研究者詳細 - 坂本　亘

論文 - 坂本　亘

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Impairment of Lhca4, a subunit of LHCI, causes high accumulation of chlorophyll and the stay-green phenotype in rice 国際誌

Yamatani, Hiroshi, Kohzuma, Kaori, Nakano, Michiharu, Takami, Tsuneaki, Kato, Yusuke, Hayashi, Yoriko, Monden, Yuki, Okumoto, Yutaka, Abe, Tomoko, Kumamaru, Toshihiro, Tanaka, Ayumi, Sakamoto, Wataru, Kusabal, Makoto

Journal of Experimental Botany 69 ( 5 ) 1027 - 1035 2018年

記述言語：英語掲載種別：研究論文（学術雑誌）

Chlorophyll is an essential molecule for acquiring light energy during photosynthesis. Mutations that result in chlorophyll retention during leaf senescence are called 'stay-green' mutants. One of the several types of stay-green mutants, Type E, accumulates high levels of chlorophyll in the pre-senescent leaves, resulting in delayed yellowing. We isolated delayed yellowing1-1 (dye1-1), a rice mutant whose yellowing is delayed in the field. dye1-1 accumulated more chlorophyll than the wild-type in the pre-senescent and senescent leaves, but did not retain leaf functionality in the 'senescent green leaves', suggesting that dye1-1 is a Type E stay-green mutant. Positional cloning revealed that DYE1 encodes Lhca4, a subunit of the light-harvesting complex I (LHCI). In dye1-1, amino acid substitution occurs at the location of a highly conserved amino acid residue involved in pigment binding; indeed, a severely impaired structure of the PSI-LHCI super-complex in dye1-1 was observed in a blue native PAGE analysis. Nevertheless, the biomass and carbon assimilation rate of dye1-1 were comparable to those in the wild-type. Interestingly, Lhcb1, a trimeric LHCII protein, was highly accumulated in dye1-1, in the chlorophyll-protein complexes. The high accumulation of LHCII in the LHCI mutant dye1 suggests a novel functional interaction between LHCI and LHCII.

DOI： 10.1093/jxb/erx468

Overexpression of the protein disulfide isomerase AtCYO1 in chloroplasts slows dark-induced senescence in Arabidopsis 国際誌

Tominaga, Jun, Nakahara, Yasutoshi, Horikawa, Daisuke, Tanaka, Ayumi, Kondo, Maki, Kamei, Yasuhiro, Takami, Tsuneaki, Sakamoto, Wataru, Unno, Kazutoshi, Sakamoto, Atsushi, Shimada, Hiroshi

Bmc Plant Biology 18 ( 1 ) 80 - 80 2018年

記述言語：英語掲載種別：研究論文（学術雑誌）

BACKGROUND: Chlorophyll breakdown is the most obvious sign of leaf senescence. The chlorophyll catabolism pathway and the associated proteins/genes have been identified in considerable detail by genetic approaches combined with stay-green phenotyping. Arabidopsis CYO1 (AtCYO1), a protein disulfide reductase/isomerase localized in the thylakoid membrane, is hypothesized to assemble the photosystem by interacting with cysteine residues of the subunits. RESULTS: In this study, we report that ectopic overexpression of AtCYO1 in leaves induces a stay-green phenotype during darkness, where oxidative conditions favor catabolism. In AtCYO1ox leaves, Fv/Fm and both chlorophyll a and chlorophyll b content remained high during dark-induced senescence. The thylakoid ultrastructure was preserved for a longer time in AtCYO1ox leaves than in wild type leaves. AtCYO1ox leaves maintained thylakoid chlorophyll-binding proteins associated with both PSII (D1, D2, CP43, CP47, LHCB2, and Cyt f) and PSI (PSA-A/B), as well as stromal proteins (Rubisco and ferredoxin-NADP+ reductase). AtCYO1ox did not affect senescence-inducible gene expression for chlorophyll catabolism or accumulation of chlorophyll catabolites. CONCLUSIONS: Our results suggest that ectopic overexpression of AtCYO1 had a negative impact on the initiation of chlorophyll degradation and proteolysis within chloroplasts. Our findings cast new light on the redox regulation of protein disulfide bonds for the maintenance of functional chloroplasts.

