Characterization of a new granule-bound starch synthase gene found in amaranth grains (Amaranthus cruentus L.)

Abstract

To clarify the mechanism underlying amylose synthesis in the amaranth pericarp, we attempted to identify new GBSS isoforms. A new GBSS-encoding gene (i.e., CrGBSSIb) was isolated from amaranth leaf tissue. The CrGBSSIb gene consists of 4699-bp, including a 1938-bp open reading frame encoding 606 amino acids. A comparison of the cDNA and genomic sequences suggested that CrGBSSIb contains 12 introns and 13 exons. Interestingly, a phylogenetic analysis revealed that the amaranth GBSSIb gene evolved independently from the other GBSSI isoforms within this crop (i.e., intraspecies) and differed from the other plant GBSSII genes. The expression patterns of two GBSS isoforms revealed that CrGBSSIb and CrGBSSI are expressed during the early and late phases of seed development, respectively. Additionally, a high CrGBSSIb transcript level was detected in leaf tissue. This result indicates that CrGBSSI and CrGBSSIb, which affect amylose synthesis in amaranth grains, are active in the perisperm and pericarp, respectively. Therefore, CrGBSSIb encodes an enzyme associated with amylose synthesis. The enzyme may be primarily responsible for amylose metabolism in pericarp tissue.