<title>Abstract</title>
PGR3 is a P-class pentatricopeptide repeat (PPR) protein required for the stabilization of petL operon RNA and the translation of the petL gene in plastids. Irrespective of its important roles in plastids, key questions have remained unanswered, including how PGR3 protein promotes translation and which plastid mRNA PGR3 activates the translation. Here, we show that PGR3 facilitates the translation from ndhG, in addition to petL, through binding to their 5′ untranslated regions (UTRs). Ribosome profiling and RNA sequencing in pgr3 mutants revealed that translation from petL and ndhG was specifically suppressed. Harnessing small RNA fragments protected by PPR proteins in vivo, we probed the PGR3 recruitment to the 5′ UTRs of petL and ndhG. The putative PGR3-bound RNA segments per se repress the translation possibly with a strong secondary structure and thereby block ribosomes’ access. However, the PGR3 binding antagonizes the effects and facilitates the protein synthesis from petL and ndhG in vitro. The prediction of the 3-dimensional structure of PGR3 suggests that the 26th PPR motif plays important roles in target RNA binding. Our data show the specificity of a plastidic RNA-binding protein and provide a mechanistic insight into translational control.

When DNA double-strand breaks occur, four-stranded DNA structures called Holliday junctions (HJs) form during homologous recombination. Because HJs connect homologous DNA by a covalent link, resolution of HJ is crucial to terminate homologous recombination and segregate the pair of DNA molecules faithfully. We recently identified Monokaryotic Chloroplast1 (MOC1) as a plastid DNA HJ resolvase in algae and plants. Although Cruciform cutting endonuclease1 (CCE1) was identified as a mitochondrial DNA HJ resolvase in yeasts, homologs or other mitochondrial HJ resolvases have not been identified in other eukaryotes. Here, we demonstrate that MOC1 depletion in the green alga Chlamydomonas reinhardtii and the moss Physcomitrella patens induced ectopic recombination between short dispersed repeats in ptDNA. In addition, MOC1 depletion disorganized thylakoid membranes in plastids. In some land plant lineages, such as the moss P. patens, a liverwort and a fern, MOC1 dually targeted to plastids and mitochondria. Moreover, mitochondrial targeting of MOC1 was also predicted in charophyte algae and some land plant species. Besides causing instability of plastid DNA, MOC1 depletion in P. patens induced short dispersed repeat-mediated ectopic recombination in mitochondrial DNA and disorganized cristae in mitochondria. Similar phenotypes in plastids and mitochondria were previously observed in mutants of plastid-targeted (RECA2) and mitochondrion-targeted (RECA1) recombinases, respectively. These results suggest that MOC1 functions in the double-strand break repair in which a recombinase generates HJs and MOC1 resolves HJs in mitochondria of some lineages of algae and plants as well as in plastids in algae and plants.

<title>Abstract</title>
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<title>Background</title>
Plastid electron transport systems are essential not only for photosynthesis but also for dissipating excess reducing power and sinking excess electrons generated by various redox reactions. Although numerous organisms with plastids have lost their photoautotrophic lifestyles, there is a spectrum of known functions of remnant plastids in non-photosynthetic algal/plant lineages; some of non-photosynthetic plastids still retain diverse metabolic pathways involving redox reactions while others, such as apicoplasts of apicomplexan parasites, possess highly reduced sets of functions. However, little is known about underlying mechanisms for redox homeostasis in functionally versatile non-photosynthetic plastids and thus about the reductive evolution of plastid electron transport systems.


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<sec>
<title>Results</title>
Here we demonstrated that the central component for plastid electron transport systems, plastoquinone/plastoquinol pool, is still retained in a novel strain of an obligate heterotrophic green alga lacking the photosynthesis-related thylakoid membrane complexes. Microscopic and genome analyses revealed that the Volvocales green alga, chlamydomonad sp. strain NrCl902, has non-photosynthetic plastids and a plastid DNA that carries no genes for the photosynthetic electron transport system. Transcriptome-based in silico prediction of the metabolic map followed by liquid chromatography analyses demonstrated carotenoid and plastoquinol synthesis, but no trace of chlorophyll pigments in the non-photosynthetic green alga. Transient RNA interference knockdown leads to suppression of plastoquinone/plastoquinol synthesis. The alga appears to possess genes for an electron sink system mediated by plastid terminal oxidase, plastoquinone/plastoquinol, and type II NADH dehydrogenase. Other non-photosynthetic algae/land plants also possess key genes for this system, suggesting a broad distribution of an electron sink system in non-photosynthetic plastids.


</sec>
<sec>
<title>Conclusion</title>
The plastoquinone/plastoquinol pool and thus the involved electron transport systems reported herein might be retained for redox homeostasis and might represent an intermediate step towards a more reduced set of the electron transport system in many non-photosynthetic plastids. Our findings illuminate a broadly distributed but previously hidden step of reductive evolution of plastid electron transport systems after the loss of photosynthesis.


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The sexual cycle of the unicellular Chlamydomonas reinhardtii culminates in the formation of diploid zygotes that differentiate into dormant spores that eventually undergo meiosis. Mating between gametes induces rapid cell wall shedding via the enzyme g-lysin; cell fusion is followed by heterodimerization of sex-specific homeobox transcription factors, GSM1 and GSP1, and initiation of zygote-specific gene expression. To investigate the genetic underpinnings of the zygote developmental pathway, we performed comparative transcriptome analysis of both pre- and post-fertilization samples. We identified 253 transcripts specifically enriched in early zygotes, 82% of which were not up-regulated in gsp1 null zygotes. We also found that the GSM1/GSP1 heterodimer negatively regulates the vegetative wall program at the posttranscriptional level, enabling prompt transition from vegetative wall to zygotic wall assembly. Annotation of the g-lysin-induced and early zygote genes reveals distinct vegetative and zygotic wall programs, supported by concerted up-regulation of genes encoding cell wall-modifying enzymes and proteins involved in nucleotide-sugar metabolism. The haploid-to-diploid transition in Chlamydomonas is masterfully controlled by the GSM1/GSP1 heterodimer, translating fertilization and gamete coalescence into a bona fide differentiation program. The fertilization-triggered integration of genes required to make related, but structurally and functionally distinct organelles-the vegetative versus zygote cell wall-presents a likely scenario for the evolution of complex developmental gene regulatory networks.

