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

A hypothesis is presented concerning a putative extra-am yloplastic location o f barley seed endosperm A DP-glucose pyrophosphorylase (AGPase), a key enzyme o f starch biosynthesis. The hypothesis is based bo th on indirect and direct evidence obtained in our laboratory as well as on data o f o ther investigators. It is proposed tha t ADP-glucose form ed by the extraam yloplastic enzyme is transported to the am yloplasts via an ADP-glucose carrier in the plastid m em brane, and then is utilized by the starch biosynthesizing m achinery o f these organelles. In addition to the extra-am yloplastic form o f AGPase, barley endosperm contains also a sec­ ond isozyme o f A G Pase, located in the am yloplasts. Presence o f isozymes o f A G Pase in cereal seed endosperm is consistent w ith biochemical, m olecular and genetic da ta on starch biosyn­ thesis in this tissue.