Microtubules in the coenocyte Phytophthora function in nuclear positioning and sustaining tip growth

The microtubule cytoskeleton consists of dynamic intracellular filaments and is involved in numerous processes, ranging from nuclear division to intracellular transport. Some of these microtubule-mediated processes are conserved in all eukaryotic lineages while others are specific for certain groups of organisms. Here, we focus on the microtubule cytoskeleton of oomycetes in the genus Phytophthora, a group of harmful plant pathogens. For visualizing microtubule organization and dynamics, we generated transgenic Phytophthora palmivora lines expressing GFP-tagged -tubulin. Besides a conserved localization in the mitotic spindle, we observed cytoplasmic microtubules originating from microtubule-organizing centers associated with nuclei in the coenocytic hyphae. These dynamic microtubules initiated long-lasting, antiparallel connections with microtubules originating from adjacent nuclei. After mitosis, these microtubules rapidly increased in length while maintaining their antiparallel interaction. Frequent buckling events suggest that microtubule-based force generation plays a role in nuclear spacing. This idea was strengthened by erratic nuclear motility and positioning in hyphae exposed to the microtubule depolymerizing drug oryzalin. Besides aberrant nuclear positioning we also observed defects in hyphal growth; in oryzalin-treated hyphae lacking microtubules, tip growth was less sustained than in non-treated hyphae. This suggests that microtubules radiating into the hyphal tip from the apical nucleus have a function in sustaining tip growth. Altogether, this study provides novel insights in the localization, dynamics and functions of the microtubule cytoskeleton in the coenocytic Phytophthora hyphae.