Introgression of useful linked genes for resistance to stem rust, leaf rust and powdery mildew and their molecular validation in wheat (Triticum aestivum L.)

Abstract

Two independent DNA segments with two linked genes each namely, Lr19/Sr25 and Sr36/Pm6 were introgressed by three cycles of backcrossing into fifteen bread wheat (Triticum aestivum L.) cultivars, susceptible to leaf rust (Lr), stem rust (Sr) and powdery mildew (Pm). Evaluation of the BC3-F5 along with their recurrent parents and controls carrying specific Lr and Sr genes under natural and artificial conditions indicated that conventionally improved newly constituted lines confer resistance to leaf rust, stem rust and powdery mildew in adult stage against the pathotypes prevailing in the Nilgiris. Some of these lines also displayed resistance to leaf rust pathotype 77-8 (253R 31) in adult stage. However, the improved lines, viz., C306*3/Cook*6/C80-1, HD2687*3/Cook*6/C80-1, MACS2496*3/ Cook*6/C80-1, PBW226*3/Cook*6/C80-1, UP2338*3/Cook*6/ C80-1 and UP262*3/Cook*6/C80-1 showed moderate susceptibility to 77-8. The leaf rust pathotype 77-8 was found avirulent on Lr26. The resistance in UP262 can be ascribed to Lr23 and some unknown gene(s) possessed by it. Since Lr19 is effective to all the pathotypes of leaf rust in India except 77-8, the differential response exhibited by BC3-F5 lines against a mixture of prevailing races at Wellington and against a new pathotype 77-8 at New Delhi indicated unambiguously the presence of Lr19 in the introgressed lines. High degree of resistance to stem rust and powdery mildew in all the BC3-F5 lines evidenced that these carry the genes Sr36 and Pm6. The conventional methodology proved successful in phenotype-based selection of resistance gene combinations in the absence of molecular markers. To ensure the correct identity of target genes, the BC3-F5 lines (2 to 9) were tested with molecular markers SCS265, SCS253 and Gb linked to leaf rust resistance gene Lr19 and SSR stm773 for stem rust resistance gene Sr36. The presence of genetically associated genes was verified by the amplification of translocated DNA segment in most of the backcross lines. With the detection and testing with a new Lr19 virulence, host-pathogen interaction test response against leaf rust and the molecular test response did not correspond in HW2004*3/Cook*6/C80-1 because of presence of an additional segment of Lr24/Sr24. The availability of a combination of major genetically diverse resistance genes in adapted wheat cultivars would facilitate the strategic deployment to achieve enhanced durable resistance. Also these lines could be used as donor and supplement the molecular mapping and tagging of genes.