Holliday junctions, four-stranded DNA structures formed during homologous recombination, are disentangled by resolvases that have been found in prokaryotes and eukaryotes but not in plant organelles. Here, we identify monokaryotic chloroplast 1 (MOC1) as a Holliday junction resolvase in chloroplasts by analyzing a green alga Chlamydomonas reinhardtii mutant defective in chloroplast nucleoid (DNA-protein complex) segregation. MOC1 is structurally similar to a bacterial Holliday junction resolvase, resistance to ultraviolet (Ruv) C, and genetically conserved among green plants. Reduced or no expression of MOC1 in Arabidopsis thaliana leads to growth defects and aberrant chloroplast nucleoid segregation. In vitro biochemical analysis and high-speed atomic force microscopic analysis revealed that A. thaliana MOC 1 (AtMOC1) binds and cleaves the core of Holliday junctions symmetrically. MOC1 may mediate chloroplast nucleoid segregation in green plants by resolving Holliday junctions.

The chloroplast (cp) genome is organized as nucleoids that are dispersed throughout the cp stroma. Previously, a cp homolog of bacterial recombinase RecA (cpRECA) was shown to be involved in the maintenance of cp genome integrity by repairing damaged chloroplast DNA and by suppressing aberrant recombination between short dispersed repeats in the moss Physcomitrella patens. Here, overexpression and knockdown analysis of cpRECA in the green alga Chlamydomonas reinhardtii revealed that cpRECA was involved in cp nucleoid dynamics as well as having a role in maintaining cp genome integrity. Overexpression of cpRECA tagged with yellow fluorescent protein or hemagglutinin resulted in the formation of giant filamentous structures that colocalized exclusively to chloroplast DNA and cpRECA localized to cp nucleoids in a heterogenous manner. Knockdown of cpRECA led to a significant reduction in cp nucleoid number that was accompanied by nucleoid enlargement. This phenotype resembled those of gyrase inhibitor-treated cells and monokaryotic chloroplast mutant cells and suggested that cpRECA was involved in organizing cp nucleoid dynamics. The cp genome also was destabilized by induced recombination between short dispersed repeats in cpRECA-knockdown cells and gyrase inhibitor-treated cells. Taken together, these results suggest that cpRECA and gyrase are both involved in nucleoid dynamics and the maintenance of genome integrity and that the mechanisms underlying these processes may be intimately related in C. reinhardtii cps.

Sex-determining regions (SDRs) or mating-type (MT) loci in two sequenced volvocine algal species, Chlamydomonas reinhardtii and Volvox carteri, exhibit major differences in size, structure, gene content, and gametolog differentiation. Understanding the origin of these differences requires investigation of MT loci from related species. Here, we determined the sequences of the minus and plus MT haplotypes of the isogamous 16-celled volvocine alga, Gonium pectorale, which is more closely related to the multicellular V. carteri than to C. reinhardtii. Compared to C. reinhardtii MT, G. pectorale MT is moderately larger in size, and has a less complex structure, with only two major syntenic blocs of collinear gametologs. However, the gametolog content of G. pectorale MT has more overlap with that of V. carteri MT than with C. reinhardtii MT, while the allelic divergence between gametologs in G. pectorale is even lower than that in C. reinhardtii. Three key sex-related genes are conserved in G. pectorale MT: GpMID and GpMTD1 in MT-, and GpFUS1 in MT+. GpFUS1 protein exhibited specific localization at the plus-gametic mating structure, indicating a conserved function in fertilization. Our results suggest that the G. pectorale-V. carteri common ancestral MT experienced at least one major reformation after the split from C. reinhardtii, and that the V. carteri ancestral MT underwent a subsequent expansion and loss of recombination after the divergence from G. pectorale. These data begin to polarize important changes that occurred in volvocine MT loci, and highlight the potential for discontinuous and dynamic evolution in SDRs.

Chloroplast (cp) DNA is compacted into cpDNA-protein complexes, called cp nucleoids. An abundant and extensively studied component of cp nucleoids is the bifunctional protein sulfite reductase (SiR). The preconceived role of SiR as the core cp nucleoid protein, however, is becoming less likely because of the recent findings that SiRs do not associate with cp nucleoids in some plant species, such as Zea mays and Arabidopsis thaliana. To address this discrepancy, we have performed a detailed phylogenetic analysis of SiRs, which shows that cp nucleoid-type SiRs share conserved C-terminally encoded peptides (CEPs). The CEPs are likely to form a bacterial ribbon helix helix DNA-binding motif, implying a potential role in attaching SiRs onto cp nucleoids. A proof-of-concept experiment was conducted by fusing the nonnucleoid-type SiR from A. thaliana (AtSiR) with the CEP from the cp nucleoid-type SiR of Phaseolus yulgaris. The addition of the CEP drastically altered the intra-cp localization of AtSiR to cp nucleoids. Our analysis supports the possible functions of CEPs in determining the localization of SiRs to cp nucleoids and illuminates a possible evolutionary scenario for SiR as a cp nucleoid protein.

Recently, the liverwort Marchantia polymorpha has received increasing attention as a basal plant model for multicellular studies. Its ease of handling, well-characterized plastome and proven protocols for biolistic plastid transformation qualify M. polymorpha as an attractive platform to study the evolution of chloroplasts during the transition from water to land. In addition, chloroplasts of M. polymorpha provide a convenient test-bed for the characterization of genetic elements involved in plastid gene expression due to the absence of mechanisms for RNA editing. While reporter genes have proven valuable to the qualitative and quantitative study of gene expression in chloroplasts, expression of green fluorescent protein (GFP) in chloroplasts of M. polymorpha has proven problematic. We report the design of a codon-optimized gfp varian, mturq2cp, which allowed successful expression of a cyan fluorescent protein under control of the tobacco psbA promoter from the chloroplast genome of M. polymorpha. We demonstrate the utility of mturq2cp in (i) early screening for transplastomic events following biolistic transformation of M. polymorpha spores; (ii) visualization of stromules as elements of plastid structure in Marchantia; and (iii) quantitative microscopy for the analysis of promoter activity.

