A suppressor screen using dwarf mutants of barley (Hordeum vulgare L.) led to the isolation of ‘overgrowth’ derivatives, which retained the original dwarfing gene but grew at a faster rate because of a new mutation. The new mutations were in the Slender1 (Sln1) gene (11/13 cases), which encodes the DELLA protein central to gibberellin (GA) signalling, showed 100% genetic linkage to Sln1 (1/13), or were in the Spindly1 (Spy1) gene (1/13), which encodes another protein involved in GA signalling. The overgrowth mutants were characterized by increased GA signalling, although the extent still depended on the background GA biosynthesis capacity, GA receptor function, and DELLA activity. A comparison between two GA responses, α-amylase production and leaf growth rate, revealed degrees of specificity for both the overgrowth allele and the GA response under consideration. Many overgrowth mutants were also isolated in a dwarf line of bread wheat (Triticum aestivum L.) and 19 new alleles were identified in the Rht-B1 gene, one of the ‘Green Revolution’ semi-dwarfing genes and the orthologue of Sln1. The sites of amino acid substitutions in the DELLA proteins of both species provide insight into DELLA function, and included examples where identical but independent substitutions were observed. In both species, the starting lines were too dwarfed to be directly useful in breeding programmes, but new overgrowth derivatives with semidwarf heights have now been characterized. The variation they exhibit in GA-influenced traits identifies novel alleles with perfect markers that are of potential use in breeding.

The sequence of Gid1 (a gene for a gibberellin (GA) receptor from rice) was used to identify a putative orthologue from barley. This was expressed in E. coli, and produced a protein that was able to bind GA in vitro with both structural specificity and saturability. Its potential role in GA responses was investigated using barley mutants with reduced GA sensitivity (gse1 mutants). Sixteen different gse1 mutants each carried a unique nucleotide substitution in this sequence. In all but one case, these changes resulted in single amino acid substitutions, and, for the remaining mutant, a substitution in the 5' untranslated region of the mRNA is proposed to interfere with translation initiation. There was perfect linkage in segregating populations between new mutant alleles and the gse1 phenotype, leading to the conclusion that the putative GID1 GA receptor sequence in barley corresponds to the Gse1 locus. Determination of endogenous GA contents in one of the mutants revealed enhanced accumulation of bioactive GA(1), and a deficit of C(20) GA precursors. All of the gse1 mutants had reduced sensitivity to exogenous GA(3), and to AC94377 (a GA analogue) at concentrations that are normally 'saturating', but, at much higher concentrations, there was often a considerable response. The comparison between barley and rice mutants reveals interesting differences between these two cereal species in GA hormonal physiology.

A suppressor screen using the dwarf Rht-B1c Della mutant of wheat (Triticum aestivum L.) led to the isolation of overgrowth mutants, which retained the original dwarfing gene but grew at a faster rate because of a new mutation elsewhere in that gene. Forty-six alleles were identified, which included amino acid substitutions, premature stop codons, and splice site alterations. The sites of amino acid substitution were primarily localised around conserved motifs in the DELLA protein, and these mutants showed a wide range in their extent of growth recovery (dwarf, semidwarf, tall). Detailed growth comparisons were made on a wide height range of backcrossed overgrowth alleles, comparing stem and spike growth, leaf size, tillering, phenological development, coleoptile length, grain dormancy and grain yield. There were large and reproducible differences between alleles for some traits, whereas others were largely unaffected or varied with growth conditions. Some of the overgrowth alleles offer promise as alternatives to the Rht-B1b and Rht-D1b dwarfing genes, allowing a wider range of height control, improved grain dormancy and equivalent grain yield. The collection of mutants will also be valuable as a resource to study the effect of height on different physiological or agronomic traits, and in elucidating DELLA protein function.