Plastidial phosphoglucose isomerase is an important determinant of seed yield through involvement in gibberellin-mediated reproductive development and biosynthesis of storage reserves in Arabidopsis

The plastid-localized phosphoglucose isomerase isoform PGI1 is an important determinant of growth in Arabidopsis, probably due to its involvement in synthesis of plastidial isoprenoid derived hormones. We have tested the possibility that it also influences seed yields, and several associated hypotheses. Gene expression analyses showed that PGI1 is strongly expressed in maturing seed embryos and vascular tissues. PGI1-null pgi1-2 plants had ca. 60% lower seed yields than wild type (WT) plants. They produced fewer inflorescences, and thus fewer siliques and seeds per plant. These traits were associated with low contents of bioactive gibberellins (GAs). Accordingly, WT phenotypes could be restored by exogenous GA application. pgi1-2 seeds were lighter and accumulated ca. 50% less fatty acids (FAs) and ca. 35% less proteins than WT seeds. Seeds of CK-deficient 35S:AtCKX1 and GA-deficient ga20ox1 ga20ox2 mutants did not accumulate low levels of FAs, and exogenous application of BAP and GAs did not rescue the reduced weight and FA content of pgi1-2 seeds. Seeds from reciprocal crosses between pgi1-2 and WT plants accumulated WT levels of FAs and proteins. Our results show that PGI1 is an important determinant of Arabidopsis seed yield due to its involvement in two processes: GA-mediated reproductive development and the metabolic conversion of plastidial glucose-6-phosphate to storage reserves in the embryo.