Title : Exploring novel leaf rust resistance genes in Aegilops speltoides and Ae. variabilis for imparting leaf rust resistance in bread wheat (Triticum aestivum L.)
Wheat (Triticum aestivum L.) is one of the three major cereal crops covering an area of 218 million hectares (mha) area globally producing a total of 771million tonnes (mt) of grains. Biotic stresses are major limiting factors affecting wheat crop from realizing its yield potential. One of the major biotic stresses affecting wheat cultivation globally is brown/leaf rust caused by Puccinia triticina Eriks. Utilization of genetic resistance is the best way to mitigate leaf rust infection. Aegilops species are wonderful germplasm resources that have significantly contributed to wheat breeding as source of resistance genes especially against wheat rusts. Ae. speltoides (SS) belongs to Sitopsis and Ae. variabilis (syn. Ae. peregrina) (UUSS) belongs to Aegilops section of the genus Aegilops. Ae. speltoides is the donor for several leaf rust resistance genes Lr28, Lr35, Lr36, Lr47 and Lr51, while Lr59 was derived from Ae. variabilis. We have identified 133 leaf rust resistant Ae. variabilis derived introgression lines and 43 leaf rust resistant Ae. speltoides derived introgression lines against the most predominant virulent race in India 77-5 (121R63-1= THTTS). Multi-pathotype testing using 22 leaf rust races was undertaken in selected resistant lines viz., four Ae. speltoides derived introgression lines Sel.2427, ET, ASD546-4, ASD753 and two Ae. variabilis derived introgression lines AVD243-4, AVD836. Ten-day-old seedlings were inoculated by spraying a solution of uredospores and incubated for 48 hrs in a humid chamber which were shifted to greenhouse benches under ambient temperature, light and relative humidity conditions. Individual seedlings were scored for leaf rust reaction at 12 days after inoculation. Sel.2427, ET, ASD546-4 and AVD243-4 were found resistant to all the races. While AVD836 showed resistance to all races except 77-8 (253R31=TGTTQ) which is typical of Lr19 gene. Another Ae. speltoides derived line ASD753 showed resistance to all races except 77-10 (377R60-1= MHTTS) which is typical of Lr28 gene. Molecular markers linked to Lr19, Lr28, Lr24 and Lr34 were utilized for confirming the uniqueness and novelty of resistance in all the resistant lines. Selection2427, ET and AVD243-4 which were found to be resistant to all races were also found to be unique from Lr24, the leaf rust resistance gene having no known virulent race in India. In Selection2427, a dominant Lr gene was mapped to the long arm of 3B chromosome. However, presence of gametocidal genes affects utilization of this gene in wheat improvement. Mutagenesis by gamma ray irradiation was exploited to break the association of leaf rust and gametocidal genes and leaf rust resistant mutants with good fertility and seed set have been developed. Genomic dot-blot hybridization and genomic in situ hybridization were utilized for detection of alien genomes in the introgression lines. Novel leaf rust resistance genes were identified in 88 Ae. variabilis and two Ae. speltoides derived lines. These genes will be an asset in wheat resistance breeding. Wild relatives of wheat are a wonderful source of rust resistance genes. Utilization of these germplasm resources will help broaden the genetic variability available in wheat and prevent genetic erosion.