Bacterial extracellular vesicles (bEVs) are emerging as key players in interkingdom communication, yet their role in delivering functional proteins to host cells during symbiosis remains unexplored. This study shows that Sinorhizobium fredii HH103 packages a PIN-like auxin transporter, AuxT, into bEVs that traffic within the peribacteroid space of soybean nodules. AuxT is chromosomally encoded and constitutively expressed, genetically uncoupled from the flavonoid-inducible auxin biosynthesis machinery located on the symbiotic plasmid. Structural prediction reveals that AuxT adopts an eight-transmembrane-helix architecture with striking homology to plant PIN auxin exporters, despite negligible sequence identity. Molecular docking demonstrates that AuxT binds indole-3-acetic acid within a central cavity, with dimerization inducing ligand-specific conformational changes consistent with transport activity. The auxT mutant exhibits significant symbiotic defects including reduced shoot biomass, nodule number, and nodule mass that are fully restored by complementation. Critically, AuxT-enriched bEVs contain elevated auxin levels, and nodules colonized by the complemented strain accumulate more auxin specifically within the bEVs peribacteroid space, while bacteroids themselves show no auxin retention. We postulate that bEV-associated AuxT mediates localized auxin export into the symbiosome, modulating the host hormonal environment to optimize symbiotic development. This work reveals a previously unrecognized mechanism of interdomain hormonal modulation, where a bacterium uses a structurally convergent transporter and vesicular delivery to actively shape host physiology and to improve the symbiotic performance.
Natalia Moreno-de Castro, Mustafa Safa Karagöz, Irene Herrero Gómez et al.· bioRxiv· 0 citations
Rhizobiaceae serve as classical models for elucidating mutualistic plant-microbe interactions yet they represent a narrow phylogenetic subgroup of Alphaproteobacteria. Studying additional lineages of Rhizobiaceae, we observed broad associations with oxygenic phototrophs beyond land plants, including early branching clades of submerged plants, multicellular and unicellular algae, as well as cyanobacteria. In particular, bacteria of the genus Hoeflea were often affiliated with cyanobacteria or microbial algae, whereas Peteryoungia spp. colonized roots of submerged plants. While both genera were originally described as nonpigmented heterotrophs, our detailed genomic, biochemical and physiological analyses revealed that most strains actually contained genes for anoxygenic photosynthesis. Under oligotrophic, oxic growth conditions, each characterized representative expressed bacteriochlorophyll a-containing functional photosynthetic complexes. Photosynthesis genes shared the highest homology among phylogenetically closest relatives, displaying topologies congruent to cognate house-keeping gene phylogenies, and maintained highly conserved gene synteny across the chromosomes of different species. Together, this indicated a vertical inheritance and long ancestral history of aerobic anoxygenic photosynthesis in Rhizobiaceae rather than multiple recent horizontal transfers. Subsequent time-scale phylogenetic analysis suggested that the last common ancestor of Rhizobiaceae was an aquatic phototroph, with different lineages of Rhizobiaceae consecutively evolving in association with algae, land plants, then later legumes. While aquatic lineages maintained photosynthetic machinery till today, Rhizobia which developed symbioses with land plants either as mutualistic endosymbiosis within root nodules or as plant pathogens, concomitantly lost photosynthetic capability. Based on our results, multiple biotic interactions with diverse oxygenic phototrophs drove the early evolution of Rhizobiaceae.
Steven B. Kuzyk, Philipp Halama, M. Saini et al.· bioRxiv· 0 citations
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