Starch biogenesis: relationship between starch structures and starch biosynthetic enzymes
The goal of this research is to gain understanding on starch biogenesis by studying starch granule formation and development. The objectives of this research were to modify the structure of cyanobacteria glycogen by replacing the endogenous glycogen synthase gene with a plant starch synthase and to study starch granule formation by creating a plant starch biosynthetic system in a cyanobacterium, and to investigate the development of starch granules in maize endosperm during maturation of the maize kernel;A potato starch synthase III (PSSIII) was expressed in Cyanobacterium Synechocystis sp. PCC6803 mutants without glycogen synthase I (M1) or glycogen synthase II (M2) to produce two new mutants PM1 and PM2. The mutants PM12 and PM21, containing only PSSIII as glycogen/starch synthase, were derived from PM1 and PM2, respectively. Results indicated that the survival of Synechocystis required the existence of glycogen/starch synthase. The disruption of GSII and the expression of PSSIII were coupled with the increase of glycogen branching enzyme activity. Compared with WT glycogen, PM1, PM2, PM12 and PM21 produced glucans with more short chains and fewer long chains, which is consistent with the increase of the branching enzyme activity in the mutants. Results indicated that there are regulatory and cooperative interactions among plant starch synthase, glycogen synthases, and glycogen branching enzyme in the process of the biosynthesis of glucans;To study starch structures and properties along the development of starch granules, endosperm and pericarp starches were isolated from maize (B73) kernels at different developmental stages. Maize endosperm starch content, granule size, and amylose content increased during the kernel development. The onset gelatinization temperature of the endosperm starch increased from 61.3°C (8DAP) to 69.0°C (14DAP) and then decreased to 67.4°C (30DAP). The amylopectin branch chain-length of the endosperm starch on 14DAP displayed the longest average chain length and the largest percentage of the long chain (DP≥37). In contrary, there were no significant changes in size, amylose content, starch content, and thermal properties of pericarp starches. Results indicated that the endosperm starch structure was not synthesized homogenously with the maturation of kernel.