Role of an endogenous nitric oxide burst in the resistance of wheat to stripe rust
The stripe rust (or yellow rust) disease caused by Puccinia striiformis Westend is a serious disease of wheat in many areas of the world. The role of NO, which is an important redox-active signalling molecule in plants, was investigated in the wheat-stripe rust system. The phenotypes from interactions of the same wheat variety, Lovrin10, with two different clones of stripe rust strains (P. striiformis Westend), namely China yellow rust (CY)22-2 and CY29-1, which are immune and susceptible reaction types, respectively. The time course of host endogenous NO detected by electron spin resonance indicated that recognition of an avirulent strain was associated with two peaks of NO production. The first peak of NO accumulated in the early infection stage whereas the second peak accumulated in the latent period; however, only a single peak of NO was observed in the latent period for the virulent strain. Furthermore, the activity of pathogen-related protein-phenylalanine ammonia-lyase was higher in the resistant system than in the susceptible system, which suggested that the first NO production was associated with resistance. Exogenous NO improved the activity of phenylalanine ammonia-lyase and induced a resistant response of Lovrin10 to the virulent strain CY29-1, thereby providing further evidence that the first peak of NO production was associated with resistance. These results indicate that the first NO burst in the immune system plays an important role in the resistant reaction of wheat to strip rust.