DOI： 10.1186/s12870-018-1294-5

Selective Elimination of Membrane-Damaged Chloroplasts via Microautophagy 国際誌

Nakamura, Sakuya, Hidema, Jun, Sakamoto, Wataru, Ishida, Hiroyuki, Izumi, Masanori

Plant Physiology 177 ( 3 ) 1007 - 1026 2018年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：American Society of Plant Biologists ({ASPB})

Plant chloroplasts constantly accumulate damage caused by visible wavelengths of light during photosynthesis. Our previous study revealed that entire photodamaged chloroplasts are subjected to vacuolar digestion through an autophagy process termed chlorophagy; however, how this process is induced and executed remained poorly understood. In this study, we monitored intracellular induction of chlorophagy in Arabidopsis (Arabidopsis thaliana) leaves and found that mesophyll cells damaged by high visible light displayed abnormal chloroplasts with a swollen shape and 2.5 times the volume of normal chloroplasts. In wild-type plants, the activation of chlorophagy decreased the number of swollen chloroplasts. In the autophagy-deficient autophagy mutants, the swollen chloroplasts persisted, and dysfunctional chloroplasts that had lost chlorophyll fluorescence accumulated in the cytoplasm. Chloroplast swelling and subsequent induction of chlorophagy were suppressed by the application of exogenous mannitol to increase the osmotic pressure outside chloroplasts or by overexpression of VESICLE INDUCING PROTEIN IN PLASTID1, which maintains chloroplast envelope integrity. Microscopic observations of autophagy-related membranes showed that swollen chloroplasts were partly surrounded by autophagosomal structures and were engulfed directly by the tonoplast, as in microautophagy. Our results indicate that an elevation in osmotic potential inside the chloroplast due to high visible light-derived envelope damage results in chloroplast swelling and serves as an induction factor for chlorophagy, and this process mobilizes entire chloroplasts via tonoplast-mediated sequestering to avoid the cytosolic accumulation of dysfunctional chloroplasts.

DOI： 10.1104/pp.18.00444

FtsH Protease in the Thylakoid Membrane: Physiological Functions and the Regulation of Protease Activity. 国際誌

Yusuke Kato, Wataru Sakamoto

Frontiers in plant science 9 855 - 855 2018年

記述言語：英語掲載種別：研究論文（学術雑誌）

Protein homeostasis in the thylakoid membranes is dependent on protein quality control mechanisms, which are necessary to remove photodamaged and misfolded proteins. An ATP-dependent zinc metalloprotease, FtsH, is the major thylakoid membrane protease. FtsH proteases in the thylakoid membranes of Arabidopsis thaliana form a hetero-hexameric complex consisting of four FtsH subunits, which are divided into two types: type A (FtsH1 and FtsH5) and type B (FtsH2 and FtsH8). An increasing number of studies have identified the critical roles of FtsH in the biogenesis of thylakoid membranes and quality control in the photosystem II repair cycle. Furthermore, the involvement of FtsH proteolysis in a singlet oxygen- and EXECUTER1-dependent retrograde signaling mechanism has been suggested recently. FtsH is also involved in the degradation and assembly of several protein complexes in the photosynthetic electron-transport pathways. In this minireview, we provide an update on the functions of FtsH in thylakoid biogenesis and describe our current understanding of the D1 degradation processes in the photosystem II repair cycle. We also discuss the regulation mechanisms of FtsH protease activity, which suggest the flexible oligomerization capability of FtsH in the chloroplasts of seed plants.

