Controlling soft rot of postharvest chilli pepper (Capsicum annuum L.) by an antagonist Bacillus amyloliquefaciens S917: Efficacy and action mode

Abstract

Pectobacterium carotovorum, the causal agent of soft rot, is responsible for significant economic damage in storage and transportation of postharvest chilli pepper. In this study, an antagonist S917, identified as Bacillus amyloliquefaciens based on the 16S rDNA sequence, was screened from 1400 strains for antagonistic activity towards Pectobacterium carotovorum subsp. brasiliense XC1 (Pcb XC1). The in vitro antibacterial result showed that cells and peptide extract of B. amyloliquefaciens S917 had good inhibitory activity against Pcb XC1. Based on draft genome sequences of B. amyloliquefaciens S917, five bacteriocins (Thusin_alpha, Thusin_beta2, Amylocyclicin, ComX1, LCI-like) were identified by BAGEL4 database and four antibacterial metabolites (Bacillaene, Macrolactin H, Bacilysin, Bacillibactin) were identified by antiSMASH database. The in vivo disease control result of chilli pepper showed that cells of B. amyloliquefaciens S917 performed better in reducing the rotting incidence and lesion diameter. Result of gas chromatography-ion mobility spectrometry (GC-IMS) showed that B. amyloliquefaciens S917 cells (BSC) treatment protected the original volatile aroma of chilli pepper. However, the control had richer volatile compounds related to soft rot, including methylbutanoate, dimethyl sulfide, methylbutanal, 3-methyl-2-butenal, pentan-1-ol, 3-methylbutan-1-ol, hexanol. Further, action mode of BSC was investigated. The number of green fluorescent protein labelled Pcb XC1 in chilli pepper was reduced by more than 99% after BSC treatment. Result of scanning electron microscopy also showed that BSC treatment prevented the damage of pepper epidermal structure caused by Pcb XC1. Moreover, the result of RNA-seq showed that BSC treatment triggered defense responses of chilli pepper. Pathways of plant pathogen interaction, phenylpropanoid biosynthesis and metabolism, plant hormone synthesis and signal transduction, MAPK signaling, WRKY transcription factor, and cell wall synthesis were activated. This study confirmed that B. amyloliquefaciens S917 can be used as an antagonist to control soft rot of postharvest chilli pepper.