Abstract:Objective To screen the microbial strains producing volatile organic compounds (VOCs) with both broad-spectrum antagonistic activity and postharvest fruit preservation potential.Methods Endophytic bacterial strains were isolated and purified by the dilution plating method from the roots and branches of wild tea plants in Guangxi, China. Candidate strains were initially selected based on the number of functional traits via six types of functional media: cellulase, amylase, siderophore, organic phosphorus, inorganic phosphorus, and nitrogen-fixing media. The antagonistic activity of the strains against seven common plant pathogenic fungi was determined by the dual-culture assay, and thus the broad-spectrum antagonistic strains were screened out. Strains with superior overall performance were further selected to evaluate their antagonistic activity against the postharvest anthracnose pathogens—Colletotrichum fructicola and Colletotrichum musae—of mangoes and bananas. An in vitro banana preservation assay was conducted with the chemical preservative prochloraz as a positive control.Results Functional screening on selective media yielded 98 strains that simultaneously possessed four or more plant growth-promoting or stress-tolerance traits, including nitrogen fixation, phosphate solubilization, and siderophore production. In dual-culture assays against seven common plant pathogenic fungi, 18 broad-spectrum antagonistic strains significantly inhibiting at least five pathogens were screened out, among which four strains exhibited stable and strong antagonistic activity against all the seven pathogens. On this basis, two key indicators, number of functional traits and broad-spectrum inhibition rate, were comprehensively evaluated, and five strains with the best overall performance were finally selected for subsequent specific antagonism assays against the pathogens causing mango and banana anthracnose and for validation of their postharvest fruit preservation effects. In dual-culture assays, the inhibition rates of the tested strains against the two anthracnose pathogens ranged from 43.36% to 83.50%. In plate-on-plate assays, the VOCs produced by these strains exhibited inhibition rates of 56.80%-99.25% against C. fructicola and 54.50%-99.85% against C. musae, with several strains showing nearly 100.00% inhibitory activity against both pathogens. In vitro fruit preservation tests demonstrated that VOCs produced by the antagonistic strains delayed the postharvest decay of mangoes and bananas to varying degrees. Strain T-1-6 showed the most pronounced effect, extending the onset of visible banana decay to 21 days (the final decay grade was grade 0), and its preservation effect was comparable to that of the chemical preservative prochloraz, achieving approximately 50% control efficacy against surface molds on mangoes. Analysis of 16S rRNA gene and gyrB sequences revealed that all the five dominant antagonistic strains belonged to the genus Bacillus, including B. amyloliquefaciens, B. thuringiensis, B. cereus, and B. subtilis.Conclusion The VOCs-producing endophytic Bacillus strains from tea plants possess multiple functional traits and broad-spectrum antagonistic activity. This study provides promising candidate strains and a theoretical basis for the green disease control and biopreservation of postharvest tropical fruits such as mangoes and bananas.
