Toxin-Antitoxin System HigBA in Xylella fastidiosa Is Expressed at Low Temperature but Does Not Impact Virulence or Grapevine Recovery in Greenhouse Trials

Abstract

Xylella fastidiosa has multiple chromosomally encoded type II toxin-antitoxin (TA) systems, several of which have unknown functions in the bacterial lifecycle. In many bacterial species, type II TA systems are associated with bacterial stress survival under various detrimental environmental conditions, as well as influencing disease associated phenotypes such as biofilm development and expression of virulence factors. Chromosomal TA systems are often found in genomic regions with evidence of phage insertion, pointing to potential acquisition of these elements by horizontal gene transfer. One of the TA systems found in X. fastidiosa, higBA is transcriptionally expressed at higher levels at 15°C than at 28°C. However, a higBA deletion mutant (ΔhigBA) in X. fastidiosa subsp. fastidiosa Stag’s Leap is not deficient in growth or viability under in vitro cold stress and is able to infect and cause disease in grapevines comparable to the wild type strain. Grapevines infected with X. fastidiosa ΔhigBA recover from infection following cold treatment at 4°C at a similar rate to wild type infected plants, suggesting this TA system does not contribute significantly to X. fastidiosa survival at low temperature in planta. Although a definitive function for HigBA in X. fastidiosa has yet to be determined, this study adds to a growing body of work describing functions and expression dynamics of TA systems in plant pathogenic bacteria.