A Genetic Approach to Studying the Morphology, Structure and Function of Starch Granules using Pea as a Model

Abstract

A series of near-isogenic lines have been developed for mutations induced and identified at six independent loci controlling steps in starch biosynthesis of pea (Pisum sativum) seeds. The review describes the methodology associated with the development of these lines and with the characterisation of the starches produced, in particular the effects that the mutations have on the morphology, structure and properties of the starch granules. The shape of starch granules from lines containing mutations at either the r or rug5 loci, encoding a starch branching enzyme and a soluble starch synthase, differ significantly from granules found in the non-mutant parental line. Mutations at each of the six loci result in characteristic changes in granular structure, which has been studied using polarised light microscopy, wide angle X-ray diffraction, solid state NMR and differential scanning calorimetry. Relatively small but significant changes were found in the proportion of crystalline material within the granules and much greater differences in the spatial distribution of the crystals within the granules. In particular, granules from rand rugs mutants do not show the characteristic 'Maltese cross' pattern when viewed under polarised light. Large differences were found between the mutants in the proportion of A- and B-type crystallites within the granules, ranging from about 60% A-type in rug3 mutants to 100% B-type in rmutant lines. It is suggested that the pea lines are an invaluable resource for answering fundamental questions on the genetic control of starch granule structure and function.