Septation and cytokinesis in fungi

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The control of cytokinesis and septation by pathogenic fungi is critical to the developmental changes associated with host invasion and fungal pathogenesis. Pathogenic processes, such as growth within host tissue, often require changes between yeast-like, determinate, isotropic growth and hyphal, polarised, anisotropic growth, which must be appropriately regulated in concert with nuclear division. In plant pathogenic fungi, a body of evidence, primarily generated from studies of the rice blast fungus M. oryzae, shows that septin-mediated remodelling of the actin cytoskeleton is fundamental to the action of its specialised infection cells, called appressoria, during plant infection. Future studies will need to ask several questions which logically follow these recent discoveries. First of all, is the role of septin GTPases in appressorium repolarisation regulated distinctly from their role in each round of septation within vegetative and invasive hyphae? For example, is there a turgor-sensing mechanism that regulates septin ring formation only once the required turgor has been generated in the infection structure to breach the plant cuticle? Is the regulated synthesis of reactive oxygen species, which is essential for septin assembly in M. oryzae appressoria, a common mechanism by which septin-dependent processes are regulated in fungi? Can this explain why regulated bursts of ROS are often associated with cellular differentiation in fungi, such as fruit body formation and sporulation? Are the processes identified in M. oryzae appressorium development highly conserved among appressorium-forming fungi, such as the rusts, anthracnose-causing fungi and the powdery mildews? If so, can we find evidence for common mechanisms that regulate appressorium-mediated plant infection, such as the conserved MAP kinase cascades (Perez-Nadales et al. 2014), that might be targeted to develop broad spectrum anti-penetrant fungicides? Finally, it is clear that there are important parallels between the way in which yeast budding operates and the operation of fungal appressoria, particularly in the processes leading up to penetration peg formation and repolarisation. Some of the most conserved components probably play similar roles. Clearly, there are important differences as well, not least of all in the enormous invasive forces deployed by appressoria, but the conserved components involved in some of these processes can provide an important roadmap to test hypotheses and define the fundamental mechanisms by which infection structures work in plant pathogenic fungi.