Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

Phosphatidylglycerol (PG) is the only major phospholipid in the thylakoid membrane of chloroplasts. PG is essential for photosynthesis, and loss of PG in Arabidopsis thaliana results in severe defects of growth and chloroplast development, with decreased chlorophyll accumulation, impaired thylakoid formation, and down-regulation of photosynthesis-associated genes encoded in nuclear and plastid genomes. However, how the absence of PG affects gene expression and plant growth remains unclear. To elucidate this mechanism, we investigated transcriptional profiles of a PG-deficient Arabidopsis mutant pgp1-2 under various light conditions. Microarray analysis demonstrated that reactive oxygen species (ROS)-responsive genes were up-regulated in pgp1-2. However, ROS production was not enhanced in the mutant even under strong light, indicating limited impacts of photooxidative stress on the defects of pgp1-2. Illumination to dark-adapted pgp1-2 triggered down-regulation of photosynthesis-associated nuclear-encoded genes (PhANGs), while plastid-encoded genes were constantly suppressed. Overexpression of GOLDEN2-LIKE1 (GLK1), a transcription factor gene regulating chloroplast development, in pgp1-2 up-regulated PhANGs but not plastid-encoded genes along with chlorophyll accumulation. Our data suggest a broad impact of PG biosynthesis on nuclear-encoded genes partially via GLK1 and a specific involvement of this lipid in plastid gene expression and plant development.

DOI： 10.1093/jxb/erac034

PubMed

Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

In the thylakoid membrane of cyanobacteria and chloroplasts, many proteins involved in photosynthesis are associated with or integrated into the fluid bilayer matrix formed by four unique glycerolipid classes, monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidylglycerol. Biochemical and molecular genetic studies have revealed that these glycerolipids play essential roles not only in the formation of thylakoid lipid bilayers but also in the assembly and functions of photosynthetic complexes. Moreover, considerable advances in structural biology have identified a number of lipid molecules within the photosynthetic complexes such as PSI and PSII. These data have provided important insights into the association of lipids with protein subunits in photosynthetic complexes and the distribution of lipids in the thylakoid membrane. Here, we summarize recent high-resolution observations of lipid molecules in the structures of photosynthetic complexes from plants, algae, and cyanobacteria, and evaluate the distribution of lipids among photosynthetic protein complexes and thylakoid lipid bilayers. By integrating the structural information into the findings from biochemical and molecular genetic studies, we highlight the conserved and differentiated roles of lipids in the assembly and functions of photosynthetic complexes among plants, algae, and cyanobacteria.

DOI： 10.1093/jxb/erac017

PubMed

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

In seed plants, phospho-base N-methyltransferase (PMT) catalyzes a key step in the biosynthesis pathway of phosphatidylcholine (PC), the most abundant phospholipid class. Arabidopsis thaliana possesses three copies of PMT, with PMT1 and PMT3 play a primary role because the pmt1 pmt3 double mutant shows considerably reduced PC content with a pale seedling phenotype. Although the function of PMT1 and PMT3 may be redundant because neither of the parental single mutants showed a similar mutant phenotype, major developmental defects and possible functional divergence of these PMTs underlying the pale pmt1 pmt3 seedling phenotype are unknown. Here, we show the major developmental defect of the pale seedlings in xylem of the hypocotyl with partial impairments in chloroplast development and photosynthetic activity in leaves. Although PMT1 and PMT3 are localized at the endoplasmic reticulum, their tissue-specific expression pattern was distinct in hypocotyls and roots. Intriguingly, the function of PMT3 but not PMT1 requires its characteristic N-terminal sequence in addition to the promoter because truncation of the N-terminal sequence of PMT3 or substitution with PMT1 driven by the PMT3 promoter failed to rescue the pale pmt1 pmt3 seedling phenotype. Thus, PMT3 function requires the N-terminal sequence in addition to its promoter, whereas the PMT1 function is defined by the promoter.