DOI： 10.3389/fpls.2018.00855

Structure and Molecular Characterization of Diadenosine Polyphosphate Hydrolase in Brachypodium distachyon 査読

Motohiro Tanaka, Igor Iamshchikov, Yusuke Kato, Rushan Sabirov, Oleg Gusev, Wataru Sakamoto, Manabu Sugimoto

Journal of Plant Biochemistry & Physiology 06 ( 03 ) 2018年

The Non-Mendelian Green Cotyledon Gene in Soybean Encodes a Small Subunit of Photosystem II 国際誌

Kohzuma, Kaori, Sato, Yutaka, Ito, Hisashi, Okuzaki, Ayako, Watanabe, Mai, Kobayashi, Hideki, Nakano, Michiharu, Yamatani, Hiroshi, Masuda, Yu, Nagashima, Yumi, Fukuoka, Hiroyuki, Yamada, Tetsuya, Kanazawa, Akira, Kitamura, Keisuke, Tabei, Yutaka, Ikeuchi, Masahiko, Sakamoto, Wataru, Tanaka, Ayumi, Kusaba, Makoto

Plant Physiology 173 ( 4 ) 2138 - 2147 2017年

記述言語：英語掲載種別：研究論文（学術雑誌）

Chlorophyll degradation plays important roles in leaf senescence including regulation of degradation of chlorophyll-binding proteins. Although most genes encoding enzymes of the chlorophyll degradation pathway have been identified, the regulation of their activity has not been fully understood. Green cotyledon mutants in legume are stay-green mutants, in which chlorophyll degradation is impaired during leaf senescence and seed maturation. Among them, the soybean (Glycine max) green cotyledon gene cytG is unique because it is maternally inherited. To isolate cytG, we extensively sequenced the soybean chloroplast genome, and detected a 5-bp insertion causing a frame-shift in psbM, which encodes one of the small subunits of photosystem II. Mutant tobacco plants (Nicotiana tabacum) with a disrupted psbM generated using a chloroplast transformation technique had green senescent leaves, confirming that cytG encodes PsbM. The phenotype of cytG was very similar to that of mutant of chlorophyll b reductase catalyzing the first step of chlorophyll b degradation. In fact, chlorophyll b-degrading activity in dark-grown cytG and psbM-knockout seedlings was significantly lower than that of wild-type plants. Our results suggest that PsbM is a unique protein linking photosynthesis in presenescent leaves with chlorophyll degradation during leaf senescence and seed maturation. Additionally, we discuss the origin of cytG, which may have been selected during domestication of soybean.

DOI： 10.1104/pp.16.01589

The Rubisco Chaperone BSD2 May Regulate Chloroplast Coverage in Maize Bundle Sheath Cells 国際誌

Salesse, Coralie, Sharwood, Robert, Sakamoto, Wataru, Sterna, David

Plant Physiology 175 ( 4 ) 1624 - 1633 2017年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：American Society of Plant Biologists ({ASPB})

In maize (Zea mays), Bundle Sheath Defective2 (BSD2) plays an essential role in Rubisco biogenesis and is required for correct bundle sheath (BS) cell differentiation. Yet, BSD2 RNA and protein levels are similar in mesophyll (M) and BS chloroplasts, although Rubisco accumulates only in BS chloroplasts. This raises the possibility of additional BSD2 roles in cell development. To test this hypothesis, transgenic lines were created that overexpress and underexpress BSD2 in both BS and M cells, driven by the cell type-specific Rubisco Small Subunit (RBCS) or Phosphoenolpyruvate Carboxylase (PEPC) promoters or the ubiquitin promoter. Genetic crosses showed that each of the transgenes could complement Rubisco deficiency and seedling lethality conferred by the bsd2 mutation. This was unexpected, as RBCS-BSD2 lines lacked BSD2 in M chloroplasts and PEPC-BSD2 lines expressed half the wild-type BSD2 level in BS chloroplasts. We conclude that BSD2 does not play a vital role in M cells and that BS BSD2 is in excess of requirements for Rubisco accumulation. BSD2 levels did affect chloroplast coverage in BS cells. In PEPC-BSD2 lines, chloroplast coverage decreased 30% to 50%, whereas lines with increased BSD2 content exhibited a 25% increase. This suggests that BSD2 has an ancillary role in BS cells related to chloroplast size. Gas exchange showed decreased photosynthetic rates in PEPC-BSD2 lines despite restored Rubisco function, correlating with reduced chloroplast coverage and pointing to CO2 diffusion changes. Conversely, increased chloroplast coverage did not result in increased Rubisco abundance or photosynthetic rates. This suggests another limitation beyond chloroplast volume, most likely Rubisco biogenesis and/or turnover rates.

