Abstract
Endophytic bacteria associated with medicinal plants possess unique strategies that enhance growth and suvival of host plants, many of which are mediated by distinctive secondary metabolites. These bacteria and their secondary metabolites are important subjects for both basic and applied research aimed at sustainable agriculture. In the present study, 114 endophytic strains isolated from the wild ethnomedicinal plant Glycyrrhiza uralensis (licorice) were screened for their in vitro antimicrobial activities against common fungal pathogens of tomato (Fusarium oxysporum f. sp., Fulvia fulva, Alternaria solani), cotton (Fusarium oxysporum f. sp. Vesinfectum, Verticillium dahliae), pomegranite (Ceratocystis fimbriata), Cymbidinium (Colletotrichum gloeosporioides), and Tsao-ko (Pestalotiopsis microspora and Fusarium graminearum) and the common bacteria Staphylococcus aureus, Bacillus cereus, Salmonella enteritidis, and Escherichia coli. Several Bacillus strains, particularly Bacillus atrophaeus and Bacillus mojavensis, had a broad spectrum of antifungal and antibacterial activity. A total of 16 strains, selected based on broad antimicrobial activity, were shown to contain at least one putative secondary metabolite-encoding gene (i.e., polyketide synthase or non-ribosomal peptide synthetase) and/or one lytic enzyme (i.e., protease, cellulase, lipase, chitinase), which may be important mediators of antagonistic activity against pathogens. Five strains, representing Bacillus atrophaeus and Bacillus mojavensis, were selected for plant growth chamber experiments based on strong in vitro antifungal activities. All five strains significantly reduced disease severity in Arabidopsis thaliana plants challenged with V. dahlia infection. Gas-chromatography/mass-spectrometry analysis of cell-free extracts of Bacillus atrophaeus strain XEGI50 showed that at least 13 compounds were produced only during co-cultivation with V. dahlia, including putative compounds known to have antimicrobial activity, such as 1,2-benzenedicarboxylic acid, bis (2-methylpropyl) ester; 9,12-octadecadienoic acid (Z,Z)-, methyl ester; 9-octadecenoic acid, methyl ester, (E)-; and decanedioic acid, bis(2-ethylhexyl) ester. To our knowledge, this study is the first to report that bacteria isolated from G. uralensis have biocontrol abilities. Our findings provide new insights into the antimicrobial activities of natural endophytes, particularly B. atrophaeus, and suggest this species may a promising candidate as a biocontrol agent to confer resistance to Verticillium wilt disease and other phytopathogens in cotton and other crops.
Highlights
Fungal disease is the main threat to both crop yields and global food security (Fisher et al, 2012)
A total of 114 endophytes isolated from wild populations of G. uralensis, representing 18 genera and 34 species, were screened for their ability to inhibit four common bacteria, representing the Gram-positive phylum Firmicutes and the Gram-negative phylum Proteobacteria (Table 1)
We provide insights about plant beneficial traits of culturable endophytic bacteria associated with the medicinal plant G. uralensis
Summary
Fungal disease is the main threat to both crop yields and global food security (Fisher et al, 2012). Vascular wilts are devastating plant diseases that cause major losses to food crops and destroy natural ecosystems (Yadeta and Thomma, 2013). Two major genera of pathogenic fungi, Fusarium and Verticillium, enter their host plants through the roots or are transmitted by beetles and cause vascular wilts. Both are characterized by a wide host range (Juzwik et al, 2008; Harwood et al, 2011). In China, approximately 3 million hectares of cotton crops are affected by Verticillium wilt, accounting for an annual loss of yield of 10–30% (Bibi et al, 2013). Xinjiang Province, located in the northwest of China, produces 11% of the global cotton fiber yield and is disproportionately harmed by Verticillium wilt disease (Peng et al, 2016)
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