Starch-Branching Enzymes Sbe1 and Sbe2 From Wheat (Triticum aestivum cv. Cheyenne): Molecular Characterization, Developmental Expression, and Homoeologue Assignment by Differential PCR*

Wheat starch-branching enzymes McCue et al. Abstract. Starch is t e mai component of the wheat kernel, and wheat flour is used for hundreds of food and nonfood products. We are exploring ways to improve wheat quality and to develop new uses for wheat based on altered starch characteristics. To understand the molecular basis for variations in the physical and chemical properties of starch, we examined transcripts for starch biosynthetic enzymes. cDNAs encoding 2 isoforms of starchbranching enzyme (Sbe1, Sbe2) were isolated from wheat endosperm. The longest Sbe1 and Sbe2 cDNAs were 2797 and 2975 bp, respectively, and they shared extensive identity with Sbe sequences reported for wheat and other species. With orthologue-specific primer pairs, homoeologue assignments to chromosome 7 were made for Sbe1#19 (TRIae:Sbe1A.1) and Sbe1#9 (TRIae:Sbe1D.1) using the wheat cv. Chinese Spring nullisomic-tetrasomic-7 lines. This strategy may prove useful for future mapping of expressed sequence tag (EST) data. The Sbe cDNAs and a granule-bound starch synthase cDNA (GbssI) (from an EST sequencing project) were used to examine the steady-state RNA levels during development of the wheat. Steady-state levels of Sbe2 mRNA were detectable 5 d postanthesis (DPA) and reached a maximum at 10 DPA. Steady-state levels of Sbe1 and GbssI began to rise at 10 DPA and peaked at 15 DPA. Levels of all messages declined rapidly at 20-25 DPA. Reported here is the first analysis of transcripts for these enzymes in the same RNA pools and demonstration that expression patterns are unique and developmentally regulated.