Allelic variation at the amylose-extender locus: Levels of soluble starch branching enzymes

Characterization of starch branching enzymes and starch synthases from homozygous Ae and ae-Ref kernels have shown ae-Ref kernels to lack detectable quantities of starch branching enzyme IIb (Boyer and Preiss 1978, BBRC 80:169). Two additional starch branching enzymes, I and IIa, as well as two fractions of soluble starch synthase, were found to be similar from kernels of the two genotypes. The soluble starch branching enzymes and starch synthases have not been examined in kernels homozygous for five additional ae alleles. These alleles are of spontaneous origin (ae-M1 and ae-B4), or arose in stocks carrying the Ac-Ds system of controlling elements (ae-i1) or in stocks after EMS treatment (ae-EMS1 and ae-EMS3). All the alleles are recessive and homozygous kernels phenotypically indistinguishable. W64A backcross conversions for all of the alleles were provided by Douglas Garwood. Enzymes were purified from 22-day-old kernels and characterized as described (Boyer and Preiss, Plant Physiol. 67:1141, 1981). No differences were found for the properties of branching enzymes I and Ha or the soluble starch synthases from kernels homozygous for different ae alleles. Based on DEAE-cellulose chromatography, extracts from kernels homozygous for any of the recessive alleles contained no branching enzyme IIb:

Table.

In some purifications, the low levels of branching enzyme activity found in IIb fractions were due to contaminating branching enzyme IIa. The variation in levels of branching enzymes I and IIa between genotype does not appear to be allele related. Because purification of branching enzyme was required before different branching enzymes could be identified and quantified, the variation between purifications of the same genotype was as great as the variation between genotypes. Therefore, all the recessive ae alleles examined fall into one class, characterized by the deficiency of starch-branching enzyme IIb.

Karen D. Hedman and Charles D. Boyer


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