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A prerequisite to exploiting soil microbes for sustainable crop production is the identification of the plant genes shaping microbiota composition in the rhizosphere. Here, the authors report QTLs and the associated candidate genes underlying rhizosphere microbiome composition in barley.

ABSTRACT The microbiota populating the rhizosphere, the interface between roots and soil, can modulate plant growth, development, and health. These microbial communities are not stochastically assembled from the surrounding soil, but their composition and putative function are controlled, at least partially, by the host plant. Here, we use the staple cereal barley as a model to gain novel insights into the impact of differential applications of nitrogen, a rate-limiting step for global crop production, on the host genetic control of the rhizosphere microbiota. Using a high-throughput amplicon sequencing survey, we determined that nitrogen availability for plant uptake is a factor promoting the selective enrichment of individual taxa in the rhizosphere of wild and domesticated barley genotypes. Shotgun sequencing and metagenome-assembled genomes revealed that this taxonomic diversification is mirrored by a functional specialization, manifested by the differential enrichment of multiple Gene Ontology terms, of the microbiota of plants exposed to nitrogen conditions limiting barley growth. Finally, a plant soil feedback experiment revealed that host control of the barley microbiota underpins the assembly of a phylogenetically diverse group of bacteria putatively required to sustain plant performance under nitrogen-limiting supplies. Taken together, our observations indicate that under nitrogen conditions limiting plant growth, host-microbe and microbe-microbe interactions fine-tune the host genetic selection of the barley microbiota at both taxonomic and functional levels. The disruption of these recruitment cues negatively impacts plant growth. IMPORTANCE The microbiota inhabiting the rhizosphere, the thin layer of soil surrounding plant roots, can promote the growth, development, and health of their host plants. Previous research indicated that differences in the genetic composition of the host plant coincide with variations in the composition of the rhizosphere microbiota. This is particularly evident when looking at the microbiota associated with input-demanding modern cultivated varieties and their wild relatives, which have evolved under marginal conditions. However, the functional significance of these differences remains to be fully elucidated. We investigated the rhizosphere microbiota of wild and cultivated genotypes of the global crop barley and determined that nutrient conditions limiting plant growth amplify the host control on microbes at the root-soil interface. This is reflected in a plant- and genotype-dependent functional specialization of the rhizosphere microbiota, which appears to be required for optimal plant growth. These findings provide novel insights into the significance of the rhizosphere microbiota for plant growth and sustainable agriculture.

Phytophthora is a genus of destructive plant pathogens. Certain species are damaging to native ecosystems, forestry, and the horticultural sector, and there is evidence of their dissemination in plant imports. Horticultural nurseries are central nodes of the plant trade and previous studies have found a high diversity of Phytophthora associated with plant nursery stock. It was subsequently hypothesized that green waste disposal sites in nurseries could harbour diverse Phytophthora communities and act as a pathogen reservoir and conduit, facilitating further Phytophthora infection of nursery stock and its spread into the wider environment. This project identified Phytophthora species associated with green waste at three Scottish nurseries by sampling material from waste piles, water run-off from piles, and roots from discarded plants. Species were identified using a baiting method and sequencing of environmental DNA. Plant nursery green waste was shown to harbour diverse and varied Phytophthora species assemblages, with differences among nurseries reflecting biosecurity management practices. Eighteen Phytophthora species were detected in the samples, including the highly destructive pathogens P. ramorum and P. austrocedri. Results suggest that the improved management of waste, for example through effective on-site composting, is essential to reduce the risk of Phytophthora pathogens spreading from nurseries into the wider environment.