Soybean nodule-associated non-rhizobial bacteria inhabit plant pathogens and induce growth promotion in tomato

Authors: TOKGOZ, SERKAN - University Of Nebraska; Lakshman, Dilip; GOZLAN, MAHMOUD - University Of Nebraska; PINAR, HASAN - University Of Nebraska; Roberts, Daniel; MITRA, AMITAVA - University Of Nebraska

Submitted to: Plants
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2020
Publication Date: 11/5/2020
Citation: Tokgoz, S., Lakshman, D.K., Gozlan, M., Pinar, H., Roberts, D.P., Mitra, A. 2020. Soybean nodule-associated non-rhizobial bacteria inhabit plant pathogens and induce growth promotion in tomato. Plants. https://doi.org/10.3390/plants9111494.
DOI: https://doi.org/10.3390/plants9111494

Interpretive Summary: Root nodule is a unique environment formed in the legume roots through a highly specific symbiotic relationship between leguminous plants and nodule-inducing bacteria of the Rhizobium species. Recent studies discovered many other groups of bacteria besides Rhizobia within legume nodules. In an effort to identify beneficial bacteria for plant disease control and growth promotion, we have explored soybean nodules for potential nodule-associated bacteria. A total of 500 bacterial colonies isolated from single nodules were individually tested on solid media plates against the tomato wilt causing bacterial pathogen Clavibacter michiganensis subsp. michiganensis (CMM) for inhibition of pathogen growth. From the initial screening, 54 colonies were selected based on significant growth inhibition of the pathogen. Fifteen selected colonies from the initial screening were identified by using MALDI-TOF mass spectrometry and 16S rRNA gene sequencing. Pseudomonas spp. was found to be the dominant bacterial group as nodule-associated bacteria within soybean nodules. Those fifteen isolates were further tested on tomato seedlings and found to impart various degrees of protection of tomato against the wilt pathogen as well as providing plant growth promotion. This study demonstrated the potentials of isolation, identification, and screening of beneficial nodule-associated bacteria for use in plant protection and growth promotion.

Technical Abstract: Root nodule is a unique environment formed in the legume roots through a highly specific symbiotic relationship between leguminous plants and nodule-inducing bacteria. Over the years, Rhizobia were presumed as the only group of bacteria residing within the nodules. However, recent studies discovered many other groups of bacteria besides Rhizobia within the legume nodules. In an effort to identify beneficial bacteria for plant disease control and growth promotion, soybean nodules were studied as the source of nodule-associated bacteria. Metagenomics analysis was conducted to determine the bacterial diversity of nodule microbiome using surface sterilized single nodules. A total of 500 colonies from 10 nodules were tested individually against the tomato wilt causing bacterial pathogen Clavibacter michiganensis subsp. michiganensis (CMM) on solid media plates for inhibition of pathogen growth. From the initial screening, 54 colonies were selected based on significant growth inhibition of the pathogen. These colonies were further tested in vitro on another bacterial pathogen, Pseudomonus syringae pv. tomato (PST) and two fungal pathogens, Rhizoctonia solani and Sclerotinia sclerotiorum on solid plates. Bacterial metabolites were extracted from 15 selected colonies out of the 54 colonies with ethanol and tested against the CMM and PST pathogens on solid plates. These 15 colonies were identified by using MALDI-TOF mass spectrometry and 16S rRNA gene sequencing; Pseudomonas spp. was found to be the dominant bacterial group as soybean nodule-associated bacteria. In the in planta testing using the 15 selected nodule-associated colonies as protectant, a number of them imparted various degrees of protection against the CMM pathogen and/or plant growth promotion on tomato seedlings. Isolation and identification of beneficial nodule-associated bacteria established the foundation for further explorartion of potential nodule-associated bacteria for plant protection and growth promotion.