative oxidase in the mitochondria.

The filamentous cyanobacterium Anabaena sp. PCC 7120 fixes dinitrogen facultatively. Upon depletion of combined nitrogen, about 10% of vegetative cells within the filaments differentiate terminally into nitrogen-fixing cells. The heterocyst has been studied as a model system of prokaryotic cell differentiation, with major focus on signal transduction and pattern formation. The fate of heterocyst differentiation is determined at about the eighth hour of induction (point of no return), well before conspicuous morphological or metabolic changes occur. However, little is known about how the initial heterocysts are selected after the induction by nitrogen deprivation. To address this question, we followed the fate of every cells on agar plates after nitrogen deprivation with an interval of 4 h. About 10% of heterocysts were formed without prior division after the start of nitrogen deprivation. The intensity of fluorescence of GFP in the transformants of hetR-gfp increased markedly in the future heterocysts at the fourth hour with respect to other cells. We also noted that the growing filaments consisted of clusters of four consecutive cells that we call quartets. About 75% of initial heterocysts originated from either of the two outer cells of quartets at the start of nitrogen deprivation. These results suggest that the future heterocysts are loosely selected at early times after the start of nitrogen deprivation, before the commitment. Such early candidacy could be explained by different properties of the outer and inner cells of a quartet, but the molecular nature of candidacy remains to be uncovered.

While it has been established that binary fission of leaf chloroplasts requires the prokaryote-derived, division site determinant protein MinE, it remains to be clarified whether chloroplast division in non-leaf tissues and the division of non-colored plastids also involve the MinE protein. In an attempt to address this issue, plastids of cotyledons, floral organs, and roots were examined in the Arabidopsis thaliana mutant of the MinE (AtMinE1) gene, which was modified to express the plastid-targeted cyan fluorescent protein constitutively, and were quantitatively compared with those in the wild type. In the cotyledons, floral organs, and root columella, the plastid size in the atminE1 mutant was significantly larger than in the wild type, while the plastid number per cell in atminE1 appeared to be inversely smaller than that in the wild type. In addition, formation of the stroma-containing plastid protrusions (stromules) in the cotyledon epidermis, petal tip, and root cells was more active in atminE1 than in the wild type.

Symmetric chloroplast division requires a prokaryote-derived division regulator protein MinD, whose subchloroplastic localization remains to be completely established. We investigated the localization and functionality of AtMinD1 (Arabidopsis thaliana MinD) fused with a dual hemagglutinin epitope (dHA) or a yellow fluorescent protein (YFP). AtMinD1-dHA, which successfully complemented the arc11/atminD1 mutant phenotype, was predominantly located at the envelope membrane and the mid-chloroplast constriction site. Meanwhile, AtMinD1-YFP was non-functional and showed suborganellar localization partly similar to that of AtMinD1-dHA. This prompts us to reevaluate earlier transgenic and transient expression studies using fluorescent protein-tagged AtMinD1.

Chloroplast division involves the tubulin-related GTPase FtsZ that assembles into a ring structure (Z-ring) at the mid-chloroplast division site, which is where invagination and constriction of the envelope membranes occur. Z-ring assembly is usually confined to the mid-chloroplast site by a well balanced counteraction of the stromal proteins MinD and MinE. The in vivo mechanisms by which FtsZ nucleates at specific sites, polymerises into a protofil-ament and organises a closed ring of filament bundles remain largely unknown. To clarify the dynamic aspects of FtsZ, we developed a living cell system for simultaneous visualisation of various FtsZ configurations, utilising the Arabidopsis thaliana overexpressor and mutant of the MinE (AtMinE1) gene, which were modified to weakly express green fluorescent protein (GFP) fused to AtFtsZ1-1. Time-lapse observation in the chloroplasts of both plants revealed disorderly movement of the dots and short filaments of FtsZ. The short filaments often appeared to emanate from the dots and to converge with a long filament, producing a thick cable. In the AtMinE1 overexpressor, we also observed spirals along the longitudinal axis of the organelle that often rolled the closed rings together. In the atminE1 mutant, we visualised the isolated rings with a maximum diameter of 2m that did not encircle the organelle periphery, but appeared to be suspended in the stroma. Our observations further demonstrated heterogeneity in chloroplast shapes and concurrently altered configurations of FtsZ in the mutant.

