Abstract:
β-amylase is a thermostable enzyme that hydrolyses starch during cooking of sweetpotato (Ipomoea batatas)
storage roots, thereby influencing eating quality. Its activity is known to vary amongst genotypes but the genetic
diversity of the beta-amylase gene (Amyβ) is not well studied. Amyβ has a highly conserved region between exon
V and VI, forming part of the enzyme's active site. To determine the gene diversity, a 2.3 kb fragment, including
the conserved region of the Amyβ gene was sequenced from 25 sweetpotato genotypes. The effect of sequence
variation on gene expression, enzyme activity, and firmness in cooked roots was determined. Six genotypes
carrying several SNPs within exon V, linked with an AT or ATGATA insertion in intron V were unique and
clustered together. The genotypes also shared an A336E substitution in the amino acid sequence, eight residues
upstream of a substrate-binding Thr344. The genotypes carrying this allele exhibited low gene expression and low
enzyme activity. Enzyme activity was negatively correlated with firmness (R ¼ 0.42) in cooked roots. This is the
first report of such an allele, associated with low enzyme activity. These results suggest that genetic variation
within the AmyB locus can be utilized to develop markers for firmness in sweet potato breeding.
