Balancing of the mitotic exit network and cell wall integrity signaling governs the development and pathogenicity in Magnaporthe oryzae

Abstract

The fungal cell wall plays an essential role in maintaining cell morphology, transmitting external signals, controlling cell growth, and even virulence. Relaxation and irreversible stretching of the cell wall are the prerequisites of cell division and development, but they also inevitably cause cell wall stress. Both Mitotic Exit Network (MEN) and Cell Wall Integrity (CWI) are signaling pathways that govern cell division and cell stress response, respectively, how these pathways cross talk to govern and coordinate cellular growth, development, and pathogenicity remains not fully understood. We have identified MoSep1, MoDbf2, and MoMob1 as the conserved components of MEN from the rice blast fungus Magnaporthe oryzae. We have found that blocking cell division results in abnormal CWI signaling. In addition, we discovered that MoSep1 targets MoMkk1, a conserved key MAP kinase of the CWI pathway, through protein phosphorylation that promotes CWI signaling. Moreover, we provided evidence demonstrating that MoSep1-dependent MoMkk1 phosphorylation is essential for balancing cell division with CWI that maintains the dynamic stability required for virulence of the blast fungus.

Author summary

The cell wall is a relatively rigid structure for supporting the cell shape and against extracellular stresses. However, it also maintains plasticity to cope with cell division, growth, and differentiation. In the rice blast pathogenic fungus Magnaporthe oryzae, such differentiation corresponds directly to its virulence. Thus, how to balance the “strong for shaping” with the “malleable for growth and virulence” poses as an important question of both basic- and applied science of significance. We here report that the protein kinase MoSep1 links the Mitotic Exit Network (MEN) to the Cell Wall Integrity (CWI) signaling through the phosphorylation of the CWI MAP kinase kinase MoMkk1. We found that the MoSep1-dependent phosphorylation of MoMkk1 relieves the cell wall stress caused by cell division and that the MEN-CWI-mediated balance of rigid and remodeling of the cell wall is important in the growth, development, and virulence of the blast fungus. Our study provides a new evidence on how the blast fungus adapts to self-generated stress for growth and virulence and it sheds new light on the crosstalk between MEN and CWI signaling.