Components of resistance in barley to stem rust: pre- and post-penetration development in seedling and adult plant leaves

Abstract

Seven barley (Hordeum vulgare) cultivars or breeding lines with different levels of resistance to stem rust and the susceptible wheat cultivar Little Club (Triticum aestivum) were inoculated with races QCC, QFC, and TPM of Puccinia graminis f.sp. tritici and examined using light microscopy to associate the sequence of histological events with phenotypic expression of resistance. There were no significant differences in urediniospore germination, appressorium formation, or substomatal penetration in any of the race–cultivar combinations in seedling leaves. Formation of primary haustorium mother cells (HMCs) was not affected by the presence or absence of the resistance gene Rpg1 when inoculated with race QCC, but was reduced considerably in lines possessing this gene after inoculation with races QFC and TPM. Development of all races was arrested during primary HMC formation to a greater extent in all barley lines than in Little Club wheat. The expression of resistance in barley to P. g. tritici was associated with the frequency of post-penetration abortion, the degree of reduction in colony growth, and the incidence of colony-associated necrosis of host cells. The rankings of the host lines based on these histological events were in agreement with the rankings for receptivity and urediniospore production determined previously on stem tissues of adult barley plants. Keywords: barley, stem rust, resistance, components, histology.