Chloroplast (cp) DNA is thought to originate from the ancestral endosymbiont genome and is compacted to form nucleoprotein complexes, cp nucleoids. The structure of cp nucleoids is ubiquitously observed in diverse plants from unicellular algae to flowering plants and is believed to be a multifunctional platform for various processes, including cpDNA replication, repair/recombination, transcription, and inheritance. Despite its fundamental functions, the protein composition for cp nucleoids in flowering plants was suggested to be divergent from those of bacteria and algae, but the evolutionary process remains elusive. In this research, we aimed to reveal the evolutionary history of cp nucleoid organization by analyzing the key organisms representing the three evolutionary stages of eukaryotic phototrophs: the chlorophyte alga Chlamydomonas reinhardtii, the charophyte alga Klebsormidium flaccidum, and the most basal land plant Marchantia polymorpha. To clarify the core cp nucleoid proteins in C. reinhardtii, we performed an LC-MS/MS analysis using highly purified cp nucleoid fractions and identified a novel SAP domain-containing protein with a eukaryotic origin as a constitutive core component. Then, homologous genes for cp nucleoid proteins were searched for in C. reinhardtii, K. flaccidum, and M. polymorpha using the genome databases, and their intracellular localizations and DNA binding activities were investigated by cell biological/biochemical analyses. Based on these results, we propose a model that recurrent modification of cp nucleoid organization by eukaryotic factors originally related to chromatin organization might have been the driving force for the diversification of cp nucleoids since the early stage of green plant evolution.

Chloroplast DNA (cpDNA) encodes essential genes for chloroplast functions, including photosynthesis. Homologous recombination occurs frequently in cpDNA; however, its significance and underlying mechanism remain poorly understood. In this study, we analyzed the role of a nuclear-encoded chloroplast-localized homolog of RecA recombinase, which is a key factor in homologous recombination in bacteria, in the moss Physcomitrella patens. Complete knockout (KO) of the P. patens chloroplast RecA homolog RECA2 caused a modest growth defect and conferred sensitivity to methyl methanesulfonate and UV. The KO mutant exhibited low recovery of cpDNA from methyl methanesulfonate damage, suggesting that RECA2 knockout impairs repair of damaged cpDNA. The RECA2 KO mutant also exhibited reduced cpDNA copy number and an elevated level of cpDNA molecule resulting from aberrant recombination between short dispersed repeats (13-63 bp), indicating that the RECA2 KO chloroplast genome was destabilized. Taken together, these data suggest a dual role for RECA2 in the maintenance of chloroplast genome stability: RECA2 suppresses aberrant recombination between short dispersed repeats and promotes repair of damaged DNA.

We previously reported Agrobacterium-mediated transformation methods for the liverwort Marchantia polymorpha using the hygromycin phosphotransferase gene as a marker for selection with hygromycin. In this study, we developed three additional markers for M. polymorpha transformation: the gentamicin 3'-acetyltransferase gene for selection with gentamicin; a mutated acetolactate synthase gene for selection with chlorsulfuron; and the neomycin phosphotransferase II gene for selection with G418. Based on these four marker genes, we have constructed a series of Gateway binary vectors designed for transgenic experiments on M. polymorpha. The 35S promoter from cauliflower mosaic virus and endogenous promoters for constitutive and heat-inducible expression were used to create these vectors. The reporters and tags used were Citrine, 3xCitrine, Citrine-NLS, TagRFP, tdTomato, tdTomato-NLS, GR, SRDX, SRDX-GR, GUS, ELuc(PEST), and 3xFLAG. These vectors, designated as the pMpGWB series, will facilitate molecular genetic analyses of the emerging model plant M. polymorpha.

The primitive red alga Cyanidioschyzon merolae inhabits acidic hot springs and shows robust resistance to heat shock treatments up to 63 A degrees C. Microarray analysis was performed to identify the key genes underlying the high temperature tolerance of this organism. Among the upregulated genes that were identified, we focused on two small heat shock proteins (sHSPs) that belong to a unique class of HSP families. These two genes are located side by side in an inverted repeat orientation on the same chromosome and share a promoter. These two genes were simultaneously and rapidly upregulated in response to heat shock treatment (&gt; 1,000-fold more than the control). Interestingly, upregulation appeared to be triggered not by a difference in temperatures, but rather by the absolute temperature. Similar sHSP structural genes have been reported in the green alga Chlamydomonas reinhardtii, but the threshold temperature for the expression of these sHSP-encoding genes in Ch. reinhardtii was different from the threshold temperature for the expression of the sHSP genes from Cy. merolae. These results indicate the possible importance of an absolute temperature sensing system in the evolution and tolerance of high-temperature conditions among unicellular microalgae.

Chlorophylls (Chls) play pivotal roles in energy absorption and transduction and also in charge separation in reaction centers in all photosynthetic organisms. In Chl biosynthesis steps, only a step for the enzymatic reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) is mediated by both nuclear- and chloroplast-encoded genes in land plants. Many plants encode the genes for light-dependent Pchlide reductase (LPOR) and light-independent Pchlide reductase (DPOR) in the nucleus and chloroplast genome, respectively. During the diversification of land plants, the reduction step of Pchlide to Chlide has become solely dependent on LPOR, and the genes for DPOR have been lost from chloroplast genome. It remains unclear why DPOR persists in some land plants, how they were eliminated from chloroplast genomes during the diversification of land plants, and under what environmental conditions DPOR was required. We demonstrate that DPOR is functional in liverwort (Marchantia polymorpha L.) and plays an important role in Chl biosynthesis. Having established a plastid transformation system in liverwort, we disrupted chlB, which encodes a subunit of DPOR in the M. polymorpha chloroplast genome. Morphological and Chl content analysis of a chlB mutant grown under different photoperiods revealed that DPOR is particularly required for Chl biosynthesis under short-day conditions. Our findings suggest that an environmental condition in the form of photoperiod is an important factor that determines the loss or retention of chloroplast-encoded genes mediating Pchlide reduction to Chlide.

