Abstract
Granule-bound starch synthase (GBSS) is responsible for amylose synthesis, but the role of GBSS genes and their encoded proteins remains poorly understood in banana. In this study, amylose content and GBSS activity gradually increased during development of the banana fruit, and decreased during storage of the mature fruit. GBSS protein in banana starch granules was approximately 55.0 kDa. The protein was up-regulated expression during development while it was down-regulated expression during storage. Six genes, designated as MaGBSSI-1, MaGBSSI-2, MaGBSSI-3, MaGBSSI-4, MaGBSSII-1, and MaGBSSII-2, were cloned and characterized from banana fruit. Among the six genes, the expression pattern of MaGBSSI-3 was the most consistent with the changes in amylose content, GBSS enzyme activity, GBSS protein levels, and the quantity or size of starch granules in banana fruit. These results suggest that MaGBSSI-3 might regulate amylose metabolism by affecting the variation of GBSS levels and the quantity or size of starch granules in banana fruit during development or storage.
Highlights
Starch is the main carbohydrate consumed for human nutrition, and is a major component of cereals, tubers, legumes, and fruits
MaGBSSI-3 was weakly expressed at the early stages but was highly upregulated at 50 d of development (Fig. 5A). These results suggest that the MaGBSSI-1, MaGBSSI-2, MaGBSSI-4, MaGBSSII-1, and MaGBSSII-2 might be involved in the early stages (0–30 d) of starch granule-filling, and MaGBSSI-3 might be involved in the later stages (30–60 d) of starch granule-filling during the development of banana fruit (Fig. 5A)
Slack and Wainwright [18] reported that amylose content increased gradually during the development of barley grain, and the granule-bound starch synthase (GBSS) enzyme plays a key role in amylose synthesis [19]
Summary
Starch is the main carbohydrate consumed for human nutrition, and is a major component of cereals, tubers, legumes, and fruits. Starch consists of a mixture of two different carbohydrates, namely amylose (20%–30%) and amylopectin (70%–80%) [1,2,3]. Amylose is a linear polymer of glucose residues joined together by a-1, 4glucosidic bonds, and its synthesis is mainly catalyzed through the activity of granule-bound starch synthase (GBSS). GBSS transfers the glucosyl residue from ADP-Glu to glucan substrates to produce relatively long-chain amylose molecules [4]. The amylose content directly affects the texture and taste of cereals grains [5]. The physical structure of amylopectin has an important impact on the crystalline properties of maize [5], and its synthesis requires soluble starch synthases (SSs), starch branching enzymes (SBEs), and starch debranching enzymes (DBEs) [4]
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