Rht18 Semidwarfism in Wheat Is Due to Increased GA 2-oxidaseA9 Expression and Reduced GA Content

Semi-dwarfing genes have contributed to major yield increases in crop species by reducing height, improving lodging resistance, and partitioning more assimilates to grain growth. In wheat, the Rht18 semi-dwarfing gene was identified and deployed in durum wheat before it was transferred into bread wheat where it was shown to have agronomic potential. Rht18, a dominant and gibberellin (GA) responsive mutant, is genetically and functionally distinct from the widely used GA insensitive semi-dwarfing genes Rht-B1b and Rht-D1b. In this study, the Rht18 gene was identified by mutagenizing the semi-dwarf durum cultivar Icaro (Rht18) and generating mutants with a range of tall phenotypes. Isolating and sequencing chromosome 6A of these 9overgrowth9 mutants showed that they contained independent mutations in the coding region of GA2oxA9, a gene predicted to encode a GA 2-oxidase that metabolises GA biosynthetic intermediates into inactive products, thereby lowering the amount of bioactive GA (GA1). Functional analysis of the GA2oxA9 protein demonstrated conversion of the intermediate GA12 to the inactive metabolite GA110. Analysis of transcript abundance and GA content showed that the expression of GA 2-oxA9 was higher, and the GA content lower, in Rht18 compared with its tall parent. These data indicate that the increased expression of GA2oxA9 in Rht18 results in a reduction of both bioactive GA content and plant height. This study describes a height reducing mechanism that can generate new genetic diversity for semi-dwarfism in wheat by combining increased expression with mutations of specific amino acid residues in the GA2oxA9 gene.