DOI： 10.1111/tpj.15741

PubMed

Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

Under nitrogen (N)-limited conditions, the non-N2-fixing cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis 6803) actively grows during early stages of starvation by performing photosynthesis but gradually stops the growth and eventually enters dormancy to withstand long-term N limitation. During N limitation, Synechocystis 6803 cells degrade the large light-harvesting antenna complex phycobilisomes (PBSs) presumably to avoid excess light absorption and to reallocate available N to essential functions for growth and survival. These two requirements may be driving forces for PBS degradation during N limitation, but how photosynthesis and cell growth affect PBS degradation remains unclear. To address this question, we examined involvements of photosynthesis and cell growth in PBS degradation during N limitation. Treatment of photosynthesis inhibitors and shading suppressed PBS degradation and caused non-bleaching of cells during N limitation. Limitations of photosynthesis after initial gene responses to N limitation suppressed PBS degradation, implying that photosynthesis affects PBS degradation in a post-translational manner. In addition, limitations of cell growth by inhibition of peptidoglycan and fatty acid biosynthesis, low growth temperature and phosphorous starvation suppressed PBS degradation during N limitation. Because decreased photosynthetic activity led to decreased cell growth, and vice versa, photosynthesis and cell growth would inseparably intertwine each other and affect PBS degradation during N limitation in a complex manner. Our data shed light on the coordination mechanisms among photosynthesis, cell growth and PBS degradation during N limitation.

DOI： 10.1093/pcp/pcab159

PubMed

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

The lipid bilayer matrix of the thylakoid membrane of cyanobacteria and chloroplasts of plants and algae is mainly composed of uncharged galactolipids, but also contains anionic lipids sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) as major constituents. The necessity of PG for photosynthesis is evident in all photosynthetic organisms examined to date, whereas the requirement of SQDG varies with species. In plants, although PG and SQDG are also found in non-photosynthetic plastids, their importance for the growth and functions of non-photosynthetic organs remains unclear. In addition, plants synthesize another anionic lipid glucuronosyldiacylglycerol (GlcADG) during phosphorus starvation, but its role in plant cells is not elucidated yet. To understand the functional relationships among PG, SQDG, and GlcADG, we characterized several Arabidopsis thaliana mutants defective in biosynthesis of these lipids. The mutants completely lacking both PG and SQDG biosynthesis in plastids showed developmental defects of roots, hypocotyls, and embryos in addition to leaves, which suggests that these lipids are pleiotropically required for the development of both photosynthetic and non-photosynthetic organs. Furthermore, our analysis revealed that SQDG, but not GlcADG, is essential for complementing the role of PG, particularly in photosynthesis under PG-deficient conditions such as phosphorus starvation.

DOI： 10.3390/ijms22094860

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

The appropriate regulation of thylakoid lipid synthesis is essential for the function of chloroplasts. In plant cells, membrane lipids synthesized in the ER are utilized as a precursor for the synthesis of chloroplast glycolipids. This pathway is thought to be mediated by the transport of glycerolipids synthesized in the ER into chloroplasts. However, we have little knowledge about the proteins involved in the lipid transfer between these organelles in plant cells. Here we show a protein, STAR2, containing the START (Steroidogenic acute regulatory protein-related lipid transfer) domain known to function as a lipid transporter, is involved in the incorporation of ER-derived fatty acids into chloroplast glycolipids in Marchantia polymorpha. We found that STAR2 localizes on the chloroplast envelope membrane as a punctuate structure and is required for the increase of C20 fatty acids, which are synthesized in the ER, in chloroplast glycolipids in response to phosphate deprivation. Our results indicate that STAR2 of M. polymorpha is likely to be involved in the lipid transfer from ER to chloroplast, presumably as a lipid transporter.

DOI： 10.1016/j.bbrc.2020.11.063

PubMed

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

Photosynthesis with highly photoreactive chlorophyll (Chl) provides energy for plant growth but with simultaneous risk of photooxidative damage and photoprotection costs. Although the leafless orchid Cymbidium macrorhizon mostly depends on mycorrhizal fungi for carbon, it accumulates Chl particularly during fruiting and may not be fully mycoheterotrophic. In fact, stable isotopic analysis suggested that the fruiting C. macrorhizon specimens obtain a significant proportion of its carbon demands through photosynthesis. However, actual photosynthetic characteristics of this leafless orchid are unknown. To reveal the functionality of photosynthetic electron transport in C. macrorhizon, we compared its photosynthetic properties with those of its relative mixotrophic orchid C. goeringii and the model plant Arabidopsis thaliana. Compared with C. goeringii and A. thaliana, maximum photochemical efficiency of PSII was substantially low in C. macrorhizon. Chl fluorescence induction kinetics revealed that the electron transport capacity of PSII was limited in C. macrorhizon. Chl fluorescence analysis at 77 K suggested partial energetic disconnection of the light-harvesting antenna from the PSII reaction center in C. macrorhizon. Despite its low PSII photochemical efficiency, C. macrorhizon showed photosynthetic electron-transport activity both in the field and under laboratory conditions. C. macrorhizon developed strong non-photochemical quenching in response to increased light intensity as did C. goeringii, suggesting the functionality of photoprotective systems in this orchid. Moreover, C. macrorhizon fruit developed stomata on the pericarp and showed net O2-evolving activity. Our data demonstrate that C. macrorhizon can perform photosynthetic electron transport in the pericarp, although its contribution to net carbon acquisition may be limited.

