'GREEN REVOLUTION' CEREAL GENES ARE HOMOLOGUES OF Arabidopsis GAI

John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, Norfolk UK

Recent decades have seen large increases in world grain yields. This so called 'Green Revolution' is in part due to the widespread adoption of varieties containing the semi-dwarfing Rht mutant alleles, and of associated changes in agricultural practise. These yield increases have enabled wheat productivity to keep pace with the demands of rising world population. Like Rht mutant alleles in wheat, the gai mutant allele of Arabidopsis confers a semi-dominant dwarf phenotype, and a dramatic reduction in responsiveness to the plant growth hormone gibberellin (GA). The Arabidopsis GAI and RGA genes have been cloned. These genes encode closely related proteins (GAI and RGA) that are members of a class of putative transcriptional co-activators defined by the Arabidopsis SCR product. Recently, we have found that the DNA sequences of Rht (and D8, the maize orthologue) are closely related to those of GAI/RGA. Analysis of mutant D8 and Rht alleles suggests that the wild-type proteins (GAI, RGA, D8, Rht) are bi-partite in structure, with an N-terminal GA specificity domain, and a C-terminal putative transcriptional co-activator domain. These proteins appear to act as repressors of stem elongation, whose activity is opposed to GA. The mutant proteins are altered in ways that make growth repression relatively resistant to the effects of GA. The possible biotechnological significance of these findings will be discussed.