Role of MoAND1-mediated nuclear positioning in morphogenesis and pathogenicity in the rice blast fungus, Magnaporthe oryzae

To cause disease on host plants, many phytopathogenic fungi undergo morphological transitions including development of reproductive structures as well as specialized infection structures called appressoria. Such morphological transitions display distinct nuclear dynamics. Here we report the developmental requirement of MoAND1-mediated nuclear positioning for pathogenesis of the rice blast fungus, Magnaporthe oryzae. The MoAND1 gene encodes a protein that shows high similarity to Num1 in Saccharomyces cerevisiae and ApsA in Aspergillus nidulans, both of which are cell cortex proteins involved in nuclear migration and positioning. Targeted deletion of MoAND1 did not affect radial growth of the fungus but impaired nuclear distribution along the hyphae, which is reminiscent of ApsA mutant. In contrast to the wild-type, which produces three to five spores in a sympodial manner on the conidiophore, only a single spore was borne on the conidiophore of ΔMoand1, resulting in ∼65% decrease in conidia production, compared to the wild-type. The mutant conidia displayed abnormalities in septation pattern and nuclear distribution, which were correlated with their inability to germinate. Spores of the mutant that did germinate were capable of differentiating appressoria but were defective in the execution of programmed nuclear migration and positioning during development. Furthermore, mutant appressoria were not fully functional, leading to delay in penetration of host plants. However, the ability of ΔMoand1 to grow inside host tissues was comparable to that of the wild-type. All these defects greatly decreased the virulence of the mutant. Taken together, our data suggest that there is a stringent but incomplete developmental requirement for proper migration and positioning of fungal nuclei mediated by MoAND1 during asexual reproduction and pre-penetration phase of fungal pathogenesis.