Variations in the root mycobiome and mycorrhizal fungi between different types of Vanilla forest farms on Réunion Island.

The mycorrhizal fungi of cultivated Vanilla spp. have mainly been studied in America, while a recent study has investigated them on Réunion Island (Indian Ocean). However, there are many different types of cultivation on Réunion, from shade-house crops to forest farms of endemic or exotic trees. Here we fill a gap in the study of the root mycobiome of Vanilla by sampling vines in forest plantations on recent lava flows in the southeast of Réunion. Specifically, we aimed to characterize the fungal communities between terrestrial and epiphytic roots, between forest farms that differ mainly in the species of trees, and between Vanilla roots and ECM-like roots of nearby trees. By sequencing fungal ITS2, we showed that the Vanilla root mycobiome is diverse and differed between the root types and forest farms. Epiphytic and terrestrial roots host endophytic fungi, while a putative rust with visible urediniospores was abundant in terrestrial roots mainly. Other pathogens were detected in epiphytic roots (Colletotrichum) with no sign of disease. Following sequencing and electron microscopy, Tulasnellaceae, characterized by imperforate parenthesomes and cell wall expansion with an amorphous matrix, were shown to be the main mycorrhizal fungi in both vanilla root types. Interestingly, the dominant Tulasnellaceae OTU was found in ECM-type roots of trees belonging to the ectomycorrhizal family Sapotaceae. Further observations are needed to confirm the ectomycorrhizal association of endemic trees with Tulasnella. Moreover, labeling experiments will be instrumental in investigating the transfer of nutrients between the trees and the Vanilla through the network of mycorrhizal associations in the soil.