To elucidate the mechanism(s) underlying dioecious flower development, the present study analyzed a SUPERMAN (SUP) homolog, SlSUP, which was identified in Silene latifolia. The sex of this plant is determined by heteromorphic X and Y sex chromosomes. It was revealed that SlSUP is a single-copy autosomal gene expressed exclusively in female flowers. Introduction of a genomic copy of SlSUP into the Arabidopsis thaliana sup (sup-2) mutant complemented the excess-stamen and infertile phenotypes of sup-2, and the overexpression of SlSUP in transgenic Arabidopsis plants resulted in reduced stamen numbers as well as the suppression of petal elongation. During the development of the female flower in S. latifolia, the expression of SlSUP is first detectable in whorls 2 and 3 when the normal expression pattern of the B-class flowering genes was already established and persisted in the stamen primordia until the ovule had matured completely. In addition, significant expression of SlSUP was detected in the ovules, suggestive of the involvement of this gene in ovule development. Furthermore, it was revealed that the de-suppression of stamen development by infection of the S. latifolia female flower with Microbotryum violaceum was accompanied by a significant reduction in SlSUP transcript levels in the induced organs. Taken together, these results demonstrate that SlSUP is a female flower-specific gene and suggest that SlSUP has a positive role in the female flower developmental pathways of S. latifolia

Plastids are maintained in cells by proliferating prior to cell division and being partitioned to each daughter cell during cell division. It is unclear, however, whether cells without plastids are generated when plastid division is suppressed. The crumpled leaf (crl) mutant of Arabidopsis thaliana is a plastid division mutant that displays severe abnormalities in plastid division and plant development. We show that the crl mutant contains cells lacking detectable plastids; this situation probably results from an unequal partitioning of plastids to each daughter cell. Our results suggest that crl has a partial defect in plastid expansion, which is suggested to be important in the partitioning of plastids to daughter cells when plastid division is suppressed. The absence of cells without detectable plastids in the accumulation and replication of chloroplasts 6 (arc6) mutant, another plastid division mutant of A. thaliana having no significant defects in plant morphology, suggests that the generation of cells without detectable plastids is one of the causes of the developmental abnormalities seen in crl plants. We also demonstrate that plastids with trace or undetectable amounts of chlorophyll are generated from enlarged plastids by a non-binary fission mode of plastid replication in both crl and arc6.

Chloroplasts are descendents of a cyanobacterial endosymbiont, but many chloroplast protein genes of endosymbiont origin are encoded by the nucleus. The chloroplastcyanobacteria relationship is a typical target of orthogenomics, an analytical method that focuses on the relationship of orthologous genes. Here, we present results of a pilot study of functional orthogenomics, combining bioinformatic and experimental analyses, to identify nuclear-encoded chloroplast proteins of endosymbiont origin (CPRENDOs). Phylogenetic profiling based on complete clustering of all proteins in 17 organisms, including eight cyanobacteria and two photosynthetic eukaryotes, was used to deduce 65 protein groups that are conserved in all oxygenic autotrophs analyzed but not in non-oxygenic organisms. With the exception of 28 well-characterized protein groups, 56 Arabidopsis proteins and 43 Synechocystis proteins in the 37 conserved homolog groups were analyzed. Green fluorescent protein (GFP) targeting experiments indicated that 54 Arabidopsis proteins were targeted to plastids. Expression of 39 Arabidopsis genes was promoted by light. Among the 40 disruptants of Synechocystis, 22 showed phenotypes related to photosynthesis. Arabidopsis mutants in 21 groups, including those reported previously, showed phenotypes. Characteristics of pulse amplitude modulation fluorescence were markedly different in corresponding mutants of Arabidopsis and Synechocystis in most cases. We conclude that phylogenetic profiling is useful in finding CPRENDOs, but the physiological functions of orthologous genes may be different in chloroplasts and cyanobacteria.