DOI： 10.1104/pp.17.01346

Essentials of Proteolytic Machineries in Chloroplasts 国際誌

Nishimura, Kenji, Kato, Yusuke, Sakamoto, Wataru

Molecular Plant 10 ( 1 ) 4 - 19 2017年

記述言語：英語掲載種別：研究論文（学術雑誌）

Plastids are unique organelles that can alter their structure and function in response to environmental and developmental stimuli. Chloroplasts are one type of plastid and are the sites for various metabolic processes, including photosynthesis. For optimal photosynthetic activity, the chloroplast proteome must be properly shaped and maintained through regulated proteolysis and protein quality control mechanisms. Enzymatic functions and activities are conferred by protein maturation processes involving consecutive proteolytic reactions. Protein abundances are optimized by the balanced protein synthesis and degradation, which is depending on the metabolic status. Malfunctioning proteins are promptly degraded. Twenty chloroplast proteolytic machineries have been characterized to date. Specifically, processing peptidases and energy-driven processive proteases are the major players in chloroplast proteome biogenesis, remodeling, and maintenance. Recently identified putative proteases are potential regulators of photosynthetic functions. Here we provide an updated, comprehensive overview of chloroplast protein degradation machineries and discuss their importance for photosynthesis. Wherever possible, we also provide structural insights into chloroplast proteases that implement regulated proteolysis of substrate proteins/peptides.

DOI： 10.1016/j.molp.2016.08.005

Chloroplast Proteases: Updates on Proteolysis within and across Suborganellar Compartments 国際誌

Kenji Nishimura, Yusuke Kato, Wataru Sakamoto

Plant Physiology 171 ( 4 ) 2280 - 2293 2016年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：American Society of Plant Biologists ({ASPB})

Chloroplasts originated from the endosymbiosis of ancestral cyanobacteria and maintain transcription and translation machineries for around 100 proteins. Most endosymbiont genes, however, have been transferred to the host nucleus, and the majority of the chloroplast proteome is composed of nucleus-encoded proteins that are biosynthesized in the cytosol and then imported into chloroplasts. How chloroplasts and the nucleus communicate to control the plastid proteome remains an important question. Protein-degrading machineries play key roles in chloroplast proteome biogenesis, remodeling, and maintenance. Research in the past few decades has revealed more than 20 chloroplast proteases, which are localized to specific suborganellar locations. In particular, two energy-dependent processive proteases of bacterial origin, Clp and FtsH, are central to protein homeostasis. Processing endopeptidases such as stromal processing peptidase and thylakoidal processing peptidase are involved in the maturation of precursor proteins imported into chloroplasts by cleaving off the amino-terminal transit peptides. Presequence peptidases and organellar oligopeptidase subsequently degrade the cleaved targeting peptides. Recent findings have indicated that not only intraplastidic but also extraplastidic processive protein-degrading systems participate in the regulation and quality control of protein translocation across the envelopes. In this review, we summarize current knowledge of the major chloroplast proteases in terms of type, suborganellar localization, and diversification. We present details of these degradation processes as case studies according to suborganellar compartment (envelope, stroma, and thylakoids). Key questions and future directions in this field are discussed.

