Competitive use of plant root exudates by Ralstonia solanacearum causing tobacco bacterial wilt and its antagonistic bacterium LX4

Abstract:

[Objective] It is important to study the nutrition relationship between tobacco bacterial wilt pathogen (Ralstonia solanacearum, Rs) and its antagonistic bacterial isolate LX4. The difference in their use of secreted chemicals from tobacco root plays a crucial role for increasing the colonization of antagonist and efficiently bio-controlling tobacco bacterial wilt. [Methods] Pathogen Rs and the antagonistic strain LX4 from tobacco field soil were isolated and identified. We examined the use of characteristic carbon and nitrogen sources in tobacco root exudates by Rs and LX4. Main chemicals in root exudates were identified with a gas chromatography-mass spectrometer (GC-MS). We compared and analyzed the capacity and intensity of utilizing the major nutrients by Rs and strain LX4 by co-culturing the two microorganisms. [Results] The isolated pathogen was identified as R. solanacearum and isolate LX4 was identified as Bacillus subtilis via phylogenetic tree analysis based on their 16S rDNA sequences. In the exudates of tobacco roots, the substances with contents of > 0.01 μg/mL were ordered descendingly as pectin > glucose > xylose > arabinose > galactose > ribose > sucrose > benzoic acid > fructose=D-mannitol > cetylic acid > fumaric acid. The content of pectin was the highest and significantly higher than all the other substances. Strain LX4 could use arabinose, xylose and ribose significantly better than R. solanacearum. The usage capacity of the former was 1.22, 1.95 and 2.17 times of those of the latter, respectively. Besides, the fructose usage by LX4 was higher than that by R. solanacearum in the first 12 h. After 24 h of co-culturing them on different substrates of carbon sources, the suppression rates of strain LX4 to R. solanacearum marked by green fluorescent protein were 18.34% (arabinose), 53.23% (xylose), 63.53% (ribose) and 52.09% (fructose). [Conclusion] In conclusion, the capacity of root exudates from LX4 was lower than that from Rs, which suggested the antagonist had disadvantage in its nutrition competition with the pathogen. However, the antagonist took the advantage of certain carbon substances secreted from tobacco roots to produce antagonistic substances, which were suppressive or toxic to the pathogen. There were both exploitation and interference competitions between the antagonist and pathogen. These results provide some theoretical evidences for the bio-control of tobacco bacterial wilt caused by R. solanacearum in the near future.