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

The actin cytoskeleton is a dynamic but well organized intracellular framework that is indispensable for the viability of eukaryotic cells. Its functions range from intracellular transport, formation of contractile rings, nuclear segregation, endocytosis and facilitating apical cell expansions. We studied the actin cytoskeleton dynamics in the filamentous oomycete plant pathogen Phytophthora infestans in transgenic lines expressing the actin binding peptide Lifeact-eGFP by fluorescence microscopy. This 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.