Microtubule organization in root cells of Medicago truncatula during development of an arbuscular mycorrhizal symbiosis with Glomus versiforme

The colonization of plants by arbuscular mycorrhizal fungi has been shown t o induce changes in cytoplasmic organization and morphology of root cells. Because of their role in a variety of cellular functions in plants, it is l ikely that microtubules are involved either in the signaling events leading to the establishment of the sumbiosis or in changes in host cell morpholog y and cytoplasmic architecture. Recent studies of the arbuscular mycorrhiza l symbiosis have shown that root cortical cells reorganize their microtubul es upon colonization. These studies, however, have focused primarily on the cells containing hyphal coils or arbuscules and did not include descriptio ns of microtubule changes in adjacent cells. To probe further into the pote ntial role of the microtubule cytoskeleton in the establishment of arbuscul ar mycorrhizal symbiosis, we examined the three-dimensional arrangement of microtubules in roots of the model legume Medicago truncatula colonized by the arbuscular mycorrhizal fungus Glomus versiforme by indirect immunofluor escence and confocal microscopy. Our data show extensive remodeling of the microtubule cytoskeleton from the early stages of arbuscule development unt il arbuscule collapse and senescence. While confirming some of the microtub ule patterns shown in other mycorrhizal systems, our results also reveal th at cortical cells adjacent to those containing arbuscules or adjacent to in tercellular hyphae reorganize their microtubules. This indicates that the c ortical cells initiate the modification of their cytoskeleton prior to entr y of the fungus and is consistent with signal exchange between the symbiont s prior to fungal penetration of the cells.