Effects of soluble starch synthase IIa allelic variation on rice grain quality with different Waxy backgrounds

A comparative evaluation of the effect of SSI and Wx allelic variation on rice grain quality and starch physicochemical properties

Near-isogenic lines Nip(Wx^b/SSI^j), Nip(Wx^b/SSIⁱ), Nip(wx/SSI^j) and Nip(wx/SSIⁱ) in the japonica rice Nipponbare (Nip) background containing allelic variation in the starch synthase gene SSI and Wx were investigated for cooked rice grain quality, starch morphology, pasting profiles, fine structure and crystallinity characteristics. Rice grains carrying the SSIⁱ allele had poor cooked rice taste in the Wx^b background. The introduction of SSIⁱ caused reduced cooked rice grain elongation, especially in the wx background. Starch granule size was reduced in SSIⁱ rice and the viscosity of flour and starch prepared from SSIⁱ rice was markedly increased. Moreover, analysis of the starch molecular structure revealed a remarkable increase in the short amylopectin chains and reduced starch relative crystallinity compared with SSI^j rice, which resulted in decreased gelatinization characteristics. These results suggest that SSI allelic variation has multiple effects on rice grain quality, as well as starch fine structure.

Variation of resistant starch content in different processing types and their starch granules properties in rice

Ninety-nine lines from recombinant inbred lines were selected to investigate the effects of starch synthesis-related genes on resistant starch (RS) content in different proceeding types. RS in raw milled rice (RSm), hot cooked rice (RSc), and retrogradation rice (RSr) showed a wide variation among the lines, especially RSm arrived at 10.61%. Divergent variability of RSm, RSc and RSr indicated that there were different regulation mechanisms for them. Waxy wildtype allele (Wx^a) could elevate RSm, RSc and RSr, but Soluble starch synthase IIa (SSIIa) only played a vital role in regulating RSm. Wx^a-indica SSIIa could increase RSm, and Wx^a-japonica SSIIa (SSIIaj) could elevate RSc and RSr. The mean diameter of Wx^a-SSIIaj was significantly bigger than others. The bigger starch granules, the higher RSc and RSr. Starch granules morphology with high-RSm would have a higher percentage in polyhedral and angular shape. The results provide new information for rice breeding with high-RS content.

Genetic Control and High Temperature Effects on Starch Biosynthesis and Grain Quality in Rice

Grain quality is one of the key targets to be improved for rice breeders and covers cooking, eating, nutritional, appearance, milling, and sensory properties. Cooking and eating quality are mostly of concern to consumers and mainly determined by starch structure and composition. Although many starch synthesis enzymes have been identified and starch synthesis system has been established for a long time, novel functions of some starch synthesis genes have continually been found, and many important regulatory factors for seed development and grain quality control have recently been identified. Here, we summarize the progress in this field as comprehensively as possible and hopefully reveal some underlying molecular mechanisms controlling eating quality in rice. The regulatory network of amylose content (AC) determination is emphasized, as AC is the most important index for rice eating quality (REQ). Moreover, the regulatory mechanism of REQ, especially AC influenced by high temperature which is concerned as a most harmful environmental factor during grain filling is highlighted in this review.

Identification of Structure-Controlling Rice Biosynthesis Enzymes

The main enzymes controlling the chain-length distributions (CLDs) of starches are starch synthases (SSs), starch branching enzymes (SBEs), and debranching enzymes (DBEs), which have various isoforms, denoted as SSI, SSII-1, etc. Different isozymes dominate the CLD in different ranges of degrees of polymerization (DPs). Models have been developed for the CLDs in terms of the activities of isoforms of these enzymes, in terms of two parameters: β_i, which is the ratio of the activity of SBE to that of SS in set i, and h_i, which is the relative activity of SS in that set. These provide good fits to data but without specifying which isozymes are in set i. Here, CLDs for amylopectin and amylose synthesis in rice endosperm are explored. Molecular weight distributions of the different chains formed in 87 rice varieties were obtained using size-exclusion chromatography following enzymatic debranching (converting a complex branched macromolecule to linear polymers), and fitted by the biosynthesis-based models. The mutants of each isoform among tested rice varieties were identified by amino-acid mutations in coding sequences based on the extraction and analysis of whole gene sequences. The significant differences between mutant groups of different isoforms indicate that SSI, SSII-3, SSIII-1, SSIII-2, and SBEI as well as GBSSI (an isozyme of granule-bound starch synthase) belong to the enzymes sets that control amylose biosynthesis. Further, GBSSI is in the enzyme sets that control amylopectin chains. This enables specification of all isozymes and the DP range, which they dominate, over the entire DP range. As the CLD controls many functional properties of rice, this can help breeders target and develop improved rice species.