Background:Isogamous organisms lack obvious cytological differences in the gametes of the two complementary mating types. Consequently, it is difficult to ascertain which of the two mating types are homologous when comparing related but sexual isolated strains or species. The colonial volvocalean algal genus Gonium consists of such isogamous organisms with heterothallic mating types designated arbitrarily as plus or minus in addition to homothallic strains. Homologous molecular markers among lineages may provide an "objective" framework to assign heterothallic mating types.Methodology/Principal Findings:Using degenerate primers designed based on previously reported MID orthologs, the "master regulator" of mating types/sexes in the colonial Volvocales, MID homologs were identified and their presence/absence was examined in nine strains of four species of Gonium. Only one of the two complementary mating types in each of the four heterothallic species has a MID homolog. In addition to heterothallic strains, a homothallic strain of G. multicoccum has MID. Molecular evolutionary analysis suggests that MID of this homothallic strain retains functional constraint comparable to that of the heterothallic strains.Conclusion/Significance:We coordinated mating genotypes based on presence or absence of a MID homolog, respectively, in heterothallic species. This scheme should be applicable to heterothallic species of other isogamous colonial Volvocales including Pandorina and Yamagishiella. Homothallism emerged polyphyletically in the colonial Volvocales, although its mechanism remains unknown. Our identification of a MID homolog for a homothallic strain of G. multicoccum suggests a MID-dependent mechanism is involved in the sexual developmental program of this homothallic species. © 2013 Hamaji et al.

Sigma factor is a subunit of plastid-encoded RNA polymerase that regulates the transcription of plastid-encoded genes by recognizing a set of promoters. Sigma factors have increased in copy number and have diversified during the evolution of land plants, but details of this process remain unknown. Liverworts represent the basal group of embryophytes and are expected to retain the ancestral features of land plants. In liverwort (Marchantia polymorpha L.), we isolated and characterized a T-DNA-tagged mutant (Mpsig1) of sigma factor 1 (MpSIG1). The mutant did not show any visible phenotypes, implying that MpSIG1 function is redundant with that of other sigma factors. However, quantitative reverse-transcription polymerase chain reaction and RNA gel blot analysis revealed that genes related to photosynthesis were downregulated, resulting in the minor reduction of some protein complexes. The transcript levels of genes clustered in the petL, psaA, psbB, psbK, and psbE operons of liverwort were lower than those in the wild type, a result similar to that in the SIG1 defective mutant in rice (Oryza sativa). Overexpression analysis revealed primitive functional divergence between the SIG1 and SIG2 proteins in bryophytes, whereas these proteins still retain functional redundancy. We also discovered that the predominant sigma factor for ndhF mRNA expression has been diversified in liverwort, Arabidopsis (Arabidopsis thaliana), and rice. Our study shows the ancestral function of SIG1 and the process of functional partitioning (subfunctionalization) of sigma factors during the evolution of land plants.

The chloroplast NADH dehydrogenase-like (NDH) complex mediates cyclic electron transport and chloro-respiration and consists of five sub-omplexes, which in angiosperms further associate with photosystem I (PSI) to form a super-complex. In Marchantia polymorpha, 11 plastid-encoded subunits and all the nuclear-encoded subunits of the A, B, membrane and ferredoxin-binding sub-complexes are conserved. However, it is unlikely that the genome of this liverwort encodes Lhca5 and Lhca6, both of which mediate NDHPSI super-complex formation. It is also unlikely that the subunits of the lumen sub-complex, PnsL1L4, are encoded by the genome. Consistent with this in silico prediction, the results of blue-native gel electrophoresis showed that NDH subunits were detected in a protein complex with lower molecular mass in Marchantia than the NDHPSI super-complex in Arabidopsis. Using the plastid transformation technique, we knocked out the ndhB gene in Marchantia. Although the wild-type genome copies were completely segregated out, the Delta ndhB lines grew like the wild-type photoautotrophically. A post-illumination transient increase in chlorophyll fluorescence, which reflects NDH activity in vivo in angiosperms, was absent in the thalli of the Delta ndhB lines. In ruptured chloroplasts, antimycin A-insensitive, and ferredoxin-dependent plastoquinone reduction was impaired, suggesting that chloroplast NDH mediates similar electron transport in Marchantia and Arabidopsis, despite its possible difference in structure. As in angiosperms, linear electron transport was not strongly affected in the Delta ndhB lines. However, the plastoquinone pool was slightly more reduced at low light intensity, suggesting that chloroplast NDH functions in redox balancing of the inter system, especially under low light conditions.

The isogamous green alga Chlamydomonas reinhardtii has emerged as a premier model for studying the genetic regulation of fertilization and sexual development. A key regulator is known to be a homeoprotein gene, GAMETE-SPECIFIC PLUS1 (GSP1), which triggers the zygotic program. In this study, we isolated a mutant, biparental31 (bp31), which lacks GSP1. bp31 mt+ gametes fuse normally to form zygotes, but the sexual development of the resulting diploid cell is arrested and pellicle/zygospore/tetrad formation is abolished. The uniparental inheritance of chloroplast (cp) and mitochondrial (mt) DNA (cytoplasmic inheritance) was also impaired. bp31 has a deletion of similar to 60 kb on chromosome 2, including GSP1. The mutant phenotype was not rescued by transformation with GSP1 alone but could be rescued by the cotransformation with GSP1 and another gene, INOSITOL MONOPHOSPHATASE-LIKE1, which is involved in various cellular processes, including the phosphatidylinositol signaling pathway. This study confirms the importance of Gsp1 in mediating the zygotic program, including the uniparental inheritance of cp/mtDNA. Moreover, the results also suggest a role for inositol metabolism in the sexual developmental program.

