Abstract
BackgroundEndosperm starch provides prime energy for cereal seedling growth. Cereal endosperm with repression of starch branching enzyme (SBE) has been widely studied for its high resistant starch content and health benefit. However, in barley and maize, the repression of SBE changes starch component and amylopectin structure which affects grain germination and seedling establishment. A high resistant starch rice line (TRS) has been developed through inhibiting SBEI/IIb, and its starch has very high resistance to in vitro hydrolysis and digestion. However, it is unclear whether the starch resists in situ degradation in seed and influences seedling growth after grain germination.ResultsIn this study, TRS and its wild-type rice cultivar Te-qing (TQ) were used to investigate the seedling growth, starch property changes, and in situ starch degradation during seedling growth. The slow degradation of starch in TRS seed restrained the seedling growth. The starch components including amylose and amylopectin were simultaneously degraded in TQ seeds during seedling growth, but in TRS seeds, the amylose was degraded faster than amylopectin and the amylopectin long branch-chains with B-type crystallinity had high resistance to in situ degradation. TQ starch was gradually degraded from the proximal to distal region of embryo and from the outer to inner in endosperm. However, TRS endosperm contained polygonal, aggregate, elongated and hollow starch from inner to outer. The polygonal starch similar to TQ starch was completely degraded, and the other starches with long branch-chains of amylopectin and B-type crystallinity were degraded faster at the early stage of seedling growth but had high resistance to in situ degradation during TRS seedling growth.ConclusionsThe B-type crystallinity and long branch-chains of amylopectin in TRS seed had high resistance to in situ degradation, which inhibited TRS seedling growth.
Highlights
Endosperm starch provides prime energy for cereal seedling growth
The amylose is only synthesized by granule bound starch synthase, and the amylopectin is mainly synthesized by soluble starch synthase, starch branching enzyme (SBE), and debranching enzyme
Suppressing or eliminating one or more SBE activities in rice, maize, wheat, and barley can significantly change amylopectin structure including of the decrease of branching degree and the elongation of branch-chain length, and increase the content of resistant starch (RS), which cannot be digested in the upper gastrointestinal tract but functions as a substrate for bacterial fermentation in the large intestine [3,4,5,6,7,8,9]
Summary
Endosperm starch provides prime energy for cereal seedling growth. Cereal endosperm with repression of starch branching enzyme (SBE) has been widely studied for its high resistant starch content and health benefit. A high resistant starch rice line (TRS) has been developed through inhibiting SBEI/IIb, and its starch has very high resistance to in vitro hydrolysis and digestion It is unclear whether the starch resists in situ degradation in seed and influences seedling growth after grain germination. Many high RS crops with long branch-chains of amylopectin have been developed via mutation of SBE gene or inhibition of SBE expression Their starches have high resistance to digestion and their derived food products can lower the glycemic and insulin responses and reduce the risk of developing type II diabetes, obesity, and cardiovascular disease [3,4,5,6,7,8,9]
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