Quantitative trait loci on chromosome 4B for coleoptile length and early vigour in wheat (Triticum aestivum L.)

The Norin-10 dwarfing genes, Rht-B1b (Rht1) and Rht-D1b (Rht2), are commonl y used to reduce plant height and increase grain yield in wheat breeding pr ograms. These dwarfing genes lower sensitivity of vegetative tissue to endo genous gibberellin to reduce cell and subsequent stem elongation. This redu ction in cell elongation capacity reportedly results in a concomitant reduc tion in coleoptile length and early vigour (leaf area) thereby affecting se edling establishment and growth. A detailed genetic map from a cross betwee n tall Halberd (Rht-B1a) and semidwarf Cranbrook (Rht-B1b) wheat cultivars was used to assess genetic factors affecting seedling growth. Parental and 150 doubled haploid progeny lines were characterised for seedling and heigh t-related traits in controlled and field environments. Genotypic variation was large and predominantly under additive genetic control with evidence fo r transgressive segregation for some traits. Narrow-sense heritabilities we re moderate to high (h(2)=0.31-0.91) indicating a strong genetic basis for differences between progeny. Molecular marker analyses identified a number of significant (P<0.05) quantitative trait loci (QTL) for each trait. A maj or QTL, mapping directly to the Rht-B1 locus on chromosome arm 4BS, account ed for up to 49% of the genotypic variance in peduncle length and plant hei ght, and 27-45% of the genotypic variance in coleoptile length across diffe rent temperatures. Another QTL, located close to the RFLP marker XksuC2 on the long arm of chromosome 4B, accounted for 15-27% of the genotypic varian ce in coleoptile length. The influence of the XksuC2-linked QTL on coleopti le length was greatest at 19<degrees>C and decreased with cooler temperatur es. The same QTL affected reductions in leaf size, and both coleoptile till er size and presence to affect overall seedling vigour. There was also some evidence for epistatic interactions influencing coleoptile tiller growth. Reductions in plant size at the Rht-B1b and XksuC2 loci were associated wit h presence of the Cranbrook 4B allele. The negative genetic effect of the R ht-B1b dwarfing gene on early growth of wheat confirms phenotypic evidence of a pleiotropic effect of Rht-B1b on establishment and early vigour. Genet ic increases in coleoptile length and early leaf area development are likel y to be limited in wheat populations containing the Rht-B1b dwarfing gene.