DOI： 10.1093/pcp/pcab006

PubMed

Repository URL： http://hdl.handle.net/10466/00017656

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

Chloroplast lipids are synthesized via two distinct pathways: the plastidic pathway and endoplasmic reticulum (ER) pathway. We previously reported that the contribution of the two pathways toward chloroplast development is different between mesophyll cells and guard cells in Arabidopsis leaf tissues, and that the ER pathway plays a major role in guard cell chloroplast development. However, little is known about the contribution of the two pathways toward chloroplast development in other tissue cells, and in this study, we focused on the root cells. Chloroplast development is normally repressed in roots but can be induced when the roots are detached from the shoots (root greening). We found that similar to guard cells, root cells exhibit a higher proportion of glycolipid from the ER pathway. Root greening was repressed in the gles1 mutant, which has a defect in ER-to-plastid lipid transportation via the ER pathway, while normal root greening was observed in the ats1 mutant, whose plastidic pathway is blocked. Lipid analysis revealed that the gles1 mutation caused drastic decrease in the ER-derived glycolipids in roots. Furthermore, the gles1 detached roots showed smaller chloroplasts containing little starch than WT. These results suggest that the ER pathway has a significant contribution toward chloroplast development in the root cells.

DOI： 10.1093/pcp/pcab009

PubMed

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

Somatic plant cells can regenerate shoots and/or roots or adventitious embryonic calluses, which may induce organ formation under certain conditions. Such regenerations occur via dedifferentiation of somatic cells, induction of organs, and their subsequent outgrowth. Despite recent advances in understanding of plant regeneration, many details of shoot induction remain unclear. Here, we artificially induced shoot stem-like green organs (SSOs) in Arabidopsis thaliana roots via simultaneous induction of two transcription factors (TFs), ARABIDOPSIS THALIANA HOMEOBOX PROTEIN 25 (ATHB25, At5g65410) and the B3 family transcription factor REPRODUCTIVE MERISTEM 7 (REM7, At3g18960). The SSOs exhibited negative gravitropism and differentiated vascular bundle phenotypes. The ATHB25/REM7 induced the expression of genes controlling shoot stem characteristics by ectopic expression in roots. Intriguingly, the restoration of root growth was seen in the consecutive and adjacent parts of the SSOs under gene induction conditions. Our findings thus provide insights into the development and regeneration of plant shoot stems.

DOI： 10.1016/j.isci.2020.101332

PubMed

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

Thylakoid membranes, the site of photochemical and electron transport reactions of oxygenic photosynthesis, are composed of a myriad of proteins, cofactors including pigments, and glycerolipids. In the non-diazotrophic cyanobacterium Synechocystis sp. PCC 6803, the size and function of thylakoid membranes are reduced under nitrogen (N) starvation but are quickly recovered after N addition to the starved cells. To understand how the functionality of thylakoid membranes is adjusted in response to N status in Synechocystis sp. PCC 6803, we examined changes in thylakoid components and the photosynthetic activity during the N starvation and recovery processes. In N-starved cells, phycobilisome content, photosystem II protein levels and the photosynthetic activity substantially decreased as compared with those in N-sufficient cells. Although the content of chlorophyll (Chl) a, total protein and total glycerolipid also decreased under the N-starved condition based on OD730 reflecting cell density, when based on culture volume, the Chl a and total protein content remained almost constant and total glycerolipid content even increased during N starvation, suggesting that cellular levels of these components decrease under the N-starved condition mainly through dilution due to cell growth. With N addition, the photosynthetic activity quickly recovered, followed by full restoration of photosynthetic pigment and protein levels. The content of phosphatidylglycerol (PG), an essential lipid constituent of both photosystems, increased faster than that of Chl a, whereas the content of glycolipids, the main constituents of the thylakoid lipid bilayer, gradually recovered after N addition. The data indicate differential regulation of PG and glycolipids during the construction of the photosynthetic machinery and regeneration of thylakoid membranes. Of note, addition of PG to the growth medium slightly accelerated the Chl a accumulation in wild-type cells during the recovery process. Because PG is required for the biosynthesis of Chl a and the formation of functional photosystem complexes, rapid PG biosynthesis in response to N acquisition may be required for the rapid formation of the photosynthetic machinery during thylakoid regeneration.

