'Overgrowth' mutants in barley and wheat: new alleles and phenotypes of the 'Green Revolution' DELLA gene

Abstract

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.

Fig. 1.

Screening for overgrowth mutants in barley. One tall seedling among dwarf siblings. The pale yellow seedling (front left) is an example of the 2–3% of non-green seedlings that are seen in barley M₂ populations.

Fig. 2.

Distribution of LER_max values of individual F₂ seedlings from different crosses. Values for parents (mean ±standard error) were WT, 35.2±1; Sln1d, 7.6±0.4; Sln1d.5, 13.7±0.4; Sln1d, spy1a, 17.5±0.4; TR103, 16.6±0.6.

Fig. 3.

Relationship between leaf elongation rate and α-amylase production for each overgrowth line. The data from Tables 2 and 3 were plotted as percentages of the GA₃-treated Himalaya control, with the α-amylase data being a mean of the 42 and 72h values. Data indicated with circles, squares, diamonds, and triangles refer, respectively, to Sln1d and its overgrowth derivatives, grd2b and its overgrowth derivatives, gse1n and its overgrowth derivative, and WT and its overgrowth derivatives.

Fig. 4.

GA₃ dose–response curves for growth of the first leaf of barley. Each graph contains four curves, representing the WT (same curve for each panel), the original dwarf line, the overgrowth derivative of the dwarf, and the overgrowth allele in a WT background.

Fig. 5.

Wheat overgrowth mutants. Overgrowth mutants and control isolines were grown to maturity in the greenhouse. Left to right: Maringá Rht-B1c, Maringá Rht-B1a, and three overgrowth mutants that differed in height.

Fig. 6.

Sites of amino acid substitutions in the DELLA C-terminal GRAS domain for overgrowth mutants of barley (upper arrows) and wheat (lower arrows). Conserved amino acid motifs are indicated.