Abstract
Cereal crops have starch in their endosperm, which has provided calories to humans and livestock since the dawn of civilization to the present day. Starch is one of the important biological factors which is contributing to the yield of cereal crops. Starch is synthesized by different enzymes, but starch structure and amount are mainly determined by the activities of starch synthase enzymes (SS) with the involvement of starch branching enzymes (SBEs) and debranching enzymes (DBEs). Six classes of SSs are found in Arabidopsis and are designated as soluble SSI-V, and non-soluble granule bound starch synthase (GBSS). Soluble SSs are important for starch yield considering their role in starch biosynthesis in cereal crops, and the activities of these enzymes determine the structure of starch and the physical properties of starch granules. One of the unique characteristics of starch structure is elongated glucan chains within amylopectin, which is by SSs through interactions with other starch biosynthetic enzymes (SBEs and DBEs). Additionally, soluble SSs also have conserved domains with phosphorylation sites that may be involved in regulating starch metabolism and formation of heteromeric SS complexes. This review presents an overview of soluble SSs in cereal crops and includes their functional and structural characteristics in relation to starch synthesis.
Highlights
Starch is the primary source of energy for human nutrition and is a main product of plant photosynthetic C fixation [1]
We provide an overview of soluble synthase enzymes (SS) and its roles in starch biosynthesis
Subtle difference in protein profile, reduced association of SSI and BEIIb in ssI mutant Increase amylose, decrease grain weight, increase in level of ADP-glucose pyrophosphorylases GC-AG intron splicing offer more variants for genetic divergence in rice SSIIa protein was totally absent in seeds, higher amylose content, Number of short chains formation increased in amylopectin Chains with DP 6 to 9 and DP 16 to 19 decreased, chains with DP 10 to 15 and DP 20 to 25 increased, amylose and amylopectin content increased Affected gel consistency, percent of retrogradation, Starch granules smaller and round as compared to wild type (WT), reduced contents of long chains
Summary
Starch is the primary source of energy for human nutrition and is a main product of plant photosynthetic C fixation [1]. Higher plants synthesize storage starch in the form of granules and store in the seeds and tubers. Starch present in these organs and accumulate during the developments of these organs and its stable for long period of time in dry condition. The degradation of starch occurs during respiration in plants and contributes to the formation of sucrose This sucrose is transported to the rest of the plant to provide energy in plant growth [4]. Soluble SSs (SSI, SSII, SSIII, and SSIV) function in the process of starch synthesis have been elucidated by mutant analysis of monocots by using cereal models and of dicots through studying potato tubers, Arabidopsis leaves, and pea embryos. The current knowledge of SSs regulation, their ability to form protein complexes with other enzymes, and their regulation by protein phosphorylation are outlined
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