The detriment of salicylic acid to the Pseudomonas savastanoi pv. phaseolicola proteome

Authors: Cooper, Bret

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2022
Publication Date: 5/25/2022
Citation: Cooper, B. 2022. The detriment of salicylic acid to the Pseudomonas savastanoi pv. phaseolicola proteome. Molecular Plant-Microbe Interactions. https://doi.org/10.1094/MPMI-05-22-0104-R.
DOI: https://doi.org/10.1094/MPMI-05-22-0104-R

Interpretive Summary: Halo blight disease, caused by a bacterium, reduces harvests of the dry, edible common bean. Natural resistance genes in the bean plant can provide protection against some strains. This resistance is controlled by the plant hormone salicylic acid that accumulates at the site of infection. Prior research results also imply that salicylic acid has antibiotic activity, but the mode of action is not clear. In this study, USDA-ARS scientists in Beltsville, Maryland, used mass spectrometry, an analytical technique, to measure the changes in proteins after bacteria were exposed to salicylic acid. The results show that the bacterium is actively engaged in trying to eliminate salicylic acid from cells and that salicylic acid decreases enzymes the bacterium needs to infect plants and to protect its metabolism from the antibiotic effects of salicylic acid. These findings will be of interest to scientists in the government, at universities, and at private institutions who want to understand how beans and other plants protect themselves from pathogen infection.

Technical Abstract: 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, Pseudomonas savastanoi pv. phaseolicola, 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 leading to a decrease in the production of glutamate, glycine betaine, trehalose, and alginate. Furthermore, there was a decrease in an enzyme needed to detoxify methylglyoxal. Metabolomic assays confirmed a reduction in alginate production and an increase in cellular methylglyoxal concentrations after salicylic acid treatment. Culture assay tests with benzoic acid derivatives demonstrated that salicylic acid altered bacterial growth curves more so than other hydroxylated benzoic acid isomers. These data reveal that salicylic is antibiotic and that P. savastanoi pv. phaseolicola significantly alters its proteome to keep salicylic out of the cell in vitro. 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.