Soybean Nodule-Associated Non-Rhizobial Bacteria Inhibit Plant Pathogens and Induce Growth Promotion in Tomato

Abstract

The root nodules are a unique environment formed on legume roots through a highly specific symbiotic relationship between leguminous plants and nodule-inducing bacteria. Previously, Rhizobia were presumed to be the only group of bacteria residing within nodules. However, recent studies discovered diverse groups of bacteria within the legume nodules. In this report soybean nodule-associated bacteria were studied in an effort to identify beneficial bacteria for plant disease control and growth promotion. Analysis of surface-sterilized single nodules showed bacterial diversity of the nodule microbiome. Five hundred non-rhizobial colonies from 10 nodules, 50 colonies per nodule, were tested individually against the tomato wilt causing bacterial pathogen Clavibacter michiganensis subsp. michiganensis (Cmm) for inhibition of pathogen growth. From the initial screening, 54 isolates were selected based on significant growth inhibition of Cmm. These isolates were further tested in vitro on another bacterial pathogen Pseudomonas syringae pv. tomato (Pst) and two fungal pathogens Rhizoctonia solani and Sclerotinia sclerotiorum. Bacterial metabolites were extracted from 15 selected isolates with ethanol and tested against pathogen Cmm and Pst. These isolates were identified by using MALDI-TOF mass spectrometry and 16S rRNA gene sequencing. Pseudomonas spp. were the dominant soybean nodule-associated non-rhizobial bacterial group. Several isolates imparted significant protection against pathogens and/or plant growth promotion on tomato seedlings. The most promising nodule-associated bacterial isolate that suppressed both Cmm and Pst in vitro and Pst in tomato seedlings was identified as a Proteus species. Isolation and identification of beneficial nodule-associated bacteria established the foundation for further exploration of potential nodule-associated bacteria for plant protection and growth promotion.

The microbiome of Lotus nodules varies with plant health in a species-specific manner

“…No reuse allowed without permission. (13,59,60). We found that Pseudomonas ASVs were characteristic of healthy, but not of starved L. burttii nodules (Figure 5) suggesting they have the potential to support plant health.…”

Two new Rhizobiales species isolated from root nodules of common sainfoin (Onobrychis viciifolia) show different plant colonization strategies

“…Although it carries a large amount of PGP genes, O. muellerharveyae TH2 T showed no effect on sainfoin plants in our inoculation experiment under nitrogen-limited conditions. O nobrychidicola muellerharveyae TH2 T might achieve better potential under biotic stress condition [9], as its highest number of genes were found in functional classes referring to bacterial fitness/ stress tolerance. In contrast to most other Rhizobiaceae , which have only one copy, O. muellerharveyae TH2 T had four copies of the tabtoxin degradation gene ( ttr ).…”

“…They are involved in different biological activities e.g. plant growth-promotion and biocontrol [9]. Nevertheless, our knowledge about the entire biological functions of nodule-associated bacteria is elusive.…”

From Microns to Meters: Exploring Advances in Legume Microbiome Diversity for Agroecosystem Benefits

“…NREs may also suppress plant pathogens. Tokgöz et al (2020) found that NREs isolated from soybean nodules have antifungal and antibacterial properties that extend beyond the nodule and interact with non-legumes. The authors found that an isolate of the genus Proteus was effective in inhibiting the growth of several bacterial and fungal pathogens in vitro.…”