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. In Phytophthora palmivora lines expressing green fluorescent protein (GFP)-tagged α-tubulin, we observed a conserved microtubule localization in the mitotic spindle and dynamic cytoplasmic microtubules between adjacent nuclei. These microtubules originated from the mitotic spindle and rapidly increased in length after mitosis. Since microtubule minus-ends are probably associated with microtubule organizing centres on the nuclear surface, the plus-ends of microtubules originating from adjacent nuclei probably maintain an antiparallel connection, which may play a role in nuclear spacing. This idea was strengthened by erratic nuclear motility and positioning after microtubule depolymerization. Besides aberrant nuclear positioning, we also observed less sustained tip growth in the absence of microtubules. This suggests that microtubules radiating into the hyphal tip from the apical nucleus 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.