Histological studies of the expression of the Lr9, Lr20 and Lr28 alleles for resistance to leaf rust in wheat

The development of the leaf rust fungus ( Puccinia recondita f.sp. tritici ) in a susceptible cultivar and three other cultivars possessing the Lr9, Lr20 and Lr28 alleles for resistance was studied by light and fluorescence microscopy. Formation of the substomatal vesicle, intercellular hypha and the first haustorial mother cell was unaffected by resistance. Lr9 and Lr28 expression was rapid, first seen as early initiation of hyphal branching at 16 h after inoculation, then reduced haustorial diameters at 19 h. Limited host cell necrosis was seen immediately afterwards. Elongation of intercellular hyphae was reduced between 20 and 24 h, and virtually ceased by about 30 h. Numbers of infection sites with a second haustorial mother cell were briefly higher at 24 h. Reduced hyphal branching and haustorial mother cell numbers were seen at 20–24 h and 36 h respectively. Lr20 expression was not seen until 36 h when reduced hyphal branching was observed, accompanied by extensive host cell necrosis. Reduced haustorial mother cell numbers were detected at 48 h. Findings suggested a secondary role for host cell necrosis in the expression of the Lr9 and Lr28 alleles. Host necrosis may play a determinant role in Lr20- based expression.