Mycorrhizal communities of Vanilla planifolia
Abstract
Vanilla is one of the most valuable spices in the world. In Madagascar and La Réunion, the world's leading producers, vanilla is of great economic and cultural importance. Like all orchids, vanilla plants associate with mycorrhizal fungi in their roots forming mutualistic associations that allow them to grow and thrive. Understanding the diversity of mycorrhizal fungi adapted to vanilla cultivation, particularly in the Indian Ocean islands where they have never previously been studied, is becoming a necessity for maintaining vanilla crops in these regions in the face of climate change and the emergence of new pathogens. The vanilla orchid (Vanilla spp.) is one of the most valuable cultivated plants worldwide. As with all orchids, vanillas form mycorrhizal associations with fungi in their roots, but their fungal partners have not been investigated outside their native geographic range in Central America. We investigated the whole fungal and mycorrhizal associations in cultivated vanilla (Vanilla planifolia) by sequencing the fungal ITS‐2 marker in the terrestrial and aerial roots using a metabarcoding approach. We selected plants cultivated in three conditions (i.e., cultivation under shade house, in openfield, or in the understory) in one locality of La Réunion island (Indian Ocean) and tested for a possible effect of cultivation practices on fungal communities. Cultivated vanillas in La Réunion mainly associate with Tulasnellaceae (75 OTUs) and Ceratobasidiaceae (8 OTUs). Among the seven most abundant Tulasnellaceae, six are similar to fungi detected in the roots of cultivated vanillas in Central America or in the roots of native orchids in La Réunion. Cultivation practices impacted both total fungal and mycorrhizal community compositions with no clear effect on fungal richness. Notably, Tulasnellaceae and Ceratobasidiaceae were scarce in aerial roots, except in the traditional cultivation in the forest understory. These results shed light on the geographical origins of mycorrhizal fungi of cultivated vanillas in La Réunion and show that they form a pool of both locally and globally distributed fungal partners. These mycorrhizal communities vary according to cultivation practices, and their potential roles in plant nutrition and resistance against pathogens await further attention.
Study Snapshot
Cultivated vanilla in La Réunion mainly associate with Tulasnellaceae and Ceratobasidiaceae fungi, which vary based on cultivation practices, and may play a role in plant nutrition and resistance against pathogens.
Population: Cultivated vanilla plants (Vanilla planifolia) in La Réunion
Methods: Metabarcoding-based observational study of fungal communities.
Outcomes: Fungal and mycorrhizal community composition in vanilla roots
Results: Vanilla roots mainly associated with Tulasnellaceae and Ceratobasidiaceae fungi, varying by cultivation method.
4.3 Impact of cultivation practices on fungal communities of vanilla
In Mexico, Johnson et al. (2021) suggested that cultivation practices could influence the fungal endophytes of vanilla, including mycorrhizal fungi. However, their study was comparing four sites distant of >50 km from each other, and the authors could not exclude that the observed differences were due to spatial effects. In our study conducted in an area of <2.5 km2, we confirmed that cultivation practices do influence fungal and mycorrhizal communities of vanilla.
Rarefaction curves showed that whole fungal communities were richer in the US compared with SH and OF. Comparisons of richness between aerial and terrestrial roots were limited by different sampling sizes, but aerial roots in the US seem to host a richer fungal community than terrestrial roots.
When considering mycorrhizal fungi only, aerial roots were hardly colonized except on living trees and shrubs in the US. However, vanillas growing in the SH or in the OF both rooted epiphytically on dead wood stake of Pinaceae and on the exotic trunks of J. curcas (Euphorbiaceae) trees, respectively. The chemistry of these tutors (e.g., latex produced by J. curcas; e.g., Igbinosa et al., 2009) might be responsible for this low colonization. When considering all sites, the majority of mycorrhizal fungi detected in the aerial roots of vanilla plants (i.e., in the US) were also detected in terrestrial roots (including in the farm), meaning that they may not be specific of the epiphytic environment. Indeed, mycorrhizal fungi might grow from the soil onto the tutors' bark and colonize aerial roots. For instance, CER9 was shared between the terrestrial and aerial vanilla roots in the US. As it was not similarly shared in other cultivation practices, it also suggests that the substrate could limit the growth of some fungi in aerial parts. Yet, the low colonization by mycorrhizal fungi of vanilla aerial roots has already been demonstrated by using microscopy (Porras-Alfaro & Bayman, 2007) or sequencing (Johnson et al., 2021) and might also be a property of this species.
The mycorrhizal communities also varied between cultivation practices, including between different substrate types in SH, even though the composition at the family level was similar. For instance, the Resinicium RES25 was almost exclusively detected in the roots of vanilla in bagasse (see above). The variation of cultivation practices could then favor OTUs with complementary niches as exemplified in the Ceratobasidiaceae family in which the two main OTUs were specific of the SH and of the OF and US, respectively. By modulating the nutrient sources such as nitrogen, different substrates could promote various mycorrhizal partners (Nurfadilah et al., 2013), which may provide specific services to the plant and sustain the diversification of cultivation systems in vanillas.