島田 貴士

Abstract Plants accumulate starch and triacylglycerols (TAGs) as carbon sources. Leaves primarily store starch in chloroplasts, with some TAGs stored in lipid droplets, but how carbon resource allocation is regulated in leaves during cellular metabolism is largely unknown. Using a forward genetics approach, we isolated an Arabidopsis thaliana mutant with more lipid droplets in its leaves than the wild type, named lipid rich 1 (liri1). The overaccumulation of lipid droplets was caused by loss of function in the causal gene, encoding an uncharacterized protein. TAG levels were 5-fold higher and starch levels 2-fold lower in the leaves of liri1 than in those of the wild type. LIRI1 localized to the chloroplasts, and contents of chloroplast membrane lipids were 20% higher in liri1 leaves than in wild-type leaves. Co-immunoprecipitation assays revealed that LIRI1 interacts with ACETYL-COENZYME A CARBOXYLASE CARBOXYLTRANSFERASE ALPHA SUBUNIT (an enzyme for fatty acid biosynthesis) and STARCH SYNTHASE 4 (an enzyme for starch biosynthesis). In isotope tracer experiments using [1-13C]sodium acetate, more 13C was incorporated into TAGs in liri1 leaves than in wild-type leaves. Moreover, liri1 plants showed growth defects and irregular chloroplasts. These results indicate that LIRI1 affects the carbon trade-off to inhibit lipid production in leaves.

Lipid droplets (LDs) are lipid storage organelles in plant leaves and seeds. Seed LD proteins are well known, and their functions in lipid metabolism have been characterized; however, many leaf LD proteins remain to be identified. We therefore isolated LDs from leaves of the leaf LD–overaccumulating mutant high sterol ester 1 (hise1) of Arabidopsis thaliana by centrifugation or co-immunoprecipitation. We then performed LD proteomics by mass spectrometry and identified 3,206 candidate leaf LD proteins. In this study, we selected 31 candidate proteins for transient expression assays using a construct encoding the candidate protein fused with green fluorescent protein (GFP). Fluorescence microscopy showed that MYOSIN BINDING PROTEIN14 (MYOB14) and two uncharacterized proteins localized to LDs labeled with the LD marker. Subcellular localization analysis of MYOB family members revealed that MYOB1, MYOB2, MYOB3, and MYOB5 localized to LDs. LDs moved along actin filaments together with the endoplasmic reticulum. Co-immunoprecipitation of myosin XIK with MYOB2-GFP or MYOB14-GFP suggested that LD-localized MYOBs are involved in association with the myosin XIK–LDs. The two uncharacterized proteins were highly similar to enzymes for furan fatty acid biosynthesis in the photosynthetic bacterium Cereibacter sphaeroides, suggesting a relationship between LDs and furan fatty acid biosynthesis. Our findings thus reveal potential molecular functions of LDs and provide a valuable resource for further studies of the leaf LD proteome.

To increase food production under the challenges presented by global climate change, the concept of de novo domestication-utilizing stress-tolerant wild species as new crops-has recently gained considerable attention. We had previously identified mutants with desired domestication traits in a mutagenized population of the legume Vigna stipulacea Kuntze (minni payaru) as a pilot for de novo domestication. Given that there are multiple stress-tolerant wild legume species, it is important to establish efficient domestication processes using reverse genetics and identify the genes responsible for domestication traits. In this study, we identified VsPSAT1 as the candidate gene responsible for decreased hard-seededness, using a Vigna stipulacea isi2 mutant that takes up water from the lens groove. Scanning electron microscopy and computed tomography revealed that the isi2 mutant has lesser honeycomb-like wax sealing the lens groove than the wild-type, and takes up water from the lens groove. We also identified the pleiotropic effects of the isi2 mutant: accelerating leaf senescence, increasing seed size, and decreasing numbers of seeds per pod. While doing so, we produced a V. stipulacea whole-genome assembly of 441 Mbp in 11 chromosomes and 30,963 annotated protein-coding sequences. This study highlights the importance of wild legumes, especially those of the genus Vigna with pre-existing tolerance to biotic and abiotic stresses, for global food security during climate change.

<title>Abstract</title>Eukaryotic cells acquired novel organelles during evolution through mechanisms that remain largely obscure. The existence of the unique oil body compartment is a synapomorphy of liverworts that represents lineage-specific acquisition of this organelle during evolution, although its origin, biogenesis, and physiological function are yet unknown. We find that two paralogous syntaxin-1 homologs in the liverwort <italic>Marchantia polymorpha</italic> are distinctly targeted to forming cell plates and the oil body, suggesting that these structures share some developmental similarity. Oil body formation is regulated by an ERF/AP2-type transcription factor and loss of the oil body increases <italic>M</italic>. <italic>polymorpha</italic> herbivory. These findings highlight a common strategy for the acquisition of organelles with distinct functions in plants, via periodical redirection of the secretory pathway depending on cellular phase transition.