DNA polymerase gamma, a mitochondrial replication enzyme of yeasts and animals, is not present in photosynthetic eukaryotes. Recently, DNA polymerases with distant homology to bacterial DNA polymerase I were reported in rice, Arabidopsis, and tobacco, and they were localized to both plastids and mitochondria. We call them plant organellar DNA polymerases (POPs). However, POPs have never been purified in the native form from plant tissues. The unicellular thermotrophic red alga Cyanidioschyzon merolae contains two genes encoding proteins related to Escherichia coli DNA polymerase I (PolA and PolB). Phylogenetic analysis revealed that PolB is an ortholog of POPs. Nonphotosynthetic eukaryotes also have POPs, which suggested that POPs have an ancient origin before eukaryotic photosynthesis. PolA is a homolog of bacterial DNA polymerase I and is distinct from POPs. PolB was purified from the C. merolae cells by a series of column chromatography steps. Recombinant protein of PolA was also purified. Sensitivity to inhibitors of DNA synthesis was different in PolA, PolB, and E. coli DNA polymerase I. Immunoblot analysis and targeting studies with green fluorescent protein fusion proteins demonstrated that PolA was localized in the plastids, whereas PolB was present in both plastids and mitochondria. The expression of PolB was regulated by the cell cycle. The available results suggest that PolB is involved in the replication of plastids and mitochondria.

Chloroplast division comprises a sequence of events that facilitate symmetric binary fission and that involve prokaryotic-like stromal division factors such as tubulin-like GTPase FtsZ and the division site regulator MinD. In Arabidopsis, a nuclear-encoded prokaryotic MinE homolog, AtMinE1, has been characterized in terms of its effects on a dividing or terminal chloroplast state in a limited series of leaf tissues. However, the relationship between AtMinE1 expression and chloroplast phenotype remains to be fully elucidated. Here, we demonstrate that a T-DNA insertion mutation in AtMinE1 results in a severe inhibition of chloroplast division, producing motile dots and short filaments of FtsZ. In AtMinE1 sense (overexpressor) plants, dividing chloroplasts possess either single or multiple FtsZ rings located at random intervals and showing constriction depth, mainly along the chloroplast polarity axis. The AtMinE1 sense plants displayed equivalent chloroplast phenotypes to arc11, a loss-of-function mutant of AtMinD1 which forms replicating mini-chloroplasts. Furthermore, a certain population of FtsZ rings formed within developing chloroplasts failed to initiate or progress the membrane constriction of chloroplasts and consequentially to complete chloroplast fission in both AtMinE1 sense and arc11/atminD1 plants. Our present data thus demonstrate that the chloroplast division site placement involves a balance between the opposing activities of AtMinE1 and AtMinD1, which acts to prevent FtsZ ring formation anywhere outside of the mid-chloroplast. In addition, the imbalance caused by an AtMinE1 dominance causes multiple, non-synchronous division events at the single chloroplast level, as well as division arrest, which becomes apparent as the chloroplasts mature, in spite of the presence of FtsZ rings.

We have developed an efficient system to detect and analyze DNA mutations induced by heavy-ion beams in Arabiopsis thaliana. In this system, a stable transgenic Arabidopsis line that constitutively expresses a yellow fluorescent protein (YFP) by a single-copy gene at a genomic locus was constructed and irradiated with heavy-ion beams. The YFP gene is a target of mutagenesis, and its loss of function or expression can easily be detected by the disappearance of YFP signals in planta under microscopy. With this system, a C-12(6+)-induced mutant with single deletion and multiple base changes was isolated.

Sulfite reductase (SiR) is an important enzyme catalyzing the reduction of sulfite to sulfide during sulfur assimilation in plants. This enzyme is localized in plastids, including chloroplasts, and uses ferredoxin as an electron donor. Ferredoxin-dependent SiR has been found in isolated chloroplast nucleoids, but its localization in vivo or in intact plastids has not been examined. Here, we report the DNA-binding properties of SiRs from pea (PsSiR) and maize (ZmSiR) using an enzymatically active holoenzyme with prosthetic groups. PsSiR binds to both double-stranded and single-stranded DNA without significant sequence specificity. DNA binding did not affect the enzymatic activity of PsSiR, suggesting that ferredoxin and sulfite are accessible to SiR molecules within the nucleoids. Comparison of PsSiR and ZmSiR suggests that ZmSiR does indeed have DNA-binding activity, as was reported previously, but the DNA affinity and DNA-compacting ability are higher in PsSiR than in ZmSiR. The tight compaction of nucleoids by PsSiR led to severe repression of transcription activity in pea nucleoids. Indirect immunofluorescence microscopy showed that the majority of SiR molecules colocalized with nucleoids in pea chloroplasts, whereas no particular localization to nucleoids was detected in maize chloroplasts. These results suggest that SiR plays an essential role in compacting nucleoids in plastids, but that the extent of association of SiR with nucleoids varies among plant species.

