Abstract
Understanding the role of chemotaxis in ecological interactions between plants and microbes in the rhizosphere is necessary to optimize biocontrol strategies targeting plant soil-borne diseases. Therefore, we examined and profiled the antagonistic endophytic bacteria (AEB) population with chemotaxis potential in the medicinal plant Panax notoginseng using a cheA gene-based approach coupled with 16S rRNA sequencing. Phylogenetic analysis of the chemotactic AEB (CAEB) community in P. notoginseng enabled the identification of 56 CAEB strains affiliated with 30 species of Actinobacteria, Firmicutes, and Proteobacteria; Firmicutes, especially Bacillus, were predominant. We then systematically quantified the chemotactic response profiles of CAEB toward five organic acid (OA) attractants: citric acid, fumaric acid (FA), malic acid, oxalic acid, and succinic acid. Further hierarchical cluster analysis revealed that the chemotaxis of CAEB to the same attractant exhibited different patterns among not only genera but also species and even strains of the same species. Following chemotaxis and hierarchical analysis, we selected the strongest chemoattractant, fumaric acid (FA), as the target for evaluating the effects of OAs on the representative CAEB strain Bacillus amyloliquefaciens subsp. plantarum YP1. Application of FA significantly stimulated the chemotaxis ability and growth of YP1, and increased the transcript levels of cheA and biocontrol-related genes in YP1. This is the first study to characterise the diversity of chemotaxis profiles toward OAs in natural bacterial assemblages of P. notoginseng and to highlight how FA promotes the biocontrol-related traits of P. notoginseng-associated CAEB.
Highlights
The objectives of the present study were as follows: (1) profile the abundance and diversity of the chemotactic-competent bacteria in an antagonistic endophytic bacteria (AEB) population of P. notoginseng with a cheA gene-based approach combined with 16S rRNA sequencing; (2) systematically evaluate the chemotaxis capabilities of chemotactic AEB (CAEB) populations in P. notoginseng using five organic acid (OA) and characterise the diversity of chemotaxis profiles of the CAEB population to tested OAs, emphasizing the genus Bacillus; (3) investigate the effects of OAs on biocontrol-related traits of P. notoginseng-associated CAEB by targeting the main chemoattractant in tested OAs to evaluate its effect on chemotaxis and the growth and transcript levels of genes involved in the biocontrol activity of representative CAEB strains
We previously reported that 104 endophytic strains of P. notoginseng screened from leaf, petiole, stem, root, and seed samples grown in Wenshan region of Yunnan Province, China exhibited antagonistic properties against at least one root-rot disease (RRD) pathogen (Fusarium oxysporum, Ralstonia sp., and Meloidogyne hapla) (Ma et al 2013), but previous studies have indicated that the abundance, diversity, and species assemblage of endophytes can be strongly influenced by the geographic regions they occur in (Deng et al 2011; Yaish et al 2015)
600 endophytic bacteria were isolated from different tissues of P. notoginseng grown in Luxi County, different ecological region from Wenshan County in Yunnan Province, China, and were further evaluated in vitro for their antagonistic activity towards the pathogens F. oxysporum, Ralstonia sp., and M. hapla
Summary
H. Chen (Araliaceae) is a well-known traditional Chinese medicinal herb that has been cultivated in southwestern China for more than 400 years (Guo et al 2010). A long growth period, and shade and humid planting conditions provide a favorable environment for a number of soil-borne pathogens to survive (Chen et al 2001; Wang et al 1998). The most destructive soil-borne disease in the P. notoginseng-growing areas of China is root-rot disease (RRD), which is caused by bacteria, fungal pathogens, and parasitic nematodes alone or simultaneously, and results in a serious reduction in the yield and quality of its raw active ingredients (Mao et al 2013; Miao et al 2006). RRD is very difficult to control, mainly because of the complicated interaction among different pathogens (Luo et al 1997); it is desirable to replace chemicals with biocontrol agents (BCAs) that are safe and environmentally friendly whenever possible
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