Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2016
Publication Date: 3/17/2017
Citation: Burbank, L.P., Stenger, D.C. 2017. The DinJ/RelE toxin-antitoxin system suppresses bacterial proliferation and virulence of Xylella fastidiosa in grapevine. Phytopathology. 107:388-394.
Interpretive Summary: Xylella fastidiosa, the bacterial pathogen causing Pierce’s disease of grapes, grows inside the water-conducting tissue of the plant leading to scorching symptoms and vine decline. Living bacterial cells are generally found in plant tissue with smaller bacterial populations and minimal scorching symptoms, whereas heavily symptomatic tissue is not conducive to pathogen survival. It appears that the pathogen has developed specific mechanisms to keep the bacterial population under control during infection of the plant to increase long-term survival and spread. One such population control mechanism which is described here involves production of a toxic protein (RelE) that reduces the bacterial growth rate under certain conditions. RelE toxin activity is controlled by production of a specific antitoxin protein DinJ. The relative amount of toxin and antitoxin produced changes under conditions similar to the plant environment leading to a reduction in pathogen growth. A strain of X. fastidiosa which is deficient in toxin and antitoxin (RelE and DinJ) grows excessively in the plant, leading to more rapid symptom development and death of the host. In a natural situation it is likely that the reduction in growth rate caused by RelE toxin is advantageous to the pathogen, as it will facilitate bacterial spread within the plant and probability of transmission by the insect vector.
Technical Abstract: Xylella fastidiosa, the causal agent of Pierce’s disease of grapes, is a slow-growing, xylem-limited, bacterial pathogen. Disease progression is characterized by systemic spread of the bacterium through xylem vessel networks, causing leaf scorching symptoms, senescence, and vine decline. It appears to be advantageous to this pathogen to avoid excessive blockage of xylem vessels, as living bacterial cells are generally found in plant tissue with low bacterial cell density and minimal scorching symptoms. The DinJ/RelE toxin-antitoxin (TA) system is characterized here for a role in controlling bacterial proliferation and population size during plant colonization. The DinJ/RelE locus is transcribed from two separate promoters, allowing for co-expression of antitoxin DinJ with endoribonuclease toxin RelE, in addition to independent expression of RelE. The ratio of antitoxin:toxin expressed is dependent on bacterial growth conditions, with lower amounts of antitoxin present under conditions designed to mimic grapevine xylem sap. A knockout mutant of DinJ/RelE exhibits a hypervirulent phenotype, with higher bacterial populations, and increased symptom development and plant decline. It is likely that DinJ/RelE acts to prevent rapid population growth, contributing to the ability of the pathogen to spread systemically and be acquired by the insect vector.