ABSTRACT
We present the results of studies of an unmodified version of the recombinant major barley (Hordeum vulgare) endosperm ADP-glucose pyrophoshorylase (AGPase) expressed in insect cells, which corroborate previous data that this isoform of the enzyme acts independently of the allosteric regulators 3-phosphoglycerate and inorganic phosphate. We also present a characterization of the individual subunits expressed separately in insect cells, showing that the SS AGPase is active in the presence of 3-phosphoglycerate and is inhibited by inorganic phosphate. As a step toward the elucidation of the role of the two AGPase isoforms in barley, the temporal and spatial expression profile of the four barley AGPase transcripts encoding these isoforms were studied. The results show that the steady-state level of beps and bepl, the transcripts encoding the major endosperm isoform, correlated positively with the rate of endosperm starch accumulation. In contrast, blps and blpl, the transcripts encoding the major leaf isoform, were constitutively expressed at a very low steady-state level throughout the barley plant. The implications of these findings for the evolution of plant AGPases are discussed.
ABSTRACT
The recent application of immunohistochemistry and molecular techniques has revealed that endosperm development depends on a genetic program that combines an ancient process for cellularization (similar to that seen in late Paleozoic seed ferns) with a mechanism for specifying asymmetric cell fates that has parallels to signaling processes in mammals. Progress has been further accelerated by the recent realization that the conserved nature of nuclear endosperm development extends beyond the grass species, to include dicots, such as Arabidopsis. It is anticipated that these ongoing studies will provide invaluable tools for the improvement of yield and grain quality in cereal crops.