Starch biosynthetic process

Abstract: Plants have developed two distinct starch biosynthetic systems composed of over 30 kinds of enzymatic reaction network in photosynthetic and non-photosynthetic cells. Higher plants have also evolved a process in which cells can accumulate huge amounts of starch as granules inside the amyloplast of the reserve organs. Primarily the coordinated expression of several sets of distinct isozymes with specific enzymatic properties enables plant cells to synthesize starch with distinct fine structure and form the starch granules with specific semicrystalline structure, granular morphology, and physicochemical properties in plastids. This chapter overviews the current status of our understanding of metabolic regulation of starch biosynthesis in reserve organs by focusing on functions of individual isozymes examined by numerous in vivo and in vitro studies that have been performed to reveal how individual isozymes contribute to the synthesis of the reserve starch. The results raised the high possibility that at least some isozymes have multiple functions in starch biosynthesis under different conditions depending on the presence of various glucans and interaction/association with other enzymes. The features of starch biosynthetic process in plant tissues are also discussed with emphasis on the biochemical mechanism(s) underlying the coordinate actions among various enzymes.

Abstract: Starch, a plant-derived insoluble carbohydrate composed of glucose polymers, is the principal carbohydrate in our diet and a valuable raw material for industry. The properties of starch depend on the arrangement of glucose units within the constituent polymers. However, key aspects of starch structure and the underlying biosynthetic processes are not well understood, limiting progress towards targeted improvement of our starch crops. In particular, the major component of starch, amylopectin, has a complex three-dimensional, branched architecture. This architecture stems from the combined actions of a multitude of enzymes, each having broad specificities that are difficult to capture experimentally. In this review, we reflect on experimental approaches and limitations to decipher the enzymes' specificities and explore possibilities for in silico simulations of these activities. We believe that the synergy between experimentation and simulation is needed for the correct interpretation of experimental data and holds the potential to greatly advance our understanding of the overall starch biosynthetic process. We furthermore propose that the formation of glucan secondary structures, concomitant with its synthesis, is a previously overlooked factor that directly affects amylopectin architecture through its impact on enzyme function.

Abstract: Background Saffron crocus (Crocus sativus) is an expensive and valuable species that presents preventive and curative effects. This study aimed to screen the key proteins affecting the floral initiation of saffron under cold stress and thus increasing yield by regulating the temperature. Results Protein expression profiles in flowering and non-flowering saffron buds were established using isobaric tags for relative or absolute quantitation (iTRAQ). A total of 5,624 proteins were identified, and 201 differentially abundant protein species (DAPs) were further obtained between the flowering and non-flowering groups. The most important functions of the upregulated DAPs were "sucrose metabolic process," "lipid transport," "glutathione metabolic process," and "gene silencing by RNA." Downregulated DAPs were significantly enriched in "starch biosynthetic process" and several oxidative stress response pathways. Three new flower-related proteins, CsFLK, CseIF4a, and CsHUA1, were identified in this study. The following eight key genes were validated by real-time qPCR in flowering and non-flowering top buds from five different growth phases: floral induction- and floral organ development-related genes CsFLK, CseIF4A, CsHUA1, and CsGSTU7; sucrose synthase activity-related genes CsSUS1 and CsSUS2; and starch synthase activity-related genes CsGBSS1 and CsPU1. These findings demonstrate the important roles played by sucrose/starch biosynthesis pathways in floral development at the mRNA level. During normal floral organ development, the sucrose contents in the top buds of saffron increased, and the starch contents decreased. In contrast, non-flowering buds showed significantly decreased sucrose contents under cold stress and no significant changes in starch contents compared with those in the dormancy stage. Conclusion In this report, the protein profiles of saffron under cold stress and a normal environment were revealed for the first time by iTRAQ. A possible "reactive oxygen species-antioxidant system-starch/sugar interconversion flowering pathway" was established to explain the phenomenon that saffron does not bloom due to low temperature treatment.