Abstract
The successful colonization of plant growth promoting rhizobacteria (PGPR) in the rhizosphere is an initial and compulsory step in the protection of plants from soil-borne pathogens. Therefore, it is necessary to evaluate the role of root exudates in the colonization of PGPR. Banana root exudates were analyzed by high pressure liquid chromatography (HPLC) which revealed exudates contained several organic acids (OAs) including oxalic, malic and fumaric acid. The chemotactic response and biofilm formation of Bacillus amyloliquefaciens NJN-6 were investigated in response to OA’s found in banana root exudates. Furthermore, the transcriptional levels of genes involved in biofilm formation, yqxM and epsD, were evaluated in response to OAs via quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). Results suggested that root exudates containing the OAs both induced the chemotaxis and biofilm formation in NJN-6. In fact, the strongest chemotactic and biofilm response was found when 50 μM of OAs were applied. More specifically, malic acid showed the greatest chemotactic response whereas fumaric acid significantly induced biofilm formation by a 20.7–27.3% increase and therefore biofilm formation genes expression. The results showed banana root exudates, in particular the OAs released, play a crucial role in attracting and initiating PGPR colonization on the host roots.
Highlights
Used as biocontrol agents and/or to induce plant growth[12,13,14]
In order to investigate the components of the root exudate which may show the attractant effects on the NJN-6 strain, the organic acids (OAs) were detected by high pressure liquid chromatography (HPLC)
We previously reported that the B. amyloliquefaciens NJN-6 strain showed an enhanced colonization of the banana root surface when plants were grown in vermiculite under sterile conditions[4]
Summary
Used as biocontrol agents and/or to induce plant growth[12,13,14]. In addition, once the PGPR reach the root system, biofilm formation is a prerequisite for successful root colonization by microorganisms[12]. It has been reported that low molecular weight OAs such as malic, citric and fumaric acid released by roots play initial roles in the recruitment of PGPR to the roots serving as carbon substrates and signaling molecules[21,24,25,26]. Organic acids, especially those involved in the tricarboxylic acid cycle have been shown to play functional roles as carbon and molecular signaling sources[27]. Results indicated that the malic acid and fumaric acid in root exudates could attract NJN-6 and promoted its biofilm formation by activating relevant genes
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