LIPID RICH 1 Modulates Allocation of Carbon between Starch and Triacylglycerol in Arabidopsis Leaves

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 the loss of function in the causal gene, encoding an uncharacterized protein. TAG levels were five-fold higher and starch levels two-fold lower in the leaves of liri1 than the wild type. LIRI1 localized to the chloroplasts, and the 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 suggest that LIRI1 affects the carbon trade-off to inhibit lipid production in leaves.