ミトコンドリア(mt)や葉緑体(cp)の母性遺伝は、ヒトをはじめとする動物から、植物、苔類、藻類、粘菌に至る様々な生物に共通する現象である。しかしその具体的な分子機構は今日に至るまで明らかでない。<br> 緑藻クラミドモナスでは、雄cpDNAの分解により母性遺伝が引き起こされ、その過程は蛍光顕微鏡で容易にモニターすることが出来る。今回我々はクラミドモナスにおいて、母性遺伝変異体biparental(bp)31の単離に成功した。bp31では、接合子形成は正常であるが、雄cpDNAの分解、ペリクルの形成、接合胞子形成といった過程全般が完全に停止する。また、接合に伴う大規模なトランスクリプトーム変化が完全に失われてしまう。<br> 原因遺伝子を探るべく解析したところ、bp31は12の予測遺伝子を含む60kbの領域を欠損していた。個々の遺伝子について、相同性、発現様式の解析、および相補実験を試みたところ、bp31は2つの雄配偶子特異的遺伝子を導入することにより相補されることが明らかになった。すなわち、接合子の生殖プログラムの引き金として機能するホメオボックス遺伝子GSP1とイノシトール代謝の鍵酵素であるイノシトールモノフォスファターゼである。<br> 以上より、生殖プログラムと母性遺伝の密接なつながりが遺伝子レベルで示され、また接合過程におけるイノシトール代謝の重要性が明らかになった。

ミトコンドリア(mt)や葉緑体(cp)の遺伝子は多くの生物において母親のみから遺伝する（母性遺伝）．これまで母性遺伝は，父母の配偶子（精子・卵子）の大きさが異なり，mt/cpDNA量に差があることが原因と考えられてきた．これに対し，雌雄同形の配偶子で生殖をおこなう単細胞緑藻クラミドモナス(Chlamydomonas reinhardtii)では，接合後６０分以内に雄の葉緑体(cp)DNAが積極的に分解されることで母性遺伝が引き起こされる．本レビューでは，この雄葉緑体DNAの急激かつ選択的な分解を明らかにしてきた生化学，細胞学，顕微分子生物学的研究の数々について俯瞰し，さらに母性遺伝の分子機構に迫りつつある遺伝学的な挑戦について述べる．

細胞質遺伝の発見から101年目にあたる2010年，日本植物学会においてJournal of Plant Research, 日本植物形態学会共催シンポジウム「細胞質遺伝の分子機構 ～101年目の挑戦～」と題した会が開催された．本特集では，それらのシンポジストを中心に寄稿して頂き，真核生物の細胞質遺伝の分子機構研究の歴史，そして最前線を俯瞰する．

Two mechanisms for chloroplast DNA replication have been revealed through the study of an unusual heteroplasmic strain of the green alga Chlamydomonas reinhardtii. Heteroplasmy is a state in which more than one genome type occurs in a mitochondrion or chloroplast. The Chlamydomonas strain spa19 bears two distinct chloroplast genomes, termed PS+ and PS- PS+ genomes predominate and are stably maintained in vegetative cells, despite their lack of known replication origins. In sexual crosses with spa19 as the mating type plus parent, however, PS+ genomes are transmitted in only similar to 25% of tetrads, whereas the PS- genomes are faithfully inherited in all progeny. In this research, we have explored the mechanism underlying this biased uniparental inheritance. We show that the relative reduction and dilution of PS+ vs. PS- genomes takes place during gametogenesis. Bromodeoxyuridine labeling, followed by immunoprecipitation and PCR, was used to compare replication activities of PS+ and PS- genomes. We found that the replication of PS+ genomes is specifically suppressed during gametogenesis and germination of zygospores, a phenomenon that also was observed when spa19 cells were treated with rifampicin, an inhibitor of the chloroplast RNA polymerase. Furthermore, when bromodeoxyuridine incorporation was compared at 11 sites within the chloroplast genome between vegetative cells, gametes, and rifampicin-treated cells by quantitative PCR, we found that incorporation was often reduced at the same sites in gametes that were also sensitive to rifampicin treatment. We conclude that a transcription-mediated form of DNA replication priming, which may be downregulated during gametogenesis, is indispensable for robust maintenance of PS+ genomes. These results highlight the potential for chloroplast genome copy number regulation through alternative replication strategies.

Comprehensive analysis of metabolites using capillary electrophoresis-mass spectrometry was carried out in harmful weeds belonging to Polygonaceae. A principal component analysis revealed clear distinctions among eight Rumex species and Fallopia japonica. Hierarchical clustering data showed that respective metabolites can be grouped due to species differences. There was a positive relationship between oxalate and citrate, oxalate and ascorbate, and oxalate and glutamine. The amount of oxalate per leaf fresh weight was not affected by increased concentrations of exogenously supplied nutrients from Hoagland&apos;s formulation in one of the most destructive weeds R. obtusifolius. The oxalate accumulation in this plant is independent of external nutrient level, where nutrient-rich environments apparently stimulate internal constituents such as amino acids and other metabolites.

An intriguing feature of most eukaryotes is that chloroplast (cp) and mitochondrial (mt) genomes are inherited almost exclusively from one parent. Uniparental inheritance of cp/mt genomes was long thought to be a passive outcome, based on the fact that eggs contain multiple numbers of organelles, while male gametes contribute, at best, only a few cp/mtDNA. However, the process is likely to be more dynamic because uniparental inheritance occurs in organisms that produce gametes of identical sizes (isogamous). In Chlamydomonas reinhardtii, the uniparental inheritance of cp/mt genomes is achieved by a series of mating type-controlled events that actively eliminate the mating type minus (mt-) cpDNA. The method by which Chlamydomonas selectively degrades mt- cpDNA has long fascinated researchers, and is the subject of this review.

Nishimura Y, Shikanai T, Tanpakushitsu kakusan koso. Protein, nucleic acid, enzyme, 2009, vol. 54, no. 16 Suppl, pp. 2098-2101, 2009

タデ科ギシギシ属エゾノギシギシRumex obtusifolius L.は世界各地の田畑や牧草地などに多く見られる多年生草本であり、駆除が困難な強害雑草として知られている。日本では、明治期に北海道にて移入が確認された後、数十年の間にほぼ全国に分布を拡大し、特に問題視されている。<br>タデ科ギシギシ属の植物は、シュウ酸含有量が高い。シュウ酸の過剰摂取は動物にとって有害である一方で、植物においてシュウ酸は重要な役割を果たしていると考えられる。というのは、シュウ酸は防御分子である過酸化水素合成の基質の1つであるため、傷害などのストレス応答との関与が推測されている。また、シュウ酸を根圏へ放出することで土壌中の金属イオンとキレートを形成し、金属イオンの毒性を打ち消すという報告がある。<br>本研究ではエゾノギシギシのシュウ酸代謝を明らかにするため、CE-MSを用いてギシギシ属植物の葉の代謝物量を測定し種間比較を行った。その結果、エゾノギシギシは他種よりもシュウ酸を高蓄積した。また、シュウ酸を高蓄積する植物ほどアミノ酸を多く含む傾向が見られた。さらに、地上部を切除し暗条件で育成したエゾノギシギシの葉および茎のメタボローム解析を行った。その結果、暗条件では茎の主要代謝物であるクエン酸が葉のシュウ酸の高蓄積に関与する可能性が示された。

