The volatile organic compounds (VOCs) produced by endophytic bacteria have a significant role in the control of phytopathogens. In this research, the VOCs produced by the endophytic bacteria Streptomyces sp. B86, Pantoea sp. Dez632, Pseudomonas sp. Bt851, and Stenotrophomonas sp. Sh622 isolated from healthy sugar beet (Beta vulgaris) and sea beet (Beta maritima) were evaluated for their effects on the virulence traits of Bacillus pumilus Isf19, the causal agent of harvested sugar beet root rot disease. The gas chromatographymass spectrometry (GC-MS) analysis revealed that B86, Dez632, Bt851, and Sh622 produced 15, 28, 30, and 20 VOCs, respectively, with high quality. All antagonistic endophytic bacteria produced VOCs that significantly reduced soft root symptoms and inhibited the growth of B. pumilus Isf19 at different levels. The VOCs produced by endophytic bacteria significantly reduced swarming, swimming, and twitching motility by B. pumilus Isf19, which are important to pathogenicity. Our results revealed that VOCs produced by Sh622 and Bt851 significantly reduced attachment of B. pumilus Isf19 cells to sugar beetroots, and also all endophytic bacteria tested significantly reduced chemotaxis motility of the pathogen toward root extract. The VOCs produced by Dez632 and Bt851 significantly upregulated the expression levels of defense genes related to soft rot resistance. Induction of PR1 and NBS-LRR2 genes in sugar beetroot slices suggests the involvement of SA and JA pathways, respectively, in the induction of resistance against pathogen attack. Based on our results, the antibacterial VOCs produced by endophytic bacteria investigated in this study can reduce soft rot incidence.

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Several bacteria play essential roles in causing post-harvest sugar beet root rot. In 2018 and 2019, a survey in field and storage revealed the presence of sugar beet dry rot of root. Seventeen bacteria were isolated from the harvested roots from various geographic locations in West Azerbaijan, Iran. Pathogenicity test in sugar beet root slices revealed that isolate Buk12 showed dry rot similar to those observed in the storage after seven days. Based on the petiole test, Buk12 produced slight discoloration in leaf petiole that extended into the leaf. Molecular identification showed that strain Buk12 had a high similarity to Pseudomonas thivervalensis based on partial nucleotide sequencing of the 16S rRNA gene. Endophytic bacteria produce secondary metabolites (SMs) and volatile organic compounds (VOCs) that have a significant role in the control of phytopathogens. In this study, the antagonistic activity of endophytic bacteria Streptomyces sp. B86, Bacillus sp. S411, Pseudomonas sp. B451, Bacillus sp. Andi542, Streptomyces sp. Sh800, and Staphylococcus sp. Bt870 were screened and their VOCs effects on the virulence traits of Pseudomonas thivervalensis Buk12 were evaluated. All endophytic bacteria tested showed antagonistic activity against the bacterial pathogen in vitro. The results of enzymatic activity, HCN production, and siderophore production were varied amongst strains. The VOCs emitted from endophytic bacteria significantly reduced dry root symptoms and the growth of P. thivervalensis Buk12 at different levels. Our results revealed that VOCs produced by all endophytic bacteria significantly reduced swarming motility, chemotaxis behavior, and attachment of P. thivervalensis Buk12 cells to sugar beet roots. The VOCs produced by endophytic bacteria except for Streptomyces sp. B86 significantly reduced twitching motility by P. thivervalensis Buk12. The influence of VOCs on biofilm formation by bacterial pathogen was varied amongst endophytic bacteria. The gas chromatography-mass spectrometry (GC-MS) analysis revealed that strains B86, S411, B451, Andi542, Sh800, and Bt870 produced 13, 13, 15, 23, 16, and 24 VOCs, respectively with high quality. Our findings provide new information regarding the potential of VOCs with antibacterial activity and as a biocontrol tool against postharvest bacterial root rot. To the best of our knowledge, this is the first report about bacteria as sugar beet dry rot agent from Iran and the isolation of endophytic bacteria from sea beet plants with biocontrol activity.