Importance of isoforms of starch-branching enzyme in determining the structure of starch in pea leaves

The impact of a mutation at the r locus of peas (Pisum sativum L.) on the structure of starch in the leaf has been studied. The mutation specifically eliminates the A class of isoform of starch-branching enzyme (SEE A) from the leaf, causing a 10-fold reduction in the total activity of the enzyme. Gel-permeation chromatography and thymol precipitation show that wild-type leaf starch consists of polymers with the general characteristics of amylose and amylopectin, although amylose is only a very minor component of the starch. High-performance anion exchange chromatography (HPAEC) of debranched amylopectin reveals that the distribution profile of branch lengths is strongly polymodal, and distinctly different from that of the amylopectin of storage starches. The mutation at the r locus results in the appearance of an amylopectin-like glucan of low molecular weight in the starch. The absorbance of the iodine complex of the amylopectin and analysis by HPAEC both indicate that the mutation causes an increase in the average branch length of the amylopectin but does not affect the polymodal nature of the distribution of branch lengths. The extent to which these effects of the mutation are specifically due to the loss of SEE A is discussed. It is suggested that differences in properties between isoforms of SEE are not the main factors that determine the polymodal distribution of branch lengths in amylopectin.