Algae play a more important role than land plants in the maintenance of the global environment and productivity. Progress in genome analyses of these organisms means that we can now obtain information on algal genomes, global annotation and gene expression. The full genome information for several algae has already been analyzed. Whole genomes of the red alga Cyanidioshyzon merolae, the green algae Ostreococcus tauri and Chlamydomonas reinhardtii, and the diatom Thalassiosira pseudonana have been sequenced. Genome composition and the features of cells among the four algae were compared. Each alga maintains basic genes as photosynthetic eukaryotes and possesses additional gene groups to represent their particular characteristics. This review discusses and introduces the latest research that makes the best use of the particular features of each organism and the significance of genome analysis to study biological phenomena. In particular, examples of post-genome studies of organelle multiplication in C. merolae based on analyzed genome information are presented.

Nishimura Y, Tanpakushitsu kakusan koso. Protein, nucleic acid, enzyme, 2005, vol. 50, no. 14 Suppl, pp. 1845-1846, 2005

In chloroplasts, the control of mRNA stability is of critical importance for proper regulation of gene expression. The Chlamydomonas reinhardtii strain Delta26pAtE is engineered such that the atpB mRNA terminates with an mRNA destabilizing polyadenylate tract, resulting in this strain being unable to conduct photosynthesis. A collection of photosynthetic revertants was obtained from Delta26pAtE, and gel blot hybridizations revealed RNA processing alterations in the majority of these suppressor of polyadenylation (spa) strains, resulting in a failure to expose the atpB mRNA 3' poly(A) tail. Two exceptions were spa19 and spa23, which maintained unusual heteroplasmic chloroplast genomes. One genome type, termed PS+, conferred photosynthetic competence by contributing to the stability of atpB mRNA; the other, termed PS-, was required for viability but could not produce stable atpB transcripts. Based on strand-specific RT-PCR, S1 nuclease protection, and RNA gel blots, evidence was obtained that the PS+ genome stabilizes atpB mRNA by generating an atpB antisense transcript, which attenuates the degradation of the polyadenylated form. The accumulation of double-stranded RNA was confirmed by insensitivity of atpB mRNA from PS+ genome-containing cells to S1 nuclease digestion. To obtain additional evidence for antisense RNA function in chloroplasts, we used strain Delta26, in which atpB mRNA is unstable because of the lack of a 3' stem-loop structure. In this context, when a 121-nucleotide segment of atpB antisense RNA was expressed from an ectopic site, an elevated accumulation of atpB mRNA resulted. Finally, when spa19 was placed in a genetic background in which expression of the chloroplast exoribonuclease polynucleotide phosphorylase was diminished, the PS+ genome and the antisense transcript were no longer required for photosynthesis. Taken together, our results suggest that antisense RNA in chloroplasts can protect otherwise unstable transcripts from 3'--&gt;5' exonuclease activity, a phenomenon that may occur naturally in the symmetrically transcribed and densely packed chloroplast genome.

Small, compact genomes of ultrasmall unicellular algae provide information on the basic and essential genes that support the lives of photosynthetic eukaryotes, including higher plants(1,2). Here we report the 16,520,305-base-pair sequence of the 20 chromosomes of the unicellular red alga Cyanidioschyzon merolae 10D as the first complete algal genome. We identified 5,331 genes in total, of which at least 86.3% were expressed. Unique characteristics of this genomic structure include: a lack of introns in all but 26 genes; only three copies of ribosomal DNA units that maintain the nucleolus; and two dynamin genes that are involved only in the division of mitochondria and plastids. The conserved mosaic origin of Calvin cycle enzymes in this red alga and in green plants supports the hypothesis of the existence of single primary plastid endosymbiosis. The lack of a myosin gene, in addition to the unexpressed actin gene, suggests a simpler system of cytokinesis. These results indicate that the C. merolae genome provides a model system with a simple gene composition for studying the origin, evolution and fundamental mechanisms of eukaryotic cells.

Changes in the distribution of organelles and organelle-DNA in Pelargonium zonale from the mature egg cell stage to the first zygotic division during the early stages of embryogenesis were investigated using electron microscopy and fluorescence microscopy. The mature egg is a large, polarized bulbous-shaped cell, tapering toward its micropylar end. The wide chalazal region has a large nucleus that is surrounded by cytoplasm containing many giant mitochondria and large amyloplasts. The mitochondria contain a large amount of mitochondrial DNA and appear as long stretched rods or complex rings, sometimes consisting of several concentric or half-concentric circles in sections. The time from pollination to cell fusion is approximately 6-9 h and it is 20-24 h until the first zygotic division. The changes in the zygote and its organelles preparatory to division occur in 3 stages. At stage 1 (6-9 h after pollination), cell fusion occurs and the zygote begins to elongate. Many vacuoles of varying size appear surrounding the nucleus. At stage 2 (9-15 h), the zygote nucleus migrates to a central position in the cell and the mitochondria form a single ring that becomes either irregularly crushed or appears as long thin strings. Amyloplasts exhibit a gradual decrease in the number of starch grains. At stage 3 (15-20 h), the vacuoles disappear, except for a few that remain in the micropylar region, and cell size decreases. Mitochondria become short, fine strings or small rings. Amyloplasts with starch grains are no longer observed, but are transformed into large proplastids. Following the first division of the zygote, approximately equal-sized apical and basal cells are formed. Short rod-shaped or small ring-shaped mitochondria are randomly distributed near the nucleus of the apical cell, whereas mitochondria in the basal cell are long and rod-shaped. In the electron microscope, two types of plastids can be distinguished: dark oval plastids originating from the sperm cell, which are observed in both the apical and basal cell, and others with a less dense, amorphous matrix, believed to originate from egg amyloplasts, which are unevenly distributed in the micropylar region of the basal cell. Fluorometry using a video-intensified microscope photon counting system reveals that, correlated with changes in mitochondrial morphology, DNA amount within the mitochondrion decreases linearly during these stages.

