Are Phytophthora infestans F-actin plaques cell wall anchors?

The actin cytoskeleton is a complex, dynamic intracellular network that is involved in a multitude of cellular processes and essential for viability of eukaryotic cells. In hyphae of the oomycete plant pathogen Phytophthora infestans actin filaments (F-actin) are organized in higher order cables and plaques while the formation of an aster-like actin configuration is triggered in the tips of hyphae that face a barrier. Unlike the ubiquitous actin cables, actin plaques are unique for oomycetes and present in every life stage except zoospores, the wall-less, flagellated motile propagules. They are predominantly stationary clusters of F-actin that localize at the cell membrane with lifetimes that can exceed an hour. To unravel the potential role of actin plaques in oomycetes we visualized their dynamics in protoplasts of a P. infestans strain expressing the live cell actin probe Lifeact-GFP. In these wall-less cells, actin cables are present in the cortical cytoplasm and plaques have the same, membrane associated localization as in walled mycelium. In some protoplasts, we observed rotating rings of actin cables in the cortical cytoplasm. The F-actin plaques display a distinct behaviour when compared to those in walled hyphae; their lifetime is reduced and their motility, particularly of those that co-localize with the rotating actin rings, strongly increases. These findings suggest that actin plaques are immobilized by a connection with the cell wall and potentially function as an anchor for positioning the actin cables.