Abiotic Stress and Agronomic Traits. Dwarfing Genes
Many of the cultivars available at the beginning of the 'Green Revolution' responded to the higher inputs of fertilizers by developing taller plants, prone to lodging and devoting a large proportion of assimilates to increment biomass, instead of grain production. In addition, the stem of taller plants could not support the grain weight obtained under the newly introduced culture conditions. The interest in breeding for shorter plants have actually begun before the onset of the Green Revolution, mainly to improve lodging tolerance.
Most of the dwarf and semi-dwarf wheat lines from Europe have the Japanese variety Akakomugi among their ancestors. This variety was the donor of the reduced height gene Rht8 and the daylight-insensitive gene Ppd-D1, both genes are closely linked and located on chromosome 2D. Borojevic et al. (1) provide an interesting historical account of the transfer of Rht8 from Japan to Western wheat lines. Genes Rht-B1 and Rht-D1 also have a Japanese origin, from variety Norin10, they were first transferred to US cultivars, then to CIMMYT lines and later to many other countries (2).
Dwarfing genes are classified according to their sensitivity to externally applied gibberellins (GA). Genes Rht-B1 and Rht-D1 are GA-insensitives genes, the alleles conferring the dwarf phenotype are Rht-B1b and Rht-D1b and the wild type alleles are Rht-B1a and Rht-D1a, while Rht8 is a GA-sensitive gene.
The use of GA-insensitive dwarfing genes is not recommended in environments that negatively affect seedling emergence, because those genes tend to restrict coleoptile elongation. Examples of these environments are low precipitation dryland regions, like some Mediterranean countries or areas of the US Great Plains and Pacific Northwest (3, 4).
Butler et al. (5) evaluated the agronomic performance of Rht-B1 and Rht-D1 genes in a series of 140 recombinant inbred lines (RILs) derived from a cross Kauz x MTRWA116 in four environments in Colorado under irrigated, partially irrigated and rainfed for two years. The highest grain yield was obtained in a irrigated location with the semidwarf combination Rht-B1b + Rht-D1a. Lines carrying both dwarfing alleles rendered lower grain yields in every environment. For stressed environments the best choice seemed to be related more with the right plant height than the combination of alleles, the best results were observed with shorter lines within the tall class, without any dwarfing allele, or taller lines within the semidwarf class carrying Rht-B1b + Rht-D1a.
Besides the major effects of Rht genes, there are QTLs which also affect lodging resistance. Verma et al (6) characterized 96 double haploid lines derived from a Milan (Rht-D1b) x Catbird (Rht-B1b) cross. They found QTLs for lodging and related parameters on chromosomes 1B, 1D, 2B, 2D, 4B, 4D, 6D and 7D,
The presence of the dwarfing alleles Rht-B1b and Rht-D1b can be determined by testing the sensitivity of the plants to applied GA, but this is a cumbersome method and sometimes difficult to evaluate. Rht8 is even more difficult to detect phenotypically, since its response is not related to GA signalling. The use of molecular markers greatly facilitates breeding for these characters. Ellis et al. (7) developed markers for Rht-B1 and Rht-D1. Korzun et al. (8), Worland et al (9, 10) and Ahmad and Sorrells (3) described the association of a 192bp allele of the microsatellite locus Xgwm261 with Rht8. However, in some cultivars derived from Norin 10 this allele cannot be used as a diagnostic toll for Rht8 (11). A set of KASP markers was developed in 2013. The information on the primers needed is also detailed in the methods section, along with the expected polymorphisms.
1. The transfer and history of 'reduced height genes' (Rht) in wheat from Japan to Europe. Borojevic K, Borojevic K. In: Journal of Heredity, 2005, 96:455-459. [abstract]
2. The genes of the Green Revolution. Hedden, P. In: Trends in Genetics, 2003, 19:5-9. [abstract]
3. Distribution of microsatellite alleles linked to Rht8 dwarfing gene in wheat. Ahmad M, Sorrells ME. In: Euphytica, 2002, 123:235-240. [abstract]
4. Covariation for microsatellite marker alleles associated with Rht8 and coleoptile length in winter wheat. Bai G, Das MK, Carver BF, Xu X, Krenzer EG. In: Crop Science, 2004, 44:1187-1194.[abstract]
5. Agronomic performance of Rht alleles in a spring wheat population across a range of moisture levels. Butler JD, Byrne PF, Mohammadi V, Chapman PL, Haley SD. In: Crop Science, 2005, 45:939-947.[abstract]
6. Identification and characterization of quantitative trait loci related to lodging resistance and associated traits in bread wheat. Verma V, Worland AJ, Sayers EJ, Fish L, Caligari PDS, Snape JW. In: Plant Breeding, 2005, 124:234-241.[abstract]
7. 'Perfect' markers for the Rht-B1b and Rht-D1b dwarfing genes in wheat. Ellis MH, Spielmeyer W, Gale KR, Rebetzke GJ, Richards RA. In: Theoretical and Applied Genetics, 2002, 105:1038-1042. [abstract]
8. Genetic analysis of the dwarfing gene (Rht8) in wheat. Part I. Molecular mapping of Rht8 on the short arm of chromosome 2D of bread wheat (Triticum aestivum L.). Korzun V, Roder MS, Ganal MW, Worland AJ, Law CN. In: Theoretical and Applied Genetics, 1998, 96:1104-1109.[abstract]
9. Genetic analysis of the dwarfing gene Rht8 in wheat. Part II. The distribution and adaptive significance of allelic variants at the Rht8 locus of wheat as revealed by microsatellite screening . Worland AJ, Korzun V, Röder MS, Ganal MW, Law CN. In: Theoretical and Applied Genetics, 1998, 96:1110-1120.[abstract]
10. Allelic variation at the dwarfing gene Rht8 locus and its significance in international breeding programmes. Worland AJ, Sayers EJ, Korzun V. In: Euphytica, 2001, 119:157-161.[abstract]
11. A 192bp allele at the Xgwm261 locus is not always associated with the Rht8 dwarfing gene in wheat (Triticum aestivum L.). Ellis MH, Bonnet DG, Rebetzke GJ. In: Euphytica, 2007, 157:209–214. DOI:10.1007/s10681-007-9413-7. [abstract]