Evolutionary conservation and expression patterns of maize starch branching enzyme I and IIb genes suggests isoform specialization.

Expression of the maize (Zea mays L.) starch branching enzyme (SBE) genes Sbe1 and Sbe2 were characterized during kernel development and in vegetative tissues. The onset of Sbe1 and Sbe2 expression during endosperm development was similar to that of other genes involved in starch biosynthesis (Wx, Sh2 and Bt2). However, the expression of Sbe2 peaked earlier than that of Sbe1 in developing endosperm and embryos resulting in a shift in the ratio of Sbe1 to Sbe2 relative message levels during kernel and embryo development. Transcripts hybridizing to the Sbe2 probe were not detectable in leaves or roots which nonetheless have SBEII enzymatic activity, suggesting that there may be another divergent SBEII-like gene(s) in maize. A similar expression pattern is shared between the maize genes and related genes in pea, which together with their evolutionary conservation, suggests that the SBE isoforms may play unique roles in starch biosynthesis during plant development.