The phylogenetic positions of bryophytes and charophytes, together with their genome features, are important for understanding early land plant evolution. Here we report the complete nucleotide sequence (105,340 bp) of the circular-mapping mitochondrial DNA of the moss Physcomitrella patens. Available evidence suggests that the multipartite structure of the mitochondrial genome in flowering plants does not occur in Physcomitrella. It contains genes for 3 rRNAs (rnl, rns, and rrn5), 24 tRNAs, and 42 conserved mitochondrial proteins (14 ribosomal proteins, 4 ccm proteins, 9 nicotinamide adenine dinucleotide dehydrogenase subunits, 5 ATPase subunits, 2 succinate dehydrogenase subunits, apocytochrome b, 3 cytochrome oxidase subunits, and 4 other proteins). We estimate that 5 tRNA genes are missing that might be encoded by the nuclear genome. The overall mitochondrial genome structure is similar in Physcomitrella, Chara vulgaris, Chaetosphaeridium globosum, and Marchantia polymorpha, with easily identifiable inversions and translocations. Significant synteny with angiosperm and chlorophyte mitochondrial genomes was not detected. Phylogenetic analysis of 18 conserved proteins suggests that the moss-liverwort clade is sister to angiosperms, which is consistent with a previous analysis of chloroplast genes but is not consistent with some analyses using mitochondrial sequences. In Physcomitrella, 27 introns are present within 16 genes. Nine of its intron positions are shared with angiosperms and 4 with Marchantia, which in turn shares only one intron position with angiosperms. The phylogenetic analysis as well as the syntenic structure suggest that the mitochondrial genomes of Physcomitrella and Marchantia retain prototype features among land plant mitochondrial genomes.

Chloroplast genes of higher plants are transcribed by two types of RNA polymerase that are encoded by nuclear (NEP (nuclear-encoded plastid RNA polymerase)) or plastid (PEP (plastid-encoded plastid RNA polymerase)) genomes. NEP is largely responsible for the transcription of housekeeping genes during early chloroplast development. Subsequent light-dependent chloroplast maturation is accompanied by repression of NEP activity and activation of PEP. Here, we show that the plastid-encoded transfer RNA for glutamate, the expression of which is dependent on PEP, directly binds to and inhibits the transcriptional activity of NEP in vitro. The plastid tRNA(Glu) thus seems to mediate the switch in RNA polymerase usage from NEP to PEP during chloroplast development.

Chloroplasts originate from ancient cyanobacteria-like endosymbiont. Several tens of chloroplast proteins are encoded by the chloroplast genome, while more than hundreds are encoded by the nuclear genome in plants and algae, but the exact number and identity of nuclear-encoded chloroplast proteins are still unknown. We describe here attempts to identify a large number of unidentified chloroplast proteins of endosymbiont origin (CPRENDOs). Our strategy consists of whole genome protein clustering by the homolog group method, which is optimized for organism number, and phylogenetic profiling that extract groups conserved in cyanobacteria and photosynthetic eukaryotes. An initial minimal set of CPRENDOs was predicted without targeting prediction and experimentally validated.

Chloroplast division is mediated by the coordinated action of a prokaryote-derived division system(s) and a host eukaryote-derived membrane fission system(s). The evolutionary conserved prokaryote-derived system comprises several nucleus-encoded proteins, two of which are thought to control division site placement at the midpoint of the organelle: a stromal ATPase MinD and a topological specificity factor MinE. Here, we show that arc11, one of 12 recessive accumulation and replication of chloroplasts (arc) mutants in Arabidopsis, contains highly elongated and multiple-arrayed chloroplasts in developing green tissues. Genomic sequence analysis revealed that arc11 contains a missense mutation in alpha-helix 11 of the chloroplast-targeted AtMinD1 changing an Ala at position 296 to Gly (A296G). Introduction of wild-type AtMinD1 restores the chloroplast division defects of arc11 and quantitative RT-PCR analysis showed that the degree of complementation was highly dependent on transgene expression levels. Overexpression of the mutant ARC11/AtMinD1 in transgenic plants results in the inhibition of chloroplast division, showing that the mutant protein has retained its division inhibition activity. However, in contrast to the defined and punctate intraplastidic localization patterns of an AtMinD1-YFP fusion protein, the single A296G point mutation in ARC11/AtMinD1 results in aberrant localization patterns inside chloroplasts. We further show that AtMinD1 is capable of forming homodimers and that this dimerization capacity is abolished by the A296G mutation in ARC11/AtMinD1. Our data show that arc11 is a loss-of-function mutant of AtMinD1 and suggest that the formation of functional AtMinD1 homodimers is paramount for appropriate AtMinD1 localization, ultimately ensuring correct division machinery placement and chloroplast division in plants.

