The structural organisation of the gene encoding class II starch synthase of wheat and barley and the evolution of the genes encoding starch synthases in plants

Wheat and barley contain at least four classes of starch synthases in the endosperm, granule bound starch synthase I (GBSSI) and starch synthases I, II and III (SSI, SSII, SSIII). In this work, SSII in barley is shown to be associated with the starch granule by using antibodies. A cDNA from barley encoding SSII and the genes for SSII from barley and Aegilops tauschii (A. tauschii, the D genome donor to wheat) are characterised. Fluorescent in situ hybridisation (FISH) and PCR were used to localise the wheat SSII gene to the short arm of chromosome 7, showing synteny with the location of the rice SSII gene to the short arm of chromosome 6. Comparison of the genes encoding SSII of A. tauschii, barley and Arabidopsis showed a conserved exon-intron structure although the size of the introns varied considerably. Extending such comparison between the genes encoding starch synthases (GBSSI, SSI, SSII and SSIII) from A. tauschii and Arabidopsis showed that the exon-intron structures are essentially conserved. Separate and distinct genes for the individual starch synthases therefore existed before the separation of monocotyledons and dicotyledons.