Spraying of Leaf-Colonizing Bacillus amyloliquefaciens Protects Pepper from Cucumber mosaic virus

Beneficial plant-associated bacteria protect host plants against pathogens, including viruses. However, leaf-associated (phyllosphere) bacteria have rarely been investigated as potential triggers of plant systemic defense against plant viruses. We found that leaf-colonizing Bacillus amyloliquefaciens strain 5B6 (isolated from a cherry tree leaf) protected Nicotiana benthamiana and pepper plants against Cucumber mosaic virus (CMV). In a field trial, treatment with strain 5B6 significantly reduced the relative contents of CMV coat protein RNA compared with the water control over a 3-year period, as revealed by quantitative reverse-transcription polymerase chain reaction. The expression of Capsicum annuum pathogenesis-related (PR) genes CaPR4, CaPR5, and CaPR10 was upregulated in field-grown pepper plants treated with strain 5B6. In addition, the accumulation of two naturally occurring viruses, Broad bean wilt virus and Pepper mottle virus, was reduced by foliar treatment with strain 5B6, which is similar to the results for benzothiadiazole treatment as a positive control. Taken together, the results suggest that strain 5B6 has strong potential for protecting plants against viruses by increasing defense priming of salicylic acid and jasmonic acid signaling in pepper under field conditions. This is the first report of the protection of a plant against viral diseases by foliar application of leaf-associated bacilli.