Temperature thresholds drive the biogeographic pattern of root endophytic fungal diversity in the Qinghai-Tibet Plateau

Studying the diversity and distribution of host-associated fungi along the temperature gradient can help us detect the potential impact of global warming on host-microbe interactions. By investigating 55 samples along the temperature gradient, our results demonstrated that temperature thresholds controlled the biogeographic pattern of fungal diversity in the root endosphere. When the mean annual temperature crossed ∼1.40 °C, or the mean temperature of the coldest quarter was over ∼−8.26 °C, the root endophytic fungal OTU richness abruptly decreased. Shared OTU richness between the root endosphere and rhizosphere soil showed similar temperature thresholds. However, the OTU richness of the fungi in the rhizosphere soil had a nonsignificant positive linear relationship with temperature. The asynchrony of alpha diversity in the rhizosphere soil and root endosphere in response to increasing temperature indicated that temperature might regulate the microbial colonization process from the rhizoplane to interior tissue. When the temperature crosses the threshold, the rapid decrease in OTU richness from soil entry to root tissue colonization may lead to a rapid decrease in root OTU richness. We further found that root endophytic fungal OTU richness was more sensitive to temperature increases under drought than non-drought conditions. We also found similar temperature thresholds influencing the root endophytic fungal beta diversity. When the temperature difference between two sampling points crossed an ∼2.2 °C temperature threshold, species replacement sharply decreased, and the species richness difference sharply increased. This investigation highlights that temperature thresholds are very important in shaping variation in the diversity of root endophytic fungi, especially in alpine ecosystems. Furthermore, it provides a preliminary framework for studying host-microbe interactions under global warming.