Mapping of durable adult plant stem rust resistance in six CIMMYT wheats to Ug99 group of races
Durable resistance to wheat stem rust fungus can Be achieved by developing and deploying varieties that have race-nonspecific, adult plant resistance (APR) conferred by multiple minor, slow rusting genes. Wheat lines ‘Kingbird, ‘Kiritati’, ‘Huirivis#1’, ‘Juchi’, ‘Muu’ and ‘Pavon 76’ showed high levels of APR to Ug99 races of stem rust fungus when tested in Kenya. The F5 and F6 generation recombinant inbred line (RIL) populations developed from the crosses of moderately susceptible ‘PBW343’ with five resistant parents were used in mapping. The non-Sr26 fraction of the ‘Avocet’ x Pavon 76 RIL population, developed earlier for leaf rust and stripe rust resistance studies, was also included. Field phenotyping of the parents and RILs were conducted at Njoro, Kenya for at least two years with Ug99+Sr24 (TTKST) race under high stem rust pressures. The continuous variation of APR in each RIL population and genetic analyses indicated quantitative nature of resistance that was likely governed by 3 or 4 minor genes. Single and joint year analyses by Inclusive Composite Interval Mapping (ICIM) using informative DArT and/or SSR markers identified consistent APR QTLs on chromosomes 1AL, 3BS, 5BL, 7A and 7DS in Kingbird; 2D, 3BS, 5BL and 7DS in Kiritati; 2B, 3BS, 4A, 5BL and 6B in Juchi; 2B, 3BS, 7B in Huirivis#1; 2B, 3BS and 5BL in Muu; and 1BL, 3BS, 5A and 6B in Pavon 76. QTLs on each genomic regions explained 10- 46% of the phenotypic variation for APR. Pseudo-black chaff phenotype associated with APR gene Sr2 on chromosome 3BS in all six resistant parents and identification of an APR QTL in the same region in all mapping populations confirmed the role of Sr2 in reducing stem rust severity. The 1BL QTL in Pavon 76 was in the same region where pleiotropic APR gene Lr46/Yr29/Pm39 is located. Similarly a 7DS QTL in Kingbird and Huirivis#1 was in the chromosomal region where pleiotropic APR gene Lr34/Yr18/Pm38 is located. These results indicate that the above two pleiotropic resistance genes confer APR to stem rust in addition to leaf rust, yellow rust and powdery mildew. Further studies are underway to saturate the genomic regions harboring new APR QTLs with additional molecular markers.