Characterization and Expression Analysis of Starch Branching Enzymes in Sweet Potato

Spatial and temporal expression patterns of Sbe1 and Sbe2 that encode starch branching enzyme (SBE) I and II, respectively, in sweet potato (Ipomoea batatas L.) were analyzed. Expression of both genes in Escherichia coli indicate that both genes encoded active SBE. Analysis with real-time quantitative polymerase chain reaction technique indicates that IbSbe1 mRNA was expressed at very low levels in leaves but was the predominant isoform in tuberous root while the reverse case was found for IbSbe2. The expression pattern of IbSbe1, closely resembles that of AGPase S, a gene coding for one of the subunits of ADP-glucose pyrophosphorylase, which is the key regulatory enzyme in the starch biosynthetic pathway. Western analysis detected at least two isoforms of SBE I in tuberous roots, those two isoforms showed adverse expression patterns with the development of the tuberous roots. Expression of the two IbSbe genes exhibited a diurnal rhythm during a 12-h cycle when fed a continuous solution of sucrose. Abscisic acid (ABA) was aother potent inducer of IbSbe expression, but bypassed the semidian oscillator.