Dynamics of the actin cytoskeleton in hyphae and infection structures of the oomycete pathogen Phytophthora infestans

Phytophthora infestans is the causal agent of late blight, a notorious disease on potato and tomato that leads to huge crop losses worldwide. The Phytophthora genus belongs to the oomycetes, a group of diverse organisms that similar to fungi, grow as mycelium and produce spores to propagate. Oomycetes are best known as plant pathogens but the group also comprises animal and microbial pathogens as well as saprophytes. As such oomycetes occupy similar ecological niches as fungi. However, during evolution the two groups evolved independently and this is reflected in differences in for example mating systems, cell wall composition and biochemical and metabolic pathways. In our aim to pinpoint potential drug targets in Phytophthora that can be exploited for disease control, we focus at identifying biological features unique for oomycetes. In that respect the actin cytoskeleton, among others, caught our attention. This intracellular framework is indispensable for the viability of eukaryotic cells and functions in a wide range of processes including intracellular transport, formation of contractile rings, nuclear segregation, endocytosis and facilitating apical cell expansions. We studied the actin cytoskeleton dynamics in P. infestans in transgenic lines expressing the actin binding peptide Lifeact-eGFP. Fluorescence microscopy showed that in hyphae actin filament cables and plaques are cortically localized. The distance between the hyphal tip and the first actin filament plaque correlated strongly with growth velocity. Upon growth termination, actin filament plaques appeared in the hyphal tip. The plaques were nearly immobile with average lifetimes exceeding one hour; much longer (over 500-fold) than the lifetimes of actin patches in fungi. Plaque assembly required ~30 seconds while disassembly took only ~10 seconds. In contrast to actin patches in yeast, plaque disassembly was not accompanied with formation and internalization of endocytic vesicles (Meijer et al. 2014, Cell. Microbiol.). We also investigated the in vivo actin dynamics during early stages of pathogenesis. At the site of contact with the plant cell a condensed transient actin structure was observed that resembles aster-like actin structures formed upon encountering hard surfaces. Our results suggest that the actin cytoskeleton has distinct functions during the P. infestans lifecycle. Future efforts will focus at identifying interactors and key regulators of the actin cytoskeleton and pinpoint features in the actin network that are unique for oomycetes.