Drivers of Protist Diversity and Functions, and Protist–Prokaryote Co-Occurrence Pattern Across Wheat Rhizocompartments

Soil protist communities and their interaction with prokaryotes in the rhizocompartment influence plant growth. However, the drivers of protist diversity and their co-occurrence and interaction with prokaryotic communities in dynamic rhizocompartments and between wheat varieties are not well understood. We hypothesized that rhizocompartment and wheat varieties, differing in root structure and pathogen resistance, impact protist community structure and diversity. Additionally, the co-occurrence of prokaryotes and protists was hypothesized to depend on wheat varieties selecting for different key protist-prokaryote interactions. We studied the protist community composition of four wheat varieties in three rhizocompartments: rhizoplane, rhizosphere, and bulk soil, and their co-occurrence with prokaryotic communities. In soil DNA from a greenhouse pot experiment, protist abundance was determined using qPCR, and community composition was described by metabarcoding of 18S rRNA and 16S rRNA genes. Protist community structure and abundance were significantly affected by the rhizocompartment and wheat varieties. Protist richness increased with distance from the root surface. Protist abundance was higher in the rhizocompartments of the Rembrandt wheat variety. Colpodea was more abundant in the rhizosphere, and Filosa-Sarcomonadea in the rhizoplane, compared to bulk soil. The trophic profile of the protist communities showed the dominance of protist predators, and a co-occurrence network analysis showed an intricate network with more nodes in the bulk soil. Rhizocompartment and wheat variety drive protist communities, consistent with the drivers of prokaryotic communities, demonstrating the interconnectivity of protist-prokaryotic co-occurrence in the soil rhizosphere and highlighted by the functional assignments.