Plastids are vital plant organelles involved in many essential biological processes. Plastids are not created de novo but divide by binary fission mediated by nuclear-encoded proteins of both prokaryotic and eukaryotic origin. Although several plastid division proteins have been identified in plants, limited information exists regarding possible division control mechanisms. Here, we describe the identification of GIANT CHLOROPLAST 1 (GC1), a new nuclear-encoded protein essential for correct plastid division in Arabidopsis. GC1 is plastid-localized and is anchored to the stromal surface of the chloroplast inner envelope by a C-terminal amphipathic helix. In Arabidopsis, GC1 deficiency results in mesophyll cells harbouring one to two giant chloroplasts, whilst GC1 overexpression has no effect on division. GC1 can form homodimers but does not show any interaction with the Arabidopsis plastid division proteins AtFtsZ1-1, AtFtsZ2-1, AtMinD1, or AtMinE1. Analysis reveals that GC1-deficient giant chloroplasts contain densely packed wild-type-like thylakoid membranes and that GC1-deficient leaves exhibit lower rates of CO(2) assimilation compared to wild-type. Although GC1 shows similarity to a putative cyanobacterial SulA cell division inhibitor, our findings suggest that GC1 does not act as a plastid division inhibitor but, rather, as a positive factor at an early stage of the division process.

Transcription in higher plant plastids is performed by two types of RNA polymerases called NEP and PEP, and expression of photosynthesis genes in chloroplasts is largely dependent on PEP, a eubacteria-type multi-subunit enzyme. The transcription specificity of PEP is modulated by six nuclear-encoded sigma factors (SIG1 to SIG6) in Arabidopsis thaliana. Here, we show that one of the six sigma factors, SIG5, is induced under various stress conditions, such as high light, low temperature, high salt and high osmotic conditions. Interestingly, transcription from the psbD blue light-responsive promoter (psbD-BLRP) was activated by not only light but also various stresses, and the transcription and the transcriptional activation of psbD-BLRP were abolished in a sig5-2 mutant. This suggests that the PEP holoenzyme containing SIG5 transcribes the psbD-BLRP in response to multiple stresses. Since the seed germination under saline conditions and recovery from damage to the PSII induced by high light were delayed in the sig5-2 mutant, we postulate that SIG5 protects plants from stresses by enhancing repair of the PSII reaction center.

Despite numerous physiological studies addressing the interactions between brassinosteroids (BRs) and auxins, little is known about the underlying molecular mechanisms. We studied the expression of IAA5 and IAA19 in response to treatment with indole acetic acid (IAA) or brassinolide (BL), the most active BR. Exogenous IAA induced these genes quickly and transiently, whereas exogenous BL induced them gradually and continuously. We also found that a fusion of DR5, a synthetic auxin response element, with the GUS (beta-glucuronidase) gene was induced with similar kinetics to those of the IAA5 and IAA19 genes in response to both IAA and BL treatment of transgenic plants. These results suggest that the IAA genes are induced by BL, at least in part, via the activation of the auxin response element. Endogenous IAA levels per gram fresh weight did not increase when seedlings of Arabidopsis wild type (WT) or the BR-deficient mutant det2 were treated with BL. Furthermore, the levels of IAA transcripts were lower in the det2 mutant than in the WT, even though endogenous IAA levels per gram fresh weight were higher in the det2 mutant than in the WT. In conclusion, the lack of evidence for auxin-mediated activation of early auxin-inducible genes in response to BL suggests that the BR and auxin signaling pathways independently activate the transcriptional system of the IAA and DR5-GUS genes.

