Expression of a yeast-derived invertase in developing cotyledons of Vicia narbonensis alters the carbohydrate state and affects storage functions

In plants the carbohydrate state provides signals to adjust metabolism to specific physiological conditions. Storage-active sink organs like seeds often contain high levels of sucrose. In order to change the sugar status during seed development a yeast-derived invertase gene was expressed in Vicia narbonensis under control of the LeguminB4 promoter. A signal sequence targeted the invertase to the apoplast in maturing embryos. In the cotyledons, sucrose was decreased whereas hexoses strongly accumulated. There was a major reduction of starch whereas proteins were less affected. Vacuoles of cotyledon cells were enlarged and dry seeds wrinkled. Transcripts and enzyme activity of sucrose synthase, the small and large subunit of ADP-glucose pyrophosphorylase as well as vicilin were downregulated. Sucrose phosphate synthase and legumin-mRNAs were not affected. Analysing single seeds with different sucrose levels revealed a positive correlation of sucrose concentration to mRNA levels of sucrose synthase and most pronounced to ADP-glucose pyrophosphorylase-mRNA levels as well as to starch content. Glucose on the other hand did not show any correlation. After feeding C-14-sucrose in vitro, the invertase-expressing cotyledons partitioned less carbon into starch compared to the wild-type. In the transgenic cotyledons, a relatively higher amount was directed into proteins compared to starch. We conclude that starch accumulation in developing cotyledons could be a function of sucrose concentration. Our results are consistent with a possible sucrose-mediated induction of storage-associated differentiation indicated by upregulation of specific genes of the starch synthesis pathway.