Abstract
Starch is synthesized in the endosperm of developing barley grain, where it functions as the primary source of stored carbohydrate. In germinated grain these starch reserves are hydrolyzed to small oligosaccharides and glucose, which are transported to the embryo to support the growth of the developing seedling. Some of the mobilized glucose is transiently stored as starch in the scutellum of germinated grain. These processes are crucial for early seedling vigor, which is a key determinant of crop productivity and global food security. Several starch synthases (SS), starch-branching enzymes (SBEs), and starch debranching enzymes (isoamylases, ISA), together with a limit dextrinase (LD), have been implicated in starch synthesis from nucleotide-sugar precursors. Starch synthesis occurs both in the developing endosperm and in the scutellum of germinated grain. For the complete depolymerization of starch to glucose, α-amylase (Amy), β-amylase (Bmy), isoamylase (ISA), limit dextrinase (LD), and α-glucosidase (AGL) are required. Most of these enzymes are encoded by gene families of up to 10 or more members. Here RNA-seq transcription data from isolated tissues of intact developing and germinated barley grain have allowed us to identify the most important, specific gene family members for each of these processes in vivo and, at the same time, we have defined in detail the spatio-temporal coordination of gene expression in different tissues of the grain. A transcript dataset for 81,280 genes is publicly available as a resource for investigations into other cellular and biochemical processes that occur in the developing grain from 6 days after pollination.
Highlights
Starch reserves in plants represent one of the most important polysaccharides for human societies
Most starches contain 70–80% amylopectin and 20–30% amylose; phospholipids and free fatty acids can be associated with the amylose and some protein is attached to the surface of the granules (Tester et al, 2004)
RNA-seq transcript data for germinated grain was from the same dataset described by Betts et al (2020), in which transcripts from 33,421 genes were quantitated from the scutellum, the residual embryo and three aleurone sections for 0–4 days after the initiation of germination
Summary
Starch reserves in plants represent one of the most important polysaccharides for human societies. Whether the source be cereal grains, tubers, nuts, fruit, vegetables, or other plants, starch constitutes a major proportion of our daily caloric intake. Starch is deposited in simple or compound granules of varying sizes and shapes in the chloroplasts of leaves and in the amyloplasts of grains and tubers. Within these granules, the major constituents of starch, namely the branched (1,4;1,6)-α-glucan amylopectin and the essentially unbranched (1,4)-α-glucan amylose, form alternating crystalline and amorphous layers (Jenkins et al, 1993). Most starches contain 70–80% amylopectin and 20–30% amylose; phospholipids and free fatty acids can be associated with the amylose and some protein is attached to the surface of the granules (Tester et al, 2004)
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