Is There an Alternative Pathway for Starch Biosynthesis in Cereal Seeds?

Abstract

A hypothesis is presented concerning a putative extra-am yloplastic location of barley seed endosperm A D P-glucose pyrophosphorylase (A G Pase), a key enzyme of starch biosynthesis. The hypothesis is based b o th on indirect and direct evidence obtained in o ur lab o rato ry as well as on d ata of o th er investigators. It is proposed th at A D P-glucose form ed by the extraam yloplastic enzyme is tran sp o rted to the am yloplasts via an A D P-glucose carrier in the plastid m em brane, and then is utilized by the starch biosynthesizing m achinery of these organelles. In ad dition to the extra-am yloplastic form of A G Pase, barley endosperm contains also a second isozyme of A G Pase, located in the am yloplasts. Presence of isozymes of A G Pase in cereal seed endosperm is consistent w ith biochem ical, m olecular and genetic d ata on starch biosynthesis in this tissue. © 1994, Walter de Gruyter. All rights reserved.