Bacterial extracellular vesicles (commonly referred to as EVs) are proteolipidic structures detached from bacterial membranes into the surrounding environment. They participate in diverse biological processes, including interdomain communication as EV encapsulation concentrates signaling molecules for efficient delivery to target eukaryotic cells. While the rhizobium-legume symbiosis represents one of nature's most intimate partnerships, the role of rhizobial EVs, particularly as carriers of symbiotic signaling molecules, remains poorly understood. The primary aim of this study was to identify Nod Factors (NFs) within rhizobial EVs, exploring their potential as molecular inoculants to enhance the rhizobium-legume symbiosis. Sinorhizobium fredii HH103 EVs were characterized to confirm their integrity and yield. Their NF content was determined and quantified, comparing genistein-induced and non-induced cultures to identify specific chemical signatures. The biological impact of these EVs was validated through nodule primordia visualization and controlled growth chamber assays in Glycine max and Lotus japonicus . A two-year field experiment was conducted in South Brazil with 10 different treatments to explore the soybean ( G. max Brasmax Zeus Ipro) responses to inoculation with plant-growth promoting bacteria ( Azospirillum brasilense AbV5 and AbV6; Bradyrhizobium japonicum CCT 4065, SEMIA 5079, SEMIA 5080 and SEMIA 586) with or without HH103 EVs. Our investigation explores the role of bacterial EVs produced by S. fredii HH103 in enhancing the symbiotic interaction between rhizobia and legume hosts, particularly soybeans. Our research demonstrates that genistein, an HH103 nod gene-inducing flavonoid, triggers hypervesiculation in this bacterium, which leads to a significant increase in the production of EVs transporting high molecular weight NFs. Interestingly, our findings indicate that adding these NF-loaded EVs not only promotes nodule primordia formation but also improves nodulation in soybean and in the model plant Lotus japonicus under controlled laboratory conditions. Notably, while EVs derived from non-induced cultures failed to enhance the number of nodules, they also exerted a positive effect on general plant growth parameters. Moreover, the application of NF-loaded EVs significantly increases soybean crop yield in field experiments, especially in the absence of water deficit. Results point to the role of these plant-growth-promoting vesicles in promoting legume yield. Therefore, our study proposes the use of rhizobial EVs as a sustainable agricultural strategy to optimize the rhizobium-legume symbiosis and boost crop productivity.
Castro et al. (Thu,) studied this question.