Salicylic acid and phytoalexin induction by a bacterium that causes halo blight in beans

Authors: Cooper, Bret; Campbell, Kimberly; Garrett, Wesley

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/8/2022
Publication Date: 2/11/2022
Citation: Cooper, B., Campbell, K., Garrett, W.M. 2022. Salicylic acid and phytoalexin induction by a bacterium that causes halo blight in beans. Phytopathology. https://doi.org/10.1094/PHYTO-12-21-0496-R.
DOI: https://doi.org/10.1094/PHYTO-12-21-0496-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 plant hormones which control the production of phytoalexins that are antibiotic to bacteria. Which hormones and which phytoalexins are produced in beans are not entirely known. In this study, USDA-ARS scientists in Beltsville, Maryland, used mass spectrometry, an analytical technique, to measure the amounts of hormones and phytoalexins in bean leaves inoculated with a halo blight strain that triggers resistance and with another strain that infects and causes disease. Salicylic acid was found to be the primary hormone induced during resistance and phytoalexins daidzein, genistein, kievitone, coumestrol, and resveratrol also were found to be induced at the same time. Bacterial culture assays revealed that salicylic acid, daidzein, genistein, kievitone, coumestrol, and resveratrol inhibited bacterial growth. Hence, the results imply that these phytoalexins likely work in combination with salicylic acid to restrict replication of P. savastanoi pv. phaseolicola in beans. These findings will be of interest to scientists in the government, at universities, and at private institutions who want to understand how beans protect themselves from halo blight infection.

Technical Abstract: Pseudomonas savastanoi pv. phaseolicola is a bacterium that causes halo blight in beans. Different varieties of beans have hypersensitive resistance to specific races of P. savastanoi pv. phaseolicola. During hypersensitive resistance, also known as effector-triggered immunity, beans produce hormones that signal molecular processes leading to the production of phytoalexins, which are presumed to be antibiotic to bacteria. To shed light on hormone and phytoalexin production during immunity, we inoculated beans with virulent and avirulent races of P. savastanoi pv. phaseolicola. We then monitored the accumulation of salicylic acid, the primary hormone that controls immunity in plants, and other plant hormones including jasmonate, methyljasmonate, indole-3-acetic acid, abscisic acid, cytokinin, gibberellic acid, and 1-aminocyclopropane-1-carboxylic acid. We found that salicylic acid, but no other examined hormone, consistently increased at sites of infection to greater levels in resistant beans compared to susceptible beans at 4 days after inoculation. We then monitored the accumulation of 10 candidate bean phytoalexins by mass spectrometry. Phytoalexins daidzein, genistein, kievitone, coumestrol, and resveratrol increased alongside salicylic acid in resistant beans but not in susceptible beans. In vitro culture assays revealed that salicylic acid, daidzein, genistein, coumestrol, and resveratrol inhibited P. savastanoi pv. phaseolicola race 5 culture growth. These results demonstrate that daidzein, genistein, coumestrol, and resveratrol may be regulated by salicylic acid during effector-triggered immunity to halo blight. This is the first report of antibiotic activity for daidzein, genistein, and resveratrol to P. savastanoi pv. phaseolicola. These phytoalexins likely work in combination with salicylic acid to restrict replication of P. savastanoi pv. phaseolicola in beans.