Novel insights into host specificity of Pyricularia oryzae and Pyricularia grisea in the infection of gramineous plant roots.

Pyricularia oryzae and Pyricularia grisea are pathogens that cause blast disease in various monocots. It has been reported that P. oryzae infects the leaves and roots of rice via different mechanisms. However, it is unclear to what extent the tissue types affect the host specificities of P. oryzae and P. grisea. Here, we evaluated the tissue-specific infection strategies of P. oryzae and P. grisea in various gramineous plants. Generally, mycelial plug inoculation caused root browning but the degree of browning did not simply follow the disease index on leaves. Interestingly, the Triticum and Digitaria pathotypes caused strong root growth inhibition in rice, wheat, and barley. Moreover, the Digitaria pathotype inhibited root branching only in rice. Culture filtrate reproduced these inhibitory effects on root, suggesting that some secreted molecules are responsible for the inhibitions. Observation of root sections revealed that most of the infection hyphae penetrated intercellular spaces and further extended into root cells, regardless of pathotype and host plant. The infection hyphae of Digitaria and Triticum pathotypes tended to localize in the outer layer of rice roots, but not in those of wheat and barley roots. The infection hyphae of the Oryza pathotype were distributed in both the intercellular and intracellular spaces of rice root cells. Pathogenesis-related genes and reactive oxygen species accumulation were induced after root inoculation with all combinations. These results suggest that resistance reactions were induced in the roots of gramineous plants against the infection with Pyricularia isolates but failed to prevent fungal invasion.