Although the active digestion of mating-type minus (mt(-)) chloroplast DNA (cpDNA) in young zygotes is considered to be the basis for the uniparental inheritance of cpDNA in Chlamydomonas reinhardtii, little is known about the underlying molecular mechanism. One model of active digestion proposes that nucleases are either synthesized or activated to digest mt(-) cpDNA. We used a native-PAGE/in gelo assay to investigate nuclease activities in chloroplasts from young zygotes, and identified a novel Ca2+-dependent nuclease activity. The timing of activation (similar to60-90 min after mating) and the localization of the nuclease activity (in mt(-) chloroplasts) coincided with the active digestion of mt(-) cpDNA. Furthermore, the activity of the nuclease was coregulated with the maturation of mating-type plus (mt(+)) gametes, which would enable the efficient digestion of mt(-) cpDNA. Based on these observations, we propose that the nuclease (designated as (M) under bart(+)-specific (DN) under bar ase, MDN) is a developmentally controlled nuclease that is activated in mt(+) gametes and participates in the destruction of mt(-) cpDNA in young zygotes, thereby ensuring uniparental inheritance of chloroplast traits.

In flowering plants, guidance of the pollen tube to the embryo sac (the haploid female gametophyte) is critical for successful fertilization. The target embryo sac may attract the pollen tube as the final step of guidance in the pistil. We show by laser Ceti ablation that two synergid cells adjacent to the egg cell attract the pollen tube. A single synergid cell was sufficient to generate an attraction signal, and two cells enhanced it. After fertilization, the embryo sac no longer attracts the pollen tube, despite the persistence of one synergid cell. This cessation of attraction might be involved in blocking polyspermy.

Chlamydomonas is an unicellular green alga that contains one cup-shaped chloroplast with about 60 copies of cpDNA. Chloroplasts (cp) multiply in the cytoplasm of the plant cell by binary division, with multiple copies of cpDNA transmitted and maintained in successive generations. The effect of cpDNA copy number on cell proliferation and aging was investigated using a C. reinhardtii moc mutant, which has an undispersed cp-nucleoid and unequal segregation of cpDNA during cell division. When the mother cell divided into four daughters, one moc daughter cell chloroplast contained about 60 copies of cpDNA, and the chloroplasts in the three other daughter cells contained the 4-7 copies of cpDNA. In liquid medium, the number of moc cells at the period of stationary phase was about one-third that of the wild type. To observe the process of proliferation and aging in the mother cell, we used solid medium. Three out of four moc cell spores were preferentially degenerated 60 days after cell transfer. To confirm this, wild-type and moc mother cells containing four daughter cells were treated with novobiocin to inhibit cpDNA replication. Cell degeneration increased only in the moc strain following novobiocin introduction. In total, our results suggest that cells possessing smaller amounts of cpDNA degenerate and age more rapidly.

ClpP is a proteolytic subunit of the ATP-dependent Clp protease, which is found in chloroplasts in higher plants. Proteolytic subunits are encoded both by the chloroplast gene, clpP, and a nuclear multi gene family. We insertionally disrupted clpP by chloroplast transformation in tobacco. However, complete segregation was impossible, indicating that the chloroplast-encoded clpP gene has an indispensable function for cell survival, In the heteroplasmic clpP disruptant, the leaf surface was rough by clumping, and the lateral leaf expansion was irregularly arrested, which led to an asymmetric, slender leaf shape, Chloroplasts consisted of two populations: chloroplasts that were similar to the wild type, and small chloroplasts that emitted high chi fluorescence. Ultrastructural analysis of chloroplast development suggested that clpP disruption also induced swelling of the thylakoid lumen in the meristem plastids and inhibition of etioplast development in the dark. Tn mature leaves, thylakoid membranes of the smaller chloroplast population consisted exclusively of large stacks of tightly appressed membranes. These results indicate that chloroplast-encoded ClpP is involved in multiple processes of chloroplast development, including a housekeeping function that is indispensable for cell survival.

We isolated 2 independent cytokinesis-defective mutants, inc-1 and inc-2, of the unicellular green alga Chlamydomonas reinhardtii through DNA insertional mutagenesis. Characterization of the mutants, using fluorescence microscopy and microfluorometry, revealed that progression of furrowing begins in inc cells after nuclear division. Unlike wild-type cells, however, the cleavage furrow of the dividing cells disappears before cytokinesis. Successive nuclear divisions with incomplete cytokinesis produce multinucleate cells. Additionally, chloroplast autofluorescence revealed that the chloroplast fails to divide throughout the process of cytokinesis in the mutant cells. We also examined the number of chloroplast nucleoids (cp-nucleoids) and chloroplast DNA (cpDNA) content per chloroplast as indicators of cell cycle. We observed cp-nucleoids through DAPI staining and measured the intensity of DAPI fluorescence with a video-intensified microscope photon-counting system (VIMPCS). Both the number of cp-nucleoids and cpDNA content per chloroplast increased under incomplete cytokinesis in inc cells. The inc mutants represent novel cytokinesis-defective mutants of C. reinhardtii.

Plastid-nuclei (plastid-nucleoids) and mitochondrial-nuclei (mitochondrial-nucleoids) were simultaneously isolated from cultured tobacco cells (Nicotiana tabacum L., line BY-2), and their activity synthesizing DNA in vitro was examined. The isolated plastid- and mitochondrial-nuclei incorporated about 20 and 50 pmol of dCTP, respectively, into DNA per microgram of template DNA during a 60-min incubation. The DNA synthetic activity of the two organelle-nuclei exhibited similar responses to various inhibitors; it was resistant to aphidicolin and sensitive to N-ethylmaleimide and high concentrations of ddCTP, which are all characteristics of gamma-like DNA polymerase. The responses of the DNA synthetic activity in the two organelle-nuclei to pH and divalent- and monovalent-metal cation concentrations were also similar. Moreover, an in situ DNA polymerase assay following SDS-polyacrylamide gel electrophoresis revealed that DNA. polymerases with an apparent molecular mass of 116 kDa were present in both the isolated plastid- and mitochondrial-nuclei, and that the two 116-kDa DNA polymerases were quite similar in terms of their sensitivity to various inhibitors, optimum assay conditions, and template preferences. These results indicate that DNA replication in plastids and mitochondria may be conducted by DNA polymerases that have quite similar characteristics. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.

