Leaf age and time of inoculation contribute to nonhost resistance to Pyricularia oryzae in Arabidopsis thaliana

The outcome of plant-pathogen interactions varies both with developmental stage and time of day. Rice blast caused by Pyricularia oryzae (syn. Magnaporthe oryzae) is a devastating disease of rice. The mechanisms of resistance to P. oryzae have been extensively studied and the rice-P. oryzae pathosystem has become a model system in plant-microbe interaction studies. However, the mechanisms of resistance to P. oryzae in nonhost remain poorly understood. To determine whether leaf age and time of inoculation would affect nonhost resistance (NHR) to P. oryzae in Arabidopsis thaliana, Columbia-0 (Col-0) and penetration2-1 (pen2-1) plants were inoculated with P. oryzae. The rate of entry of P. oryzae into Arabidopsis pen2-1 old leaves was significantly higher than that into young leaves after inoculation at dusk. However, there was no difference in the rates after inoculation at dawn. These results suggest that leaf age and time of inoculation are involved in nonhost resistance to P. oryzae in Arabidopsis.