The Detriment of Salicylic Acid to the Pseudomonas savastanoi pv. phaseolicola Proteome

Salicylic acid, a natural product, is the major hormonal regulator of the plant immune system. Salicylic acid also has antibacterial activity that is not completely elucidated. To gain a better understanding of this, pv. , a bacterial pathogen of beans, was exposed to subinhibitory amounts of salicylic acid and then examined using quantitative mass spectrometry. Among the 2,185 proteins quantified, there were pronounced increases in p-hydroxybenzoic acid efflux pumps and multidrug efflux pumps. By contrast there were significant decreases in porin proteins, high-osmolarity response proteins, and protein components of the type VI secretion system. In addition, there were alterations in enzymes likely affecting the production of alginate which is needed for infection. Furthermore, there was a decrease in an enzyme needed to detoxify methylglyoxal. Assays confirmed a reduction in alginate production and an increase in cellular methylglyoxal concentrations after salicylic acid treatment. Culture assays demonstrated that salicylic acid altered bacterial growth curves more so than other hydroxylated benzoic acid isomers. These data reveal that salicylic acid is antibiotic and that pv. significantly alters its proteome in response to salicylic acid . Similar alterations to the bacterial proteome occur in beans during an immune reaction when salicylic acid increases at the site of infection. Thus, in beans, salicylic acid likely deters bacterial infection by adversely altering the bacterial proteome.