Each wild-type Chlamydomonas reinhardtii cell has one large chloroplast containing several nuclei (nucleoids). We used DNA insertional mutagenesis to isolate Chlamydomonas mutants which contain a single, large chloroplast (cp) nucleus and which we named moc (monokaryotic chloroplast). DAPI-fluorescence microscopy and microphotometry observations revealed that moc mutant cells only contain one cp-nucleus throughout the cell division cycle, and that unequal segregation of cpDNA occurred during cell division in the moc mutant. One cell with a large amount of cpDNA and another with a small amount of cpDNA were produced after the first cell division. Unequal segregation also occurred in the second cell division, producing one cell with a large amount (about 70 copies) of cpDNA and three other cells with a small amount (only 2-8 copies) of cpDNA, However, most individual moc cells contained several dozen cpDNA copies 12 h after the completion of cell division, suggesting that cpDNA synthesis was activated immediately after chloroplast division. In contrast to the cpDNA, the mitochondrial (mt) DNA of the moc mutants was observed as tiny granules scattered throughout the entire cell. These segregated to each daughter cell equally during cell division. Electron-microscopic observation of the ultrastructure of moc mutants showed that a low-electron-density area, which was identified as the cp-nucleus by immunoelectron microscopy with anti-DNA antibody, existed near the pyrenoid. However, there were no other structural differences between the chloroplasts of wild-type cells and moc mutants. The thylakoid membranes and pyrenoid were identical. Therefore, we propose that the novel moc mutants are only defective in the dispersion and segregation of cpDNA. This strain should be useful to elucidate the mechanism for the segregation of cpDNA.

Plastid-nuclei (plastid-nucleoids) and mitochondrial-nuclei (mitochondrial-nucleoids) were simultaneously isolated from cultured tobacco cells (Nicotiana tabacum L., line BY-2), and their activity synthesizing DNA in vitro was examined. The isolated plastid- and mitochondrial-nuclei incorporated about 20 and 50 pmol of dCTP, respectively, into DNA per microgram of template DNA during a 60-min incubation. The DNA synthetic activity of the two organelle-nuclei exhibited similar responses to various inhibitors; it was resistant to aphidicolin and sensitive to N-ethylmaleimide and high concentrations of ddCTP, which are all characteristics of gamma-like DNA polymerase. The responses of the DNA synthetic activity in the two organelle-nuclei to pH and divalent- and monovalent-metal cation concentrations were also similar. Moreover, an in situ DNA polymerase assay following SDS-polyacrylamide gel electrophoresis revealed that DNA. polymerases with an apparent molecular mass of 116 kDa were present in both the isolated plastid- and mitochondrial-nuclei, and that the two 116-kDa DNA polymerases were quite similar in terms of their sensitivity to various inhibitors, optimum assay conditions, and template preferences. These results indicate that DNA replication in plastids and mitochondria may be conducted by DNA polymerases that have quite similar characteristics. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.

In the isogamous green alga Chlamydomonas reinhardtii, the chloroplast genome is transmitted from the mt(+) parent, while the mitochondrial genes are believed to be inherited from the mt(-) parent. Chloroplast nucleoids have been visualized by DAPI (4,6-diamidino-2-phenylindole) staining, and the preferential digestion of the mt chloroplast nucleoids has been observed in young zygotes. However, the mitochondrial nucleoids have never been visualized, and their behavior is only deduced from genetic and biochemical studies.
We discovered that the mitochondrial and chloroplast genomes can be visualized simultaneously in living cells, using the fluorescent dye SYBR GreenI. The ability to visualize the mitochondrial and chloroplast genome in vivo permits the direct observation of the number, distribution and behavior of the chloroplast and mitochondrial nucleoids in young zygotes. Using this method, the biparental transmission of the mitochondrial genome was revealed.

In the isogamous green alga Chlamydomonas reinhardtii, the chloroplast genome is transmitted from the mt(+) parent, while the mitochondrial genes are believed to be inherited from the mt(-) parent. Chloroplast nucleoids have been visualized by DAPI (4,6-diamidino-2-phenylindole) staining, and the preferential digestion of the mt chloroplast nucleoids has been observed in young zygotes. However, the mitochondrial nucleoids have never been visualized, and their behavior is only deduced from genetic and biochemical studies.
We discovered that the mitochondrial and chloroplast genomes can be visualized simultaneously in living cells, using the fluorescent dye SYBR GreenI. The ability to visualize the mitochondrial and chloroplast genome in vivo permits the direct observation of the number, distribution and behavior of the chloroplast and mitochondrial nucleoids in young zygotes. Using this method, the biparental transmission of the mitochondrial genome was revealed.

We attempted to amplify a specific region of mitochondrial DNA (mtDNA) using the polymerase chain reaction (PCR) from fewer than ten mitochondria isolated individually by microdissection or use of an optical tweezer. We selected preliminarily isolated mitochondria from Physarum polycephalum as the model materials and tried to amplify the mtDNA region corresponding to the specific mitochondrial plasmid of this true slime mould. For separation of a few mitochondria from the mitochondrial population, we initially used a destruction method in which excluded mitochondria were disrupted by a UV laser. However, mtDNA was still amplified, although weakly, from mitochondria that had been destroyed by the UV laser. Therefore, we used an optical tweezer to trap individual mitochondria and separate them from the others. The required number of mitochondria were separated from the mitochondrial suspension through a narrow canal of isolation buffer and used directly for PCR amplification. The results showed that the mtDNA could be amplified from at least 9 mitochondria trapped by the optical tweezer.