Plant Physiol, March 2001, Vol. 125, pp. 1314-1324

Comparison of Starch-Branching Enzyme Genes Reveals Evolutionary Relationships Among Isoforms. Characterization of a Gene for Starch-Branching Enzyme IIa from the Wheat D Genome Donor Aegilops tauschii¹

Commonwealth Scientific and Industrial Research Organization Plant Industry, P.O. Box 1600, Australian Capital Territory 2601, Australia (S.R., Z.L., B.K.H., S.A., M.K.M.); Plant Breeding Institute, University of Sydney, Cobbitty, New South Wales 2570, Australia (A.R.); Department of Biological Sciences, Osaka Kyoiku University, 4-698-1 Asahigaoka, Kashiwara, Osaka 582, Japan (Y.M.); and Kansai Womens College, Kashiwara, Osaka 582, Japan (M.Y.)

Genes and cDNAs for starch-branching enzyme II (SBEII) have been isolated from libraries constructed from Aegilops tauschii and wheat (Triticum aestivum) endosperm, respectively. One class of genes has been termed wSBEII-DA1 and encodes the N terminus reported for an SBEII from wheat endosperm. On the basis of phylogenetic comparisons with other branching enzyme sequences, wSBEII-DA1 is considered to be a member of the SBEIIa class. The wSBEII-DA1 gene consists of 22 exons with exons 4 to 21 being identical in length to the maize (Zea mays) SBEIIb gene, and the gene is located in the proximal region of the long arm of chromosome 2 at a locus designated sbe2a. RNA encoding SBEIIa can be detected in the endosperm from 6 d after flowering and is at its maximum level from 15 to 18 d after anthesis. Use of antibodies specific for SBEIIa demonstrated that this protein was present in both the soluble and granule bound fractions in developing wheat endosperm. We also report a cDNA sequence for SBEIIa that could arise by variant transcription/splicing. A second gene, termed wSBEII-DB1, was isolated and encodes an SBEII, which shows greater sequence identity with SBEIIb-type sequences than with SBEIIa-type sequences. Comparisons of SBEII gene structures among wheat, maize, and Arabidopsis indicate the lineage of the SBEII genes.