Studies on the expression of resistance to stem rust of wheat caused by Puccinia graminis f.sp. tritici.

Abstract

The endogenous cytokinin levels of healthy primary leaves and seeds of a stem-rust susceptible wheat cultivar Little Club were compared with those of Little Club containing the stem rust resistance gene Sr25. Use was made of paper, column and high performance liquid chromatography techniques to separate the endogenous cytokinins in the plant material, and the soybean callus bioassay was used to test for cytokinin-like activity of the chromatography fractions. Leaf material of the resistant Little Club Sr25 had a higher level of total cytokinin activity than Little Club, whereas seed material of Little Club Sr25 did not always have higher levels of cytokinins than Little Club. A number of cultivars would have to be tested before the usefulness of cytokinin levels as an indicator of resistance could be determined. The development of urediospore-derived infection structures of Puccinia graminis f.sp. tritici in wheat, barley, sorghum and maize was examined by scanning electron microscopy (SEM). Infection on and in the four species followed a similar pattern up to, and including, primary infection hyphae formation. In wheat, barley and maize, when a primary infection hypha abutted onto a host epidermal cell, a septum was laid down delimiting a primary haustorial mother cell (HMC); primary HMCs did not form in sorghum. Secondary infection hyphae arose on the substomatal vesicle side of the primary HMC septum; infection did not progress further in maize, but in wheat and barley secondary infection hyphae branched, and proliferated intercellularly forming the fungal thallus. Secondary HMCs were delimited when an intercellular hypha abutted onto host cells. In all four species atypical infection structures were also observed. In an attempt to determine the timing and expression of stem rust resistance gene Sr5, infection structure development of Puccinia graminis f.sp. tritici race 2SA2 in a resistant line (ISr5Ra) and a susceptible line (ISr8Ra) was compared quantitatively using a fluorescence microscopy technique. The results indicated that there were no significant differences in numbers of specific infection structures observed in the two near-isogenic lines up to, and including, 48 hpi, by which time race 2SA2 had successfully formed secondary HMCs in both lines.