Abstract
Like other eukaryotes, fungi use MAP kinase (MAPK) pathways to mediate cellular changes responding to external stimuli. In the past two decades, three well-conserved MAP kinase pathways have been characterized in various plant pathogenic fungi for regulating responses and adaptations to a variety of biotic and abiotic stresses encountered during plant infection or survival in nature. The invasive growth (IG) pathway is homologous to the yeast pheromone response and filamentation pathways. In plant pathogens, the IG pathway often is essential for pathogenesis by regulating infection-related morphogenesis, such as appressorium formation, penetration, and invasive growth. The cell wall integrity (CWI) pathway also is important for plant infection although the infection processes it regulates vary among fungal pathogens. Besides its universal function in cell wall integrity, it often plays a minor role in responses to oxidative and cell wall stresses. Both the IG and CWI pathways are involved in regulating known virulence factors as well as effector genes during plant infection and mediating defenses against mycoviruses, bacteria, and other fungi. In contrast, the high osmolarity growth (HOG) pathway is dispensable for virulence in some fungi although it is essential for plant infection in others. It regulates osmoregulation in hyphae and is dispensable for appressorium turgor generation. The HOG pathway also plays a major role for responding to oxidative, heat, and other environmental stresses and is overstimulated by phenylpyrrole fungicides. Moreover, these three MAPK pathways crosstalk and coordinately regulate responses to various biotic and abiotic stresses. The IG and CWI pathways, particularly the latter, also are involved in responding to abiotic stresses to various degrees in different fungal pathogens, and the HOG pathway also plays a role in interactions with other microbes or fungi. Furthermore, some infection processes or stress responses are co-regulated by MAPK pathways with cAMP or Ca2+/CaM signaling. Overall, functions of individual MAP kinase pathways in pathogenesis and stress responses have been well characterized in a number of fungal pathogens, showing the conserved genetic elements with diverged functions, likely by rewiring transcriptional regulatory networks. In the near future, applications of genomics and proteomics approaches will likely lead to better understanding of crosstalk among the MAPKs and with other signaling pathways as well as roles of MAPKs in defense against other microbes (biotic interactions).
Highlights
Plant pathogenic fungi are exposed to a variety of abiotic and biotic stresses in their natural habitats as well as during infectious growth, such as hyperosmolarity, extreme pH or temperature, mycoparasitic organisms, oxidative burst, and phytoalexins
In the past two decades, mitogen-activated protein (MAP) kinase (MAPK) pathways have been functionally characterized in various plant pathogenic fungi that differ in dispersal, survival, host ranges, and infection mechanisms
Mutants deleted of the Slt2 ortholog and its upstream MEK or MEK kinase (MEKK) are significantly reduced in virulence or are non-pathogenic, indicating a conserved role of the cell wall integrity (CWI) MAPK pathway during plant infection
Summary
Plant pathogenic fungi are exposed to a variety of abiotic and biotic stresses in their natural habitats as well as during infectious growth, such as hyperosmolarity, extreme pH or temperature, mycoparasitic organisms, oxidative burst, and phytoalexins. It is important for responses to oxidative stress through transcription factor Atf1, which may be activated by other MAPKs or protein kinases because the atf1 deletion mutant is defective in plant infection.
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