A hypothesis is presented concerning a putative extra-am yloplastic location of barley seed endosperm ADP-glucose pyrophosphorylase (AGPase), a key enzyme of starch biosynthesis. The hypothesis is based both on indirect and direct evidence obtained in our laboratory as well as on data of other investigators. It is proposed that ADP-glucose form ed by the extraamyloplastic enzyme is transported to the am yloplasts via an ADP-glucose carrier in the plastid membrane, and then is utilized by the starch biosynthesizing machinery of these organelles. In addition to the extra-am yloplastic form of AGPase, barley endosperm contains also a second isozyme of AGPase, located in the amyloplasts. Presence of isozymes of AGPase in cereal seed endosperm is consistent with biochemical, molecular and genetic data on starch biosynthesis in this tissue
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Villand et al. (1994) studied this question.