DOI： 10.1104/pp.16.00330

Rice CYO1, an ortholog of Arabidopsis thaliana cotyledon chloroplastbiogenesis factor AtCYO1, is expressed in leaves and involved in photosynthetic performance 国際誌

Tominaga, Jun, Mizutani, Haruka, Horikawa, Daisuke, Nakahara, Yasutoshi, Takami, Tsuneaki, Sakamoto, Wataru, Sakamoto, Atsushi, Shimada, Hiroshi

Journal of Plant Physiology 207 78 - 83 2016年

記述言語：英語掲載種別：研究論文（学術雑誌）

In the dicotyledonous plant Arabidopsis thaliana, the cotyledon chloroplast biogenesis factor AtCYO1 is crucial for the biogenesis of cotyledon chloroplasts. Arabidopsis mutants lacking AtCYO1 have pale cotyledons but develop normal mature leaves. In the monocotyledonous plant Oryza sativa, the gene OsCYO1 has high sequence identity to AtCYO1, but its function is unknown. We examined the role of OsCYO1 in O. sativa. We first confirmed that transformation with OsCYO1 could recover the phenotype of the Arabidopsis cyo1 mutant. Similar to AtCYO1, recombinant OsCYO1 has protein disulfide reductase (PDR) activity, which increased as a function of dieosin glutathione disulfide concentration with an apparent Km of 3.2μM and Kcat of 0.53min-1. The PDR activity was reduced when NADPH or NADH was used as an electron donor; however, PDR activity was observed with OsCYO1 and glutathione, suggesting that glutathione may serve as a reducing agent for OsCYO1 in vivo. In O. sativa, the OsCYO1 transcript level was higher in leaves compared with the coleoptile, which is the first leaf-like organ that forms during rice embryogenesis. Many OsCYO1 mutant lines defective in RNA interference had green leaves, however, three mutant lines had not only albino coleoptile but also albino leaves. Those having green leaves reduced photosynthetic performance in leaves. Our results demonstrate that OsCYO1 is enzymatically equivalent to AtCYO1 but that the physiological role of OsCYO1 in monocotyledonous plants may differ from that of AtCYO1 in dicotyledonous plants.

DOI： 10.1016/j.jplph.2016.10.005

Protection of Chloroplast Membranes by VIPP1 Rescue sAberrant Seedling Development in Arabidopsis nyc1 Mutant 国際誌

Zhang, Lingang, Kusaba, Makoto, Tanaka, Ayumi, Sakamoto, Wataru

Frontiers in Plant Science 7 533 - 533 2016年

記述言語：英語掲載種別：研究論文（学術雑誌）

Chlorophylls (Chl) in photosynthetic apparatuses, along with other macromolecules in chloroplasts, are known to undergo degradation during leaf senescence. Several enzymes involved in Chl degradation, by which detoxification of Chl is safely implemented, have been identified. Chl degradation also occurs during embryogenesis and seedling development. Some genes encoding Chl degradation enzymes such as Chl b reductase (CBR) function during these developmental stages. Arabidopsis mutants lacking CBR (NYC1 and NOL) have been reported to exhibit reduced seed storability, compromised germination, and cotyledon development. In this study, we examined aberrant cotyledon development and found that NYC1 is solely responsible for this phenotype. We inferred that oxidative damage of chloroplast membranes caused the aberrant cotyledon. To test the inference, we attempted to trans-complement nyc1 mutant with overexpressing VIPP1 protein that is unrelated to Chl degradation but which supports chloroplast membrane integrity. VIPP1 expression actually complemented the aberrant cotyledon of nyc1, whereas stay-green phenotype during leaf senescence remained. The swollen chloroplasts observed in unfixed cotyledons of nyc1, which are characteristics of chloroplasts receiving envelope membrane damage, were recovered by overexpressing VIPP1. These results suggest that chloroplast membranes are a target for oxidative damage caused by the impairment in Chl degradation. Trans-complementation of nyc1 with VIPP1 also suggests that VIPP1 is useful for protecting chloroplasts against oxidative stress.

