Common wheat cultivars have been protected from stem rust for several decades worldwide by the Sr31 resistance gene transferred from Secale cereale L. (cv. Petkus). In 1998, Sr31 was overcome in Uganda by the Ug99 race of Puccinia graminis f. sp. tritici Eriks. & Henn. (Pgt). The Ug99 race and its derivatives have spread widely in Africa, neighboring regions and Europe. However, Sr31 remains effective in other areas of the world, including Russia. To breed wheat with durable resistance, it is promising to research the resistance mechanisms of nonhost species and introgressive cultivars. The aim of the research was to estimate the resistance of S. cereale and Triticum aestivum cultivars with Sr31 to stem rust and to study the mechanisms of incompatibility of Pgt with plants at the cellular level. The research was carried out in Western Siberia (Russia, Omsk region) in 2018–2022. Rye and wheat with Sr31 (cvs. Kavkaz, Seri 82, Bacanora (=Kauz’s’), NIL Thatcher TcLr26/Sr31) were resistant at the stages of seedling and adult plant, and cv. PWB343 was more susceptible to disease. Cytological studies have shown that Pgt died on the rye plants on the surface, and cv. Petkus intensively suppressed the development of the appressoria necessary to penetrate into tissues. Wheat cultivars inhibited the Pgt development mainly on the surface and while it attempted to penetrate into the stomata (pre-haustorial resistance). It has been demonstrated that Pgt has to adapt step-by-step to the surface and tissue properties for compatible interaction, which may be the reason for the durable effectiveness of Sr31.

The damage to wheat crops by stem rust poses a threat to the food security of the world’s population. The species Thinopyrum ponticum (Podpěra, 1902) (Z.-W. Liu and R.-C. Wang, 1993) is a non-host for the stem rust fungus Puccinia graminis f. sp. tritici Eriks. and Henn. (Pgt). The Sr24, Sr25, and Sr26 genes, transferred from the Th. ponticum to the wheat gene pool, protect cultivars from the disease in different regions of the world. The study of the non-host resistance (NHR) of Th. ponticum and the effects of the introgressed Sr24, Sr25, and Sr26 genes in wheat is important for breeding cultivars with durable resistance to stem rust. The aim of the research is to study the interaction of Pgt with Th. ponticum and common wheat lines with the Sr24, Sr25, and Sr26 genes, in addition to determining the role of ROS in resistance. Wheat lines with Sr24, Sr25, and Sr26 were resistant to the West Siberian Pgt population. Using cytological methods, it was found that the NHR of Th. ponticum and Sr24, Sr25, and Sr26 led to inhibition of the most inoculumdevelopment on the plant surface. This was mainly due to the suppression of the appressoria development and their death at the stage of penetration into the stomata. Upon contact of Pgt appressoria with stomatal guard cells, the generation of the superoxide anion O2•− was revealed. This interaction is similar to the stomatal immunity of Arabidopsis thaliana to non-pathogenic bacteria. The results of our studies show that the Sr24, Sr25, and Sr26 genes reproduce the action of some NHR mechanisms in wheat.

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