BackgroundPlants are capable of building up beneficial rhizosphere communities as is evidenced by disease-suppressive soils. However, it is not known how and why soil bacterial communities are impacted by plant exposure to foliar pathogens and if such responses might improve plant performance in the presence of the pathogen. Here, we conditioned soil by growing multiple generations (five) of Arabidopsis thaliana inoculated aboveground with Pseudomonas syringae pv tomato (Pst) in the same soil. We then examined rhizosphere communities and plant performance in a subsequent generation (sixth) grown in pathogen-conditioned versus control-conditioned soil. Moreover, we assessed the role of altered root exudation profiles in shaping the root microbiome of infected plants.ResultsPlants grown in conditioned soil showed increased levels of jasmonic acid and improved disease resistance. Illumina Miseq 16S rRNA gene tag sequencing revealed that both rhizosphere and bulk soil bacterial communities were altered by Pst infection. Infected plants exhibited significantly higher exudation of amino acids, nucleotides, and long-chain organic acids (LCOAs) (C > 6) and lower exudation levels for sugars, alcohols, and short-chain organic acids (SCOAs) (C ≤ 6). Interestingly, addition of exogenous amino acids and LCOA also elicited a disease-suppressive response.ConclusionCollectively, our data suggest that plants can recruit beneficial rhizosphere communities via modification of plant exudation patterns in response to exposure to aboveground pathogens to the benefit of subsequent plant generations.Electronic supplementary materialThe online version of this article (10.1186/s40168-018-0537-x) contains supplementary material, which is available to authorized users.
Bacillus amyloliquefaciens NJN-6 produces volatile compounds (VOCs) that inhibit the growth and spore germination of Fusarium oxysporum f. sp. cubense. Among the total of 36 volatile compounds detected, 11 compounds completely inhibited fungal growth. The antifungal activity of these compounds suggested that VOCs can play important roles over short and long distances in the suppression of Fusarium oxysporum. Microbial antagonist strains capable of producing both nonvolatile compounds and volatile compounds (VOCs), which exhibit strong inhibitory activity against plant pathogens, have received much attention (4, 14). These antagonists include bacteria, such as Pseudomonas spp. (5), and nonpathogenic fungi like Trichoderma spp. (11). The release of VOCs by soil microbes has been reported to promote plant growth (13), display nematicidal activity (7), and induce systemic resistance in crops (3). Previous researchers also found that VOCs produced by bacteria could inhibit the growth (6) and the spore germination of pathogenic fungi (10), suggesting that VOCs produced by bacteria could be a mechanism of biocontrol against some soilborne fungal diseases.Fusarium oxysporum is a well-known soilborne fungus, and some strains of F. oxysporum are pathogenic to plants and are difficult to control; however, biological methods may be a reliable alternative to chemical methods for controlling soilborne fungal growth. For applications in agriculture, the Bacillus species are considered important biological control agents. Bacillus amyloliquefaciens (NJN-6), isolated from the rhizosphere soil of healthy banana plants, acts as an efficient antagonist against F. oxysporum f. sp. cubense by producing several antibiotics (15,16). In this study, we characterized the volatile organic compounds produced by strain NJN-6. We used solid-phase microextraction (SPME) combined with gas chromatography-mass spectrometry (GC-MS) to extract and identify the VOCs. Finally, we identified antagonistic VOCs as those that reduced the growth and inhibited the spore germination of F. oxysporum.Microorganisms and culture conditions. The antagonistic strain NJN-6 was identified as B. amyloliquefaciens (CGMCC [China General Microbiology Culture Collection Center] accession no. 3183) by 16S rRNA sequencing (15). The fungal strain F. oxysporum f. sp. cubense, which exhibited high virulence in banana plants, was used as the target fungus.Antagonistic assay of VOCs against fungi. One compartment of the divided plates containing modified MS medium (with 1.5% [wt/vol] agar, 1.5% [wt/vol] sucrose, and 0.4% [wt/vol] TSA [3]) was inoculated with NJN-6, except for control plates. Another compartment containing PDA medium was used for F. oxysporum to test growth inhibition, or 100 l of spore solution (10 8 CFU/ ml) was spread evenly to test the ability of the VOCs to inhibit the spore germination of fungi, or 10 g of diseased banana field soil from Ledong, Hainan Province, was added to one compartment. The plates were incubated at 28°C for 3 days, and then the diame...
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.
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