DOI： 10.3389/fpls.2016.00533

VIPP1 Has a Disordered C-Terminal Tail Necessary for Protecting Photosynthetic Membranes against Stress 国際誌

Zhang, Lingang, Kondo, Hideki, Kamikubo, Hironari, Kataoka, Mikio, Sakamoto, Wataru

Plant Physiology 171 ( 3 ) 1983 - 1995 2016年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：American Society of Plant Biologists ({ASPB})

Integrity of biomembranes is vital to living organisms. In bacteria, PspA is considered to act as repairing damaged membrane by forming large supercomplexes in Arabidopsis (Arabidopsis thaliana). Vulnerable to oxidative stress, photosynthetic organisms also contain a PspA ortholog called VIPP1, which has an additional C-terminal tail (Vc). In this study, Vc was shown to coincide with an intrinsically disordered region, and the role of VIPP1 in membrane protection against stress was investigated. We visualized VIPP1 by fusing it to GFP (VIPP1-GFP that fully complemented lethal vipp1 mutations), and investigated its behavior in vivo with live imaging. The intrinsically disordered nature of Vc enabled VIPP1 to form what appeared to be functional particles along envelopes, whereas the deletion of Vc caused excessive association of the VIPP1 particles, preventing their active movement for membrane protection. Expression of VIPP1 lacking Vc complemented vipp1 mutation, but exhibited sensitivity to heat shock stress. Conversely, transgenic plants over-expressing VIPP1 showed enhanced tolerance against heat shock, suggesting that Vc negatively regulates VIPP1 particle association and acts in maintaining membrane integrity. Our data thus indicate that VIPP1 is involved in the maintenance of photosynthetic membranes. During evolution, chloroplasts have acquired enhanced tolerance against membrane stress by incorporating a disordered C-terminal tail into VIPP1.

DOI： 10.1104/pp.16.00532

Amyloplast Membrane Protein SUBSTANDARD STARCH GRAIN6 Controls Starch Grain Size in Rice Endosperm 国際誌

Matsushima, Ryo, Maekawa, Masahiko, Kusano, Miyako, Tomita, Katsura, Kondo, Hideki, Nishimura, Hideki, Crofts, Naoko, Fujita, Naoko, Sakamoto, Wataru

Plant Physiology 170 ( 3 ) 1445 - 59 2016年

記述言語：英語掲載種別：研究論文（学術雑誌）

Starch is a biologically and commercially important polymer of glucose. Starch is organized into starch grains (SGs) inside amyloplasts. The SG size differs depending on the plant species and is one of the most important factors for industrial applications of starch. There is limited information on genetic factors regulating SG sizes. In this study, we report the rice (Oryza sativa) mutant substandard starch grain6 (ssg6), which develops enlarged SGs in endosperm. Enlarged SGs are observed starting at 3 d after flowering. During endosperm development, a number of smaller SGs appear and coexist with enlarged SGs in the same cells. The ssg6 mutation also affects SG morphologies in pollen. The SSG6 gene was identified by map-based cloning and microarray analysis. SSG6 encodes a protein homologous to aminotransferase. SSG6 differs from other rice homologs in that it has a transmembrane domain. SSG6-green fluorescent protein is localized in the amyloplast membrane surrounding SGs in rice endosperm, pollen, and pericarp. The results of this study suggest that SSG6 is a novel protein that controls SG size. SSG6 will be a useful molecular tool for future starch breeding and applications.

DOI： 10.1104/pp.15.01811

C2-P-07Functions of phototropins and photosynthesis in the light induced mitochondria chloroplasts co-localization inArabidopsis thaliana

Islam Md. Sayeedul, Nguyen Van Toan, Kato Yusuke, Sakamoto Wataru, Takagi Shingo

Microscopy 64 ( suppl 1 ) i126.1 - i126 2015年11月

Physical interaction between peroxisomes and chloroplasts elucidated by in situ laser analysis (vol 1, 15035, 2015) 査読

Kazusato Oikawa, Shigeru Matsunaga, Shoji Mano, Maki Kondo, Kenji Yamada, Makoto Hayashi, Takatoshi Kagawa, Akeo Kadota, Wataru Sakamoto, Shoichi Higashi, Masakatsu Watanabe, Toshiaki Mitsui, Akinori Shigemasa, Takanori Iino, Yoichiroh Hosokawa, Mikio Nishimura

