Mycoheterotrophic plants in the genus Thismia (Thismiaceae) depend entirely on arbuscular mycorrhizal fungi for carbon and nutrient acquisition, but the environmental conditions and microbial communities governing their seasonal occurrence remain poorly understood. We investigated how seasonal variation in soil properties and microbial communities correlates with the emergence of Thismia gardneriana in a lowland tropical rainforest of southern Thailand. We compared soil physicochemical properties between wet and dry seasons when flowers of T. gardneriana were present and absent, respectively. Using ITS2 and 16S rRNA gene amplicon sequencing, we characterized fungal and bacterial communities across three compartments: endosphere, rhizosphere, and bulk soil. Arbuscular mycorrhizal fungal (AMF) identity was confirmed through phylogenetic analyses of ITS2 and SSU-ITS-LSU ribosomal DNA sequences, supported by microscopic observation of mycorrhizal structures in root tissues. Here, we showed T. gardneriana flowered at the onset of the wet season in soils with elevated moisture, potassium, magnesium, and calcium, but reduced total and available phosphorus. AMF spore density was significantly higher when flowers emerged. The endosphere harbored a distinct microbial assemblage dominated by Glomeromycota, specifically Rhizophagus species forming Paris-type arbuscules. In contrast, Ascomycota and Basidiomycota dominated soil and rhizosphere fungal communities. Bacterial communities showed compartment-specific differentiation, with the endosphere enriched in nitrogen-fixing taxa, especially Rhizobium and chitin-degrading Puia bacteria in Chitinophagaceae, while the rhizosphere exhibited the highest microbial diversity. These findings reveal that T. gardneriana flowering is associated with seasonal soil conditions and that this mycoheterotrophic plant actively harbors specialized microbial communities in its roots and rhizosphere, suggesting complex multi-kingdom interactions essential for its survival in tropical rainforest ecosystems.
Noirungsee et al. (Tue,) studied this question.