Variation in Granule Bound Starch Synthase I (GBSSI) loci amongst Australian wild cereal relatives (Poaceae)

A complex cascade of enzymes is responsible for the development of starch granules in grain endosperm. Granule Bound Starch Synthase I (GBSSI), encoded by the Waxy gene, is a key enzyme of starch synthesis and determines the accumulation of amylose in the starch granules. The complete genomic GBSSI sequence was ascertained for eight Australian cereal wild relatives (CWR) to determine diversity within the gene. A phylogeny derived from the coding sequence of the entire Waxy gene was compared to established phylogenetic relationships. Starch granule morphology observed in conjunction with this phylogeny suggests that small polygonal starch granules arranged as compound granules are the ancestral state, evolving subsequently to bimodal starch granules and to larger simple granules. Genomic sequence length varied within the species from 2800 to 3572 bp. Most variation occurred within the intron sequences, the largest insertion showing strong homology to a retrotransposon. One wild species was determined to have a deletion in the 3′-end of exon 1 resulting in a putatively non-functional allele. Alignment of the amino acid sequence showed strong homology throughout the central fragments of the gene but broad variation in the transit peptides. All putative functional alleles maintained the reported active sites for glycogen synthesis, though with variations in other highly conserved areas of the gene. These variations within the wild relatives of cultivated cereals may provide novel sources of genetic diversity for future cereal improvement programs.