CELL-TYPE-SPECIFIC, COORDINATE EXPRESSION OF 2 ADP-GLUCOSE PYROPHOSPHORYLASE GENES IN RELATION TO STARCH BIOSYNTHESIS DURING SEED DEVELOPMENT OF VICIA-FABA L

Several cDNA clones encoding two different ADP-glucose pyrophosphoryla se (AGPase, EC 2.7.7.27) polypeptides denoted VfAGPC and VfAGPP were i solated from a cotyledonary library of Vicia faba L. Both sequences ar e closely related to AGPase small-subunit sequences from other plants. Whereas mRNA levels of VfAGPP were equally high in developing cotyled ons and leaves, the mRNA of VfAGPC was present in considerable amounts only in cotyledons. During development of cotyledons, both mRNAs accu mulated until the beginning of the desiccation phase and disappeared a fterwards. The increase of AGPase activity in cotyledons during the ph ase of storage-product synthesis was closely followed by the accumulat ion of starch. The AGPase activity in crude extracts of cotyledons was insensitive to 3-phosphoglycerate whereas the activity from leaves co uld be activated more than five-fold. Inorganic phosphate inhibited th e enzyme from both tissues but was slightly more effective on the leaf enzyme. There was a correlation at the cellular level between the dis tribution of VfAGPP and VfAGPC mRNAs and the accumulation of starch, a s studied by in-situ hybridisation and by histochemical staining in pa rallel tissue sections of developing seeds, respectively. During the e arly phase of seed development (12-15 days after fertilization) VfAGPa se mRNA and accumulation of starch were detected transiently in the hy podermal, chlorenchymal and outer parenchymal cell layers of the seed coat but not in the embryo. At 25 days after fertilization both synthe sis of VfAGPase mRNA and biosynthesis of starch had started in parench yma cells of the inner adaxial zone of the cotyledons. During later st ages, the expression of VfAGPase and synthesis of starch extended over most of the cotyledons but were absent from peripheral cells of the a baxial zone, provascular and procalyptral cells.