Mutant genes at the r and rb loci affect the structure and physico-chemical properties of pea seed starches

Abstract

Mutant genes at two loci, r and rb, known to encode genes affecting the starch biosynthetic pathway, were studied for their effect on the structure and gelatinization of pea seed starches. Comparisons were made using starches from four lines (RRRbRb, rrRbRb, RRrbrb, and rrrbrb), near-isogenic except for genes at these two loci. All the starches had C-type X-ray diffraction patterns, but different contents of 'A' and 'B' polymorphs. The presence of a mutation at either locus increased the 'B' polymorph content in the starches, although the influence of the r mutation was much greater than that of rb. Differences were discovered in the crystalline stucture of the rrRbRb starch which correlated with a high content of amorphous phase as well as with the changes in amylopectin structure. In addition, changes in the crystalline structure of this sample correlated with a lack of co-operative transition during starch gelatinization in excess water. The RRrbrb starch had a greatly increased enthalpy of gelatinization in excess water compared with the wild-type starch. It is proposed that this effect is connected with specific charge interactions between the molecules in the starch granule. The rrrbrb starch had parameters of crystalline structure and gelatinization which reflected the different influences of the two genes. With regard to gelatinization, this starch had relatively wide co-operative transition and low enthalpy and a very high peak temperature of transition.