Late maturity a-amylase (LMA) in wheat is a defect where high-isoelectric point (pI) a-amylase accumulates in the ripening grain. Wheat genotypes vary in expression from zero to high levels of a-amylase, the latter with detrimental consequences on their use for value-added end products. Expression in each genotype is characterised by varying numbers of grains affected and different levels in each grain. Analysis of a doubled haploid (DH) population (188 lines) from
WW1842Whistler has identified significant QTL on chromosomes 2DL, 3A, 3B, 3D, 4B, 4D, 5DS and 5BL. The 4B LMA allele (P < 0.0001) from Whistler is closely linked to the QTL for the ‘tall’ allele (P < 0.0001) of the Rht-B1 gene. The 4D LMA QTL (P < 0.0001) in WW1842 co-locates with the QTL for the ‘tall’ allele (P < 0.0001) of the Rht-D1 gene. This study has shown for the first time that a DH cross between two semi-dwarf cultivars with low or no LMA produces ~25% of progeny lines of the ‘tall’ genotypes with a high frequency of LMA. This is attributed to the large additive positive effects from the combination of one recessive ‘tall’ Rht-B1 gene and one recessive ‘tall’ Rht-D1 gene. Highyielding semi-dwarf genotypes with different combinations of Rht-B1 and Rht-D1 alleles which have very low or nonexistent LMA expression (e.g. WW1842 and Whistler) may meet industry criteria for registration as commercial wheat
varieties. However, when they are used as breeding lines, the cross produces some progeny genotypes with severe levels of LMA. These LMA genotypes comprise the gibberellic acid-sensitive ‘tall’ progenies and a very small proportion of semidwarfs. Thus, it is of paramount importance to screen the defect in wheat breeding programs. The suite of QTL identified for LMA will enable the use of marker assisted selection in the pyramiding of the beneficial QTL to maximise yield and minimise (or eliminate) LMA in semi-dwarf genotypes.