Research On Characteristics And Physiological Mechanism For Formation Of Starch Granule Size Distribution In Wheat Grain

Wheat cultivars released in the years from 1950s to 1990s were selected to evaluate the difference on starch granule size among diverse genotypes in the field experiment. The representative cultivars were used for the subsequent experiment. In addition, the most basal grain and the most distal grain on each spikelet of wheat cultivar Yangmai 158 were detached and identified as superior grain and inferior grain, respectively. The difference in starch granule size distribution was then compared between superior and inferior grains, the corresponding mechanism was also studied. Finally, wheat cultivar Yangmai 158 and Yangmai 11 (with Yangmai 158 as recurrent parent) were subjected to 33% less intercept radiation (S1, non-shading as control, SO) from jointing to maturity. Effect of shading on starch granule size distribution and its mechanism were then investigated. Based on the results here, the starch synthase activities and their coded gene expression were found to play important regulation roles in starch granule size distribution formation in wheat grain. Here are the main results.1. Observed the development progress of starch granule size distribution in wheat endosperm during grain fillingAnalysis of 4 DAA endosperm revealed a single population (destined to become the large A granules). At 8 DAA starch granule size exhibited a bimodal distribution with a maximum diameter more than 20μm. The new class of starch population suggested the initial formation of the small B-type granules. At 20 DAA, a new different class of starch granule population, defined as the C-type granules, was synthesized. From 24 to maturity, starch granules exhibited three distinct populations. At maturity three distinct size groups of starch granules were present: large type A granules with diameters greater than 10μm, type B granules with diameters between 2 and 10μm, and small type C granules with diameters less than 2μm.2. Characterized the difference in starch granule size distribution in wheat endosperm among diverse genotypic cultivars Starch samples isolated from the wheat cultivars released in the years from 1950s to 1990s were examined. A clear trimodal distribution of granule sizes was shown by all starch samples. Highly significant differences were seen among the cultivars from different years for volume percentage of starch granules. Following with the years evolution, volume percentage of A-type granules increased and that of B- and C-type granules decreased. Starch granule surface area and number also differed within cultivars. Mean amylose and amylopectin content varied significantly among cultivars. Amylose and amylopectin content were positively correlated with the volume percentage of granules with diameters greater than 10μm. Amylopectin content was negatively correlated with surface area and number of starch granules. Results suggested that significant differences existed in granules size distribution and starch content between wheat cultivars from different years, especially between 1950s and 1990s.3. Revealed the difference in starch granule size distribution in wheat endosperm between superior and inferior grain Compared with inferior grains, superior grains showed higher starch content as well as amylose and amylopectin. Based on volume, the formation of A-, B-, C-type starch granules initiated at 4,8,20 DAA, respectively. The mean diameter of starch granules achieved at 16 DAA in superior and inferior grains, and it was higher in inferior grains during later grain filling. The mode diameter of starch granules increased with grain filling process in superior grain, as well as inferior grains. The maximum diameter of starch granules increased slightly after 16 DAA, even becoming close to the terminative diameter at 20 DAA and not varying after this. Based on number, the small starch granules, between 0.5-2μm, were in the predominant position, composing above 90% of total number. Based on surface area, dynamic variation tendency of size distribution was similar to that based on volume, and the population of B-type and C-type starch granules was major component. However, the process of starch granule size distribution was faster in superior grain, and inferior grain had temporal delay in initiation and enlargement of granule at early stage and development of small granules at late stage during grain filling.4. Evaluated the effect long-term shading on starch granule size distribution in wheat endosperm Long-term shading showed no effect on amylose content while significantly reduced amylopectin content, which resulted in the significant decrease in the ratio of amylopectin to amylase content in grain. Long-term shading also reduced the mean diameters by volume, number and surface area. Under the long-term shading conditions, the development of starch granule was suppressed during the mid to late grain filling stage, as a result of decline in peak diameter, reduced range of max diameter and enlargement of small granules. Based on the response of starch granule size distribution to shading, Yangmai 158 showed the better tolerance than Yangmai 11.5. Explored the physiological mechanism for development of starch granule size distribution in wheat endosperm The activities of soluble and granule-bound starch synthases (designated SSS and GBSS) peaked on 20 and 24 DAA. Our results showed that the ratio of large to small starch granules in volume frequency percentage, at least partially, was related to the interplay between the SSS and GBSS activities. Genes encoding isoforms of starch synthases expressed at different grain filling period. SS I was generally expressed over the grain filling stage; the SS II and SS III were expressed over the early and mid grain filling stage, and the GBSS I was expressed during the mid to late grain filling stage. In addition, the activities of starch synthases and expression of starch synthase encoding genes temporally shared the dynamics of the starch granule size distribution. It was concluded that the starch granule size distribution in the grains of wheat was associated with the activities of the starch synthases, which were regulated at the transcription levels of the genes encoding the starch synthases.6. Debated the regulation approached for starch granule size distribution in wheat endosperm In this study, depending on the variances in starch size distribution from cultivars and treatments, some key indices for accelerating the progress of target breeding and better cultivation were found as below:1) the fitting time for regulation of starch size distribution during grain filling; 2) the bound of diameter and volume ratio between the large and small starch granules in high quality wheat grains; 3) the details of changes in size distribution of the populations of large and small starch granules under controlled conditions; 4) more effects of genotype than growth environment on the indices which the bound of diameter in 10μm and volume accumulation rate in 40%.In conclusion, the variances in starch granule size distribution follow with diverse genotype, ecological environment and grain position. However, size distribution was regulated by starch synthase activities and their coding gene expression. It was possible that large and small starch granule population (cutoff at 10μm) could be regulated during mid and late grain filling stage.