Abstract
Generally, under normal conditions plants are resistant to many of the incompatible pathogens (viral, fungal and bacterial), and this is named “non-host resistance phenomenon”. To understand this phenomenon, different types of food crops (faba bean, squash, barley and wheat) were inoculated with compatible and incompatible pathogens. Strong resistance symptoms were observed in the non-host/incompatible pathogen combinations as compared with host/compatible pathogen combinations, which showed severe infection (susceptibility). Reactive oxygen species (ROS) mostly hydrogen peroxide and superoxide were significantly increased early 24 and 48 h after inoculation (hai) in the non-host plants comparing to the host. Antioxidant enzymes activity (catalase, polyphenol oxidase and peroxidase) were not increased at the same early time 24, 48 hai in the non-host resistant and host resistant plants, however, it increased later at 72 and 168 hai. Electrolyte leakage decreased significantly in non-host resistant and host resistant/pathogen combinations. Catalase and peroxidase genes were significantly expressed in non-host resistant and in host resistant plants as compared to the host susceptible one, which did not show expression using RT-PCR technique. Furthermore, Yr5, Yr18 and Yr26 resistant genes were identified positively using PCR in all treatments either host susceptible or non-host resistant plants in which prove that no clear role of these resistant genes in resistance. Early accumulation of ROS could have a dual roles, first role is preventing the growth or killing the pathogens early in the non-host, second, stimulating the gene appearance of related genes in addition the activition of antioxidant enzymes later on which thereby, neutralize the harmful effect of ROS and consequently suppressing disease symptoms. The new finding from this study supporting the plant breeders with new source of resistance to develop new resistant cultivars and/or stop the breakdown of resistance in resistant cultivars.