DOI： 10.3389/fpls.2020.00432

PubMed

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

Galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are the predominant lipid classes in the thylakoid membrane of chloroplasts. These lipids are also major constituents of internal membrane structures called prolamellar bodies (PLBs) and prothylakoids (PTs) in etioplasts, which develop in the cotyledon cells of dark-grown angiosperms. Analysis of Arabidopsis mutants defective in the major galactolipid biosynthesis pathway revealed that MGDG and DGDG are similarly and, in part, differently required for membrane-associated processes such as the organization of PLBs and PTs and the formation of pigment-protein complexes in etioplasts. After light exposure, PLBs and PTs in etioplasts are transformed into the thylakoid membrane, resulting in chloroplast biogenesis. During the etioplast-to-chloroplast differentiation, galactolipids facilitate thylakoid membrane biogenesis from PLBs and PTs and play crucial roles in chlorophyll biosynthesis and accumulation of light-harvesting proteins. These recent findings shed light on the roles of galactolipids as key facilitators of several membrane-associated processes during the development of the internal membrane systems in plant plastids.

DOI： 10.3390/plants8100357

PubMed

Publishing type：Research paper (scientific journal)   International / domestic magazine：Domestic journal  

Etioplasts developed in angiosperm cotyledon cells in darkness rapidly differentiate into chloroplasts with illumination. This process involves dynamic transformation of internal membrane structures from the prolamellar bodies (PLBs) and prothylakoids (PTs) in etioplasts to thylakoid membranes in chloroplasts. Although two galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are predominant lipid constituents of membranes in both etioplasts and chloroplasts, their roles in the structural and functional transformation of internal membranes during etioplast-to-chloroplast differentiation are unknown. We previously reported that a 36% loss of MGDG by an artificial microRNA targeting major MGDG synthase (amiR-MGD1) only slightly affected PLB structures but strongly impaired PT formation and protochlorophyllide biosynthesis. Meanwhile, strong DGDG deficiency in a DGDG synthase mutant (dgd1) disordered the PLB lattice structure in addition to impaired PT development and protochlorophyllide biosynthesis. In this study, thylakoid biogenesis after PLB disassembly with illumination was strongly perturbed by amiR-MGD1. The amiR-MGD1 expression impaired the accumulation of Chl and the major light-harvesting complex II protein (LHCB1), which may inhibit rapid transformation from disassembled PLBs to the thylakoid membrane. As did amiR-MGD1 expression, dgd1 mutation impaired the accumulation of Chl and LHCB1 during etioplast-to-chloroplast differentiation. Furthermore, unlike in amiR-MGD1 seedlings, in dgd1 seedlings, disassembly of PLBs after illumination was retarded. Because DGDG but not MGDG prefers to form the bilayer lipid phase in membranes, the MGDG-to-DGDG ratio may strongly affect the transformation of PLBs to the thylakoid membrane during etioplast-to-chloroplast differentiation.

DOI： 10.1093/pcp/pcz041

PubMed

Kind of work：Joint Work  

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

Plants have evolved mechanisms to improve utilization efficiency or acquisition of inorganic phosphate (Pi) in response to Pi deficiency, such as altering root architecture, secreting acid phosphatases, and activating the expression of genes related to Pi uptake and recycling. Although many genes responsive to Pi starvation have been identified, transcription factors that affect tolerance to Pi deficiency have not been well characterized. We show here that the ectopic expression of B-BOX32 (BBX32) and the mutation of ELONGATED HYPOCOTYL 5 (HY5), whose transcriptional activity is negatively regulated by BBX32, resulted in the tolerance to Pi deficiency in Arabidopsis. The primary root lengths of 35S:BBX32 and hy5 plants were only slightly inhibited under Pi deficient condition and the fresh weights were significantly higher than those of wild type. The Pi deficiency-tolerant root phenotype of hy5 was similarly observed when grown on the medium without Pi. In addition, a double mutant, hy5 slr1, without lateral roots, also showed a long primary root phenotype under phosphate deficiency, indicating that the root phenotype of hy5 does not result from an increase of external Pi uptake. Moreover, we found that blue light may regulate Pi deficiency-dependent primary root growth inhibition through activating peroxidase gene expression, suggesting the Pi-deficiency tolerant root phenotype of hy5 may be due to blockage of blue light responses. Altogether, this study points out light quality may play an important role in the regulation of Pi deficiency responses. It may contribute to regulate plant growth under Pi deficiency through proper illumination.

DOI： 10.3389/fpls.2019.01803

PubMed

Publishing type：Research paper (scientific journal)  

DOI： 10.2139/ssrn.3387683

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J-GLOBAL

Authorship：Last author,　Corresponding author   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)   International / domestic magazine：Domestic journal  

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Venue：つくば（オンライン）  

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