Histopathological characterization of R-gene mediated resistance to stripe rust in wheat
Wheat yellow (stripe) rust is a recurrent problem throughout the world, and resistant varieties are an efficient means of managing the disease. Therefore, characterization of diverse sources of resistance is of prime importance for wheat breeding. The objective of the study was to investigate variation in host response in incompatible interactions conferred by different R-genes. Epifluorescence and confocal microscopic methods were utilized for histopathological investigation of six yellow rust R-genes (Yr1, Yr5, Yr6, Yr15, Yr17 and Yr27) in Avocet S background, with Avocet S as the control. Fungal colony size and area of hypersensitive response (HR) were assessed for each interaction at 4, 8 and 16 days post inoculation (dpi). The pattern for Avocet Yr15 was distinct, because HR arrested the pathogen very early and rapidly restricted pathogen growth. Avocet Yr1 and Avocet Yr5 showed a less rapid HR and restriction of pathogen growth, but most colonies were completely surrounded by HR at 8 and 16 dpi. In Avocet Yr6 the size of colonies and the extent of HR were highly variable with continuous change up to 16 dpi. More extensive pathogen growth was observed in Avocet Yr17 and Avocet Yr27, where HR induction was delayed, resulting in large intermingled colonies at 16 dpi. All interactions were clearly different from the susceptible control. Thus each R-gene produced a different temporal and spatial distribution of fungal colonies and HR response. Colony size distributions and HR response patterns are potential parameters for characterization of host resistances with different modes of action in wheat against Pst. The results also expand our comprehension of host resistance in wheat against P. striiformis.