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.
Highlights
The rhizosphere is a narrow zone of soil encompassing plant roots and various organic compounds that have a direct influence on plant growth and performance [1]
As the number of colonies was very high in the maceration extract from combined soybean root nodules, a single nodule was used for each isolation of bacterial colonies
Since Rhizobium colonies do not appear before 3–4 days of plating in the liquid Modified Arabinose Gluconate (MAG)
Summary
The rhizosphere is a narrow zone of soil encompassing plant roots and various organic compounds that have a direct influence on plant growth and performance [1]. It is regarded as one of the most dynamic and complex interfaces due to numerous interactions within organisms residing in the rhizosphere. These interactions can be beneficial, including symbiotic relationships with beneficial microbes such as rhizobia, mycorrhiza, and plant growth-promoting rhizobacteria (PGPR), or they can Plants 2020, 9, 1494; doi:10.3390/plants9111494 www.mdpi.com/journal/plants. Nitrogen fixing Rhizobia were assumed to be the only bacteria residing in the nodules of legumes [6]
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