Callus (formed when a plant tissue is wounded) is a promising system for studying starch biosynthesis and bioengineering; however the molecular structure of callus starch has been poorly characterized. Size-exclusion chromatography was used in this study to characterize the starch structure in rice calli from two cultivars and a mutant of one cultivar lacking starch branching enzyme IIb. There were major qualitative differences in the chain-length and whole-molecule size distributions between starch from grain and from callus. However callus starch was found to be able to simulate the starch metabolism from both leaves and endosperm and reveal the structural development of starch granules, and this was dependent on the culture system. During synthesis, trans-lamellar amylopectin chains in callus are synthesized earlier than single-lamella chains, while enzymatic degradation starts from outer to inner amylopectin chains. The outer layers of the callus-starch granules have larger molecules with lower amylose content and shorter amylopectin chains compared to further inside the callus-starch granules. Controlling starch granular number and size thus has potential for improving both the quantity and quality of plant starch.
Keywords: Callus; Molecular size distribution; Size-exclusion chromatography (SEC); Starch.
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