NATURE PLANTS 1 ( 4 ) 2015年4月

Possible function of VIPP1 in maintaining chloroplast membranes 国際誌

Zhang, Lingang, Sakamoto, Wataru

Biochimica Et Biophysica Acta-Bioenergetics 1847 ( 9 ) 831 - 7 2015年

記述言語：英語掲載種別：研究論文（学術雑誌）

A protein designated as VIPP1 is found widely in organisms performing oxygenic photosynthesis, but its precise role in chloroplasts has remained somewhat mysterious. Based on its structural similarity, it presumably has evolved from bacterial Phage shock protein A (PspA) with a C-terminal extension of approximately 40 amino acids. Both VIPP1 and PspA are membrane-associated despite the lack of transmembrane helices. They form an extremely large homo-complex that consists of an oligomeric ring unit. Although PspA is known to respond to membrane stress and although it acts in maintaining proton motive force through membrane repair, the multiple function of VIPP1, such as vesicle budding from inner envelope to deliver lipids to thylakoids, maintenance of photosynthetic complexes in thylakoid membranes, biogenesis of Photosystem I, and protective role of inner envelope against osmotic stress, has been proposed. Whatever its precise function in chloroplasts, it is an important protein because depletion of VIPP1 in mutants severely affects photoautotrophic growth. Recent reports of the relevant literature describe that VIPP1 becomes highly mobile when chloroplasts receive hypotonic stress, and that VIPP1 is tightly bound to lipids, which implies a crucial role of VIPP1 in membrane repair through lipid transfer. This review presents a summary of our current knowledge related to VIPP1, particularly addressing the dynamic behavior of complexes against stress and its property of lipid binding. Those data altogether suggest that VIPP1 acts a priori in chloroplast membrane maintenance through its activity to transfer lipids rather than in thylakoid formation through vesicles. This article is part of a Special Issue titled: Chloroplast Biogenesis.

DOI： 10.1016/j.bbabio.2015.02.013

Geometrical Formation of Compound Starch Grains in Rice Implements Voronoi Diagram

Matsushima, Ryo, Maekawa, Masahiko, Sakamoto, Wataru

Plant and Cell Physiology 56 ( 11 ) 2150 - 7 2015年

記述言語：英語掲載種別：研究論文（学術雑誌）

Starch forms transparent grains, called starch grains (SGs), in amyloplasts. One of the major morphological SG forms in Poaceae, called a compound SG, is formed by assemblies of small starch granules in an amyloplast. Starch granules assemble as a well-ordered structure; however, the mechanism that regulates this organization has not been identified. In this study, we examined how starch granules grow and converge into the final SG morphology. First, we found that the number of starch granules in an amyloplast is almost constant from the early developmental stage until endosperm maturity. Next, we quantitatively evaluated the geometrical similarities between starch granules and a Voronoi diagram, which is a mathematical tessellation of space based on the distance to a specific set of points in the space. The in silico growth simulation showed that the geometrical patterns of compound SGs resembling a Voronoi diagram is determined by physical interactions among the free-growing starch granules and the amyloplast envelope membrane. The geometrical similarity between compound SGs and a Voronoi diagram is likely a result of maximum loading and storage of starch in the amyloplast. The simulation described in this study provides a greater understanding of how compound SGs are formed and also has the potential to explain morphological variations of SGs.

DOI： 10.1093/pcp/pcv123

Physical interaction between peroxisomes and chloroplasts elucidated by in situ laser analysis 国際誌

Oikawa, Kazusato, Matsunaga, Shigeru, Mano, Shoji, Kondo, Maki, Yamada, Kenji, Hayashi, Makoto, Kagawa, Takatoshi, Kadota, Akeo, Sakamoto, Wataru, Higashi, Shoichi, Watanabe, Masakatsu, Mitsui, Toshiaki, Shigemasa, Akinori, Iino, Takanori, Hosokawa, Yoichiroh, Nishimura, Mikio

