Abstract
BackgroundThorough understanding of seed starch biosynthesis and accumulation mechanisms is of great importance for agriculture and crop improvement strategies. We conducted the first comprehensive study of the dynamic development of starch granules and the regulation of starch biosynthesis in Brachypodium distachyon and compared the findings with those reported for common wheat (Chinese Spring, CS) and Aegilops peregrina.ResultsOnly B-granules were identified in Brachypodium Bd21, and the shape variation and development of starch granules were similar in the B-granules of CS and Bd21. Phylogenetic analysis showed that most of the Bd21 starch synthesis-related genes were more similar to those in wheat than in rice. Early expression of key genes in Bd21 starch biosynthesis mediate starch synthesis in the pericarp; intermediate-stage expression increases the number and size of starch granules. In contrast, these enzymes in CS and Ae. peregrina were mostly expressed at intermediate stages, driving production of new B-granules and increasing the granule size, respectively. Immunogold labeling showed that granule-bound starch synthase (GBSSI; related to amylose synthesis) was mainly present in starch granules: at lower levels in the B-granules of Bd21 than in CS. Furthermore, GBSSI was phosphorylated at threonine 183 and tyrosine 185 in the starch synthase catalytic domain in CS and Ae. peregrina, but neither site was phosphorylated in Bd21, suggesting GBSSI phosphorylation could improve amylose biosynthesis.ConclusionsBd21 contains only B-granules, and the expression of key genes in the three studied genera is consistent with the dynamic development of starch granules. GBSSI is present in greater amounts in the B-granules of CS than in Bd21; two phosphorylation sites (Thr183 and Tyr185) were found in Triticum and Aegilops; these sites were not phosphorylated in Bd21. GBSSI phosphorylation may reflect its importance in amylose synthesis.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-014-0198-2) contains supplementary material, which is available to authorized users.
Highlights
Thorough understanding of seed starch biosynthesis and accumulation mechanisms is of great importance for agriculture and crop improvement strategies
In Bd21, the starch granules appeared ~8 days post-anthesis (DPA); their diameter remained less than 10 μm throughout growth and were classified as B-granules (Figure 1B)
The starch granules in Chinese Spring (CS) grew rapidly from 6 to 8 DPA but remained less than 10 μm in diameter; growth slowed from 8 to 12 DPA and yielded granules of diameter greater than 10 μm; these were classified as A-granules
Summary
Thorough understanding of seed starch biosynthesis and accumulation mechanisms is of great importance for agriculture and crop improvement strategies. Starch is the major storage carbohydrate in the seeds of cereal crops. Amylose is a relatively linear molecule consisting of (1–4)-linked units of D-glucopyranosyl, whereas amylopectin mainly consists of long chains of (1–4)-linked. D-glucopyranosyl units with occasional branching (1–6) linkages that yield tandem linked clusters (~9–10 nm long each) [2]. Amylopectins from different species exhibit different chain length distributions with periodic occurrence of varying degrees of polymerization (DP). These chains are grouped into four fractions with DP in intervals 6–12 (A-chain), 13–24 (B1-chain), 25–36 (B2-chain), and >37 (B3- or more advanced chains) [4]
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