Limited information is available concerning the interactions between the brassinosteroid (BR) and auxin signaling pathways. The expression pattern of the SAUR-AC1 gene, an early auxin-inducible gene in Arabidopsis, was studied in response to brassinolide (BL), in the presence of a BR-biosynthesis inhibitor, in a BR-deficient mutant, and in combination with auxin. The results suggested that the SAUR-AC1 gene is regulated by BRs independently of auxin levels, and that it is important in BR-mediated elongation. The axr1 (auxin insensitive 1) mutant was less sensitive to BL-induced elongation and BL-induced SAUR-AC1 expression, suggesting that a ubiquitin ligase-mediated system is involved in BR-mediated elongation.

The functions of two previously identified chitin synthase genes in Aspergillus nidulans, chsB and chsD, were analysed. First, a conditional chsB mutant was constructed in which the expression of chsB is under the control of a repressible promoter, the alcA promoter, of A. nidulans. Under repressing conditions, the mutant grew slowly and produced highly branched hyphae, supporting the idea that chsB is involved in normal hyphal growth. The involvement of chsB in conidiation was also demonstrated. Next, double mutants of chsB and chsD were constructed, in which chsB was placed under the control of the alcA promoter and chsD was replaced with the argB gene of A. nidulans. These double mutants grew more slowly than the chsB single mutant under high-osmolarity conditions. The hyphae of the double mutant appeared to be more disorganized than those of the chsB single mutant. It was also found that ChsD was functionally implicated in conidiation when the expression of chsB was limited. These results indicate the importance of the ChsD function in the absence of chsB expression. The roles of ChsB and ChsD in hyphal growth and in conidiation were supported by the analysis of the spatial expression patterns of chsB and chsD, using lacZ of Escherichia coli as a reporter gene.

A eubacteria-type RNA polymerase (PEP) plays crucial roles for chloroplast development in higher plants. The core subunits are encoded on plastid DNA (rpo genes) while the regulatory sigma factors are encoded on the nuclear DNA (SIG genes). However, the definite gene specificity of each sigma factor is unknown. We recently identified an Arabidopsis recessive pale-green mutant abc1 in which T-DNA is inserted in SIG2 (sigB). In this mutant, almost normal etioplasts were developed under dark conditions while the small chloroplasts with poor thylakoid membranes and stacked lamellar were developed under light conditions. The sig2-1 mutant was deficient in accumulating enough photosynthetic and photosynthesis-related proteins as well as chlorophyll. However, mRNAs of their structural genes were not significantly reduced. Further analyses revealed that several plastid-encoded tRNAs including trnE-UUC that has dual function for protein and ALA biosyntheses were drastically reduced in the sig2-1 mutant. In contrast, nucleus-encoded T7 phage-type RNA polymerase (NEP)-dependent gene transcripts were steadily accumulated in the mutant. These results indicate that progress of chloroplast development requires SIG2-dependent expression of plastid genes, particularly some of the tRNA genes.

We have isolated and characterized two genes from Nicotiana tabacum, whose products function as putative sigma factors for plastid RNA polymerase. Since the amino acid sequence deduced from the DNA sequences of both genes showed highly similar to that of the SigA protein of Arabidopsis thaliana, we termed the corresponding genes sigA1 and sigA2, respectively. Transient expression assay using a green fluorescent protein (GFP) fusion construct indicated that the N-terminal region of the sigA2 gene product could function as a transit peptide for import into chloroplasts. The gel-blot analysis of RNAs revealed that the sum of the sigA1 and sigA2 transcripts fluctuated apparently with an endogenous rhythm after 12-h-light, 12-h-dark entrainment in photomixotrophically cultured tobacco cells. RT-PCR based northern analysis revealed that the sigA1 and sigA2 transcripts increased along with the cell growth in cultured cells, and were most abundant in mature leaves and shoot meristems with very young leaves in tobacco plants. Immunoblot analysis of the cell extracts of tobacco plants also supports this notion. These results suggest that the sigma factors encoded by sigA1 and sigA2 play a role in chloroplast development and regulation of gene expression in matured chloroplasts.

Development of plastids into chloroplasts, the organelles of photosynthesis, is triggered by light. However, little is known of the factors involved in the complex coordination of light-induced plastid gene expression, which must be directed by both nuclear and plastid genomes. We have isolated an Arabidopsis mutant, abc1, with impaired chloroplast development, which results in a pale green leaf phenotype. The mutated nuclear gene encodes a sigma factor, SigB, presumably for the eubacterial-like plastid RNA polymerase. Our results provide direct evidence that a nuclear-derived prokaryotic-like SigB protein, plays a critical role in the coordination of the two genomes for chloroplast development.