CSIRO PUBLISHING www.publish.csiro.au/journals/fpb Functional Plant Biology, 2004, 31, 583–589 The effect of different height reducing genes on the early growth of wheat Marc H. Ellis A,B,C , Greg J. Rebetzke A , Peter Chandler A , David Bonnett A,B , Wolfgang Spielmeyer A,B and Richard A. Richards A A CSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia. B Graingene, 65 Canberra Avenue, Griffith, ACT 2603, Australia. C Corresponding author; email: marc.ellis@csiro.au Abstract. Genes that reduce height without compromising seedling vigour or coleoptile length have great potential for wheat improvement. We therefore investigated the effects of various reduced height (Rht) genes on the early stages of plant development, using a combination of near isogenic, recombinant, mutant and wild type comparisons. Gibberellin (GA) insensitivity caused by Rht-B1b or Rht-D1b was associated with reduced leaf elongation rate and coleoptile length. Similar results were found for two other sources of dwarfing, Rht11 and Rht17. We found one class of Rht genes (e.g. Rht8) which had no effect on coleoptile length, leaf elongation rate or responsiveness to GA, indicating that these dwarfing genes may act later in wheat development to reduce height and increase harvest index, without affecting early growth. A third class of Rht genes was found in three durum backgrounds. These had reduced coleoptile lengths and leaf elongation rates, but had a greater response to GA than the corresponding tall varieties. We discuss these results in relation to the possible mechanisms underlying the reduction in height and the suitability of the different Rht genes for wheat improvement. Keywords: coleoptile length, dwarfing genes, GA response, gibberellins. Introduction Reduced height of cereals is associated with increased yield potential due to a greater partitioning of assimilates to the grain (i.e. harvest index) and a lowering of the centre of gravity of the plant to reduce the risk of lodging (Evans, 1993). In wheat, the most common dwarfing genes are Rht-B1b (formerly Rht1) and Rht-D1b (formerly Rht2). These are effective in reducing height and have been widely adopted in wheat breeding programmes since their introduction in the 1960s (Gale et al. 1985; Bonnett et al. 2001). The mechanisms by which Rht-B1b and Rht-D1b effect a reduction in height are relatively well understood. These genes are homoeoalleles encoding negative regulators of the response to GA growth hormones (Peng et al. 1999). In plants with the Rht-B1b or Rht-D1b mutations, the GA response is no longer triggered by the hormone signal, and plants are reduced in growth and are insensitive to the application of GA. Because these genes affect GA-signalling, and GAs are involved in many developmental processes, Rht-B1b and Rht-D1b have a range of effects on the plant in addition to reducing plant height (see Gale et al. 1985 for review). In Abbreviations used: DH, doubled haploid; GA, gibberellin; LER, leaf elongation rate; LER max , maximal leaf elongation rate; NILs, near isogenic lines; Rht, reduced height; RILs, recombinant inbred lines. particular, Rht-B1b and Rht-D1b are associated with shorter coleoptiles and reduced early seedling vigour (Rebetzke et al. 2001). These side-effects are undesirable under certain conditions (Allan 1989; Richards 1992). For instance, longer coleoptiles may be required if seeds are sown deep when the surface layers of the soil are dry. Early vigour confers greater competitiveness with weeds and increases leaf area development to shade the soil and reduce surface evaporation. For these reasons, Rebetzke and Richards (2000) suggested that alternative dwarfing genes may be more suitable to reduce final plant height without compromising early plant growth. One suitable alternative to Rht-B1b and Rht-D1b is Rht8. This gene has been used in wheat breeding since the 1920s, when it was introduced into Italian wheats from the Japanese variety, Akagomugi, by the Italian wheat breeder N. Strampelli (Lorenzetti 2000). Rht8 is present in many of the Strampelli wheat varieties, which have been extensively used in southern Europe. In studies of both near-isogenic and recombinant inbred lines, Rht8 had no effect on either coleoptile length or seedling vigour (Rebetzke et al. 1999; Rebetzke and Richards 2000). For these reasons, © CSIRO 2004 10.1071/FP03207 1445-4408/04/060583