Profiling sorghum-microbe interactions with a specialized photoaffinity probe identifies key sorgoleone binders in Acinetobacter pittii

Abstract

Sorghum ( Sorghum bicolor ) is a major food and bioenergy grass species cultivated worldwide. To promote more robust and sustainable growth of this important crop, we need a deeper understanding of the plant-microbe interactions between sorghum and soil microbial communities that benefit plant host resiliency and enhance nutrient acquisition. The release of specific metabolites from plant roots, or root exudation, drives these plant-microbe interactions, but the molecular pathways by which root exudates shape the sorghum rhizosphere microbiome require further elucidation. To investigate these complex interkingdom interactions in the sorghum rhizosphere, we developed a photoaffinity probe based on sorgoleone, a hydrophobic secondary metabolite and allelochemical produced in sorghum seedling root exudates. Here, we apply a new synthetic sorgoleone diazirine alkyne photoaffinity probe (SoDA-PAL) to the identification of sorgoleone-binding proteins in Acinetobacter pittii SO1, a potential plant growth promoting microbe derived from Sorghum bicolor rhizosphere soil. Competitive photoaffinity labeling of A. pittii whole cell lysates with SoDA-PAL identified 137 statistically enriched proteins that were complementary to a previously identified gene cluster involved in sorgoleone catabolism. Proteins identified by SoDA-PAL included a select set of putative transporters, transcription regulators, and a subset of proteins with lipid and secondary metabolic activities. We confirm binding of SoDA-PAL to a putative hydrolase in the α/β fold family (OH685_09420) through structural bioinformatics and in-vitro recombinant protein analysis. This photoaffinity labeling approach using metabolite-based probes can be extended in the future to proteomic profiling of complex rhizosphere microbiomes to discover genes that can be leveraged to promote beneficial plant-microbe interactions. Importance: Here we demonstrate a photoaffinity-based chemical probe modeled after sorgoleone, a known secondary metabolite released from the roots of sorghum, can be used to dissect complicated plant-microbe interactions. Applying this probe to the sorghum-associated bacterium Acinetobacter pittii identified diverse proteins that directly interact with sorgoleone. We show that probe labeling is dose-dependent and is sensitive to competition with purified sorgoleone, demonstrating the probe is selective for protein targets that directly interact with sorgoleone. By using the probe to broadly profile proteins that interact with sorgoleone, we identified bacterial catabolic pathways, unintuitive transcriptional regulation pathways, and vital exchange mechanisms involving transporters that may be involved in sorgoleone utilization and cellular response toward this plant metabolite. We envision that this workflow will expand our understanding of the sorghum root exudate interactome and elucidate the molecular mechanisms by which specific metabolites shape the sorghum rhizosphere microbiome.