Altered starch turnover in the maternal plant has major effects on Arabidopsis fruit growth and seed composition

Mature seeds of both the high-starch starch-excess1 (sex1) mutant and the almost starchless phosphoglucomutase1 mutant of Arabidopsis (Arabidopsis thaliana) have 30% to 40% less lipid than seeds of wild-type plants. We show that this is a maternal effect and is not attributable to the defects in starch metabolism in the embryo itself. Low lipid contents and consequent slow postgerminative growth are seen only in mutant embryos that develop on maternal plants with mutant phenotypes. Mutant embryos that develop on plants with wild-type starch metabolism have wild-type lipid contents and postgerminative growth. The maternal effect on seed lipid content is attributable to carbohydrate starvation in the mutant fruit at night. Fruits on sex1 plants grow more slowly than those on wild-type plants, particularly at night, and have low sugars and elevated expression of starvation genes at night. Transcript levels of the transcription factor WRINKLED1, implicated in lipid synthesis, are reduced at night in sex1 but not in wild-type seeds, and so are transcript levels of key enzymes of glycolysis and fatty acid synthesis. sex1 embryos develop more slowly than wild-type embryos. We conclude that the reduced capacity of mutant plants to convert starch to sugars in leaves at night results in low nighttime carbohydrate availability in the developing fruit. This in turn reduces the rate of development and expression of genes encoding enzymes of storage product accumulation in the embryo. Thus, the supply of carbohydrate from the maternal plant to the developing fruit at night can have an important influence on oilseed composition and on postgerminative growth.