Nature Plants 1 ( 4 ) 15035 - 15035 2015年

記述言語：英語掲載種別：研究論文（学術雑誌）

Life on earth relies upon photosynthesis, which consumes carbon dioxide and generates oxygen and carbohydrates. Photosynthesis is sustained by a dynamic environment within the plant cell involving numerous organelles with cytoplasmic streaming. Physiological studies of chloroplasts, mitochondria and peroxisomes show that these organelles actively communicate during photorespiration, a process by which by-products produced by photosynthesis are salvaged. Nevertheless, the mechanisms enabling efficient exchange of metabolites have not been clearly defined. We found that peroxisomes along chloroplasts changed shape from spherical to elliptical and their interaction area increased during photorespiration. We applied a recent femtosecond laser technology to analyse adhesion between the organelles inside palisade mesophyll cells of Arabidopsis leaves and succeeded in estimating their physical interactions under different environmental conditions. This is the first application of this estimation method within living cells. Our findings suggest that photosynthetic-dependent interactions play a critical role in ensuring efficient metabolite flow during photorespiration.

DOI： 10.1038/NPLANTS.2015.35

A Mutation in GIANT CHLOROPLAST Encoding a PARC6 Homolog Affects Spikelet Fertility in Rice

Kamau, Peter K., Sano, Shingo, Takami, Tsuneaki, Matsushima, Ryo, Maekawa, Masahiko, Sakamoto, Wataru

Plant and Cell Physiology 56 ( 5 ) 977 - 91 2015年

記述言語：英語掲載種別：研究論文（学術雑誌）

Chloroplasts are not generated de novo but proliferate from a pre-existing population of plastids present in meristematic cells. Chloroplast division is executed by the co-ordinated action of at least two molecular machineries: internal machinery located on the stromal side of the inner envelope membrane and external machinery located on the cytosolic side of the outer envelope membrane. To date, molecular studies of chloroplast division in higher plants have been limited to several species such as Arabidopsis. To elucidate chloroplast division in rice, we performed forward genetics and isolated a mutant displaying large chloroplasts among an ethyl methanesulfonate (EMS)-mutagenized Oryza sativa spp japonica Nipponbare population. Using a map-based approach, this mutation, termed giant chloroplast (gic), was allocated in a gene that encodes a protein that is homologous to Paralog of ARC6 (PARC6), which is known to play a role in chloroplast division. GIC is unique in that it has a long C-terminal extension that is not present in other PARC6 homologs. Characterization of gic phenotypes in a rice field showed that gic exhibited defective growth in seed setting, suggesting that the gic mutant negatively affects the reproductive stage. This report is the first describing a chloroplast division mutant in monocotyledons and its effect on plant development.

DOI： 10.1093/pcp/pcv024

D1 fragmentation in photosystem II repair caused by photo-damage of a two-step model 国際誌

Kato, Yusuke, Ozawa, Shin-ichiro, Takahashi, Yuichiro, Sakamoto, Wataru

Photosynthesis Research 126 ( 2-3 ) 409 - 16 2015年

記述言語：英語掲載種別：研究論文（学術雑誌）

Light energy drives photosynthesis, but it simultaneously inactivates photosynthetic mechanisms. A major target site of photo-damage is photosystem II (PSII). It further targets one reaction center protein, D1, which is maintained efficiently by the PSII repair cycle. Two proteases, FtsH and Deg, are known to contribute to this process, respectively, by efficient degradation of photo-damaged D1 protein processively and endoproteolytically. This study tested whether the D1 cleavage accomplished by these proteases is affected by different monochromic lights such as blue and red light-emitting-diode light sources, remaining mindful that the use of these lights distinguishes the current models for photoinhibition: the excess-energy model and the two-step model. It is noteworthy that in the two-step model, primary damage results from the absorption of light energy in the Mn-cluster, which can be enhanced by a blue rather than a red light source. Results showed that blue and red lights affect D1 degradation differently. One prominent finding was that D1 fragmentation that is specifically generated by luminal Deg proteases was enhanced by blue light but not by red light in the mutant lacking FtsH2. Although circumstantial, this evidence supports a two-step model of PSII photo-damage. We infer that enhanced D1 fragmentation by luminal Deg proteases is a response to primary damage at the Mn-cluster.

DOI： 10.1007/s11120-015-0144-7

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