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The ecnA antitoxin is not only important for human pathogens: Evidences of its role in the plant pathogen Xathomonas citri subsp. citri

Xanthomonas citri subsp. citri causes citrus canker disease worldwide in most commercial varieties of citrus. Its transmission occurs mainly through wind-driven rain. Once on the leaf X. citri can epiphytically survive forming biofilm, which enhances persistence of the bacteria to different environmental stresses and play an important role in the early stages of host infection. Therefore, the study of genes involved in biofilm formation has been an important step towards the understanding of the bacterial strategy to survive and infect the plant host. In this work we show that ecnAB a Toxin-Antitoxin (TA) system, previously identified only in human bacterial pathogen, is conserved in many Xanthomonas spp. In general TA systems consist of a pair of genes in operon that encodes a stable toxin and an unstable antitoxin that, under normal conditions, binds to the toxin and blocks its activity. On the other hand, under stress the antitoxin is degraded, allowing the toxin to act decreasing cell growth and metabolism. When normal growth conditions are re-established, the antitoxin is produced, blocking the toxin and allowing the cells to grow. Thus, this mechanism represents an important bacterial strategy of survival under stress conditions. In this work, we show that in X. citri ecnAB is regulated by quorum sensing and it is involved in important processes such as biofilm formation, EPS production, and motility. In addition, we show that ecnAB plays a role in X. citri survival and virulence in plant host. IMPORTANCE Very little is known about TA systems in phytopathogenic bacteria. The ecnAB in special has been only studied in bacterial human pathogens. Here we showed that it is present in a wide range of the phytopathogen Xanthomonas spp., and moreover this is the first work that investigated the functional role of this TA system in Xanthomonas citri biology suggesting an important new role in adaptation and survival with implications in the bacterial pathogenicity.