Cell cycle-mediated regulation of plant infection by the rice blast fungus

To gain entry to plants, many pathogenic fungi develop specialized infection structures called appressoria. Here, we demonstrate that appressorium morphogenesis in the rice blast fungus Magnaporthe oryzae is tightly regulated by the cell cycle. Shortly after a fungus spore lands on the rice (Oryza sativa) leaf surface, a single round of mitosis always occurs in the germ tube. We found that initiation of infection structure development is regulated by a DNA replication-dependent checkpoint. Genetic intervention in DNA synthesis, by conditional mutation of the Never-in-Mitosis 1 gene, prevented germ tubes from developing nascent infection structures. Cellular differentiation of appressoria, however, required entry into mitosis because nimA temperature-sensitive mutants, blocked at mitotic entry, were unable to develop functional appressoria. Arresting the cell cycle after mitotic entry, by conditional inactivation of the Blocked-in-Mitosis 1 gene or expression of stabilized cyclinB-encoding alleles, did not impair appressorium differentiation, but instead prevented these cells from invading plant tissue. When considered together, these data suggest that appressorium-mediated plant infection is coordinated by three distinct cell cycle checkpoints that are necessary for establishment of plant disease.