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

Abstract

24Xanthomonas citri subsp. citri causes citrus canker disease worldwide in most commercial 25 varieties of citrus. Its transmission occurs mainly through wind-driven rain. Once on the leaf X. 26 citri can epiphytically survive forming biofilm, which enhances persistence of the bacteria to 27 different environmental stresses and play an important role in the early stages of host infection. 28 Therefore, the study of genes involved in biofilm formation has been an important step towards 29 the understanding of the bacterial strategy to survive and infect the plant host. In this work we 30 show that ecnAB a Toxin-Antitoxin (TA) system, previously identified only in human bacterial 31 pathogen, is conserved in many Xanthomonas spp. In general TA systems consist of a pair of 32 genes in operon that encodes a stable toxin and an unstable antitoxin that, under normal 33 conditions, binds to the toxin and blocks its activity. On the other hand, under stress the antitoxin 34 is degraded, allowing the toxin to act decreasing cell growth and metabolism. When normal 35 growth conditions are re-established, the antitoxin is produced, blocking the toxin and allowing 36 the cells to grow. Thus, this mechanism represents an important bacterial strategy of survival 37 under stress conditions. In this work, we show that in X. citri ecnAB is regulated by quorum 38 sensing and it is involved in important processes such as biofilm formation, EPS production, and 39 motility. In addition, we show that ecnAB plays a role in X. citri survival and virulence in plant 40 host. 41 42 IMPORTANCE 43Very little is known about TA systems in phytopathogenic bacteria. The ecnAB in special has 44 been only studied in bacterial human pathogens. Here we showed that it is present in a wide 45 range of the phytopathogen Xanthomonas spp., and moreover this is the first work that 46 investigated the functional role of this TA system in Xanthomonas citri biology suggesting an 47 important new role in adaptation and survival with implications in the bacterial pathogenicity. 48 49 KEYWORDS: TA system, quorum sensing, biofilm, stress tolerance 50 51 52Phytopathogenic bacteria such as Xanthomonas spp. cause huge damages in many 53 different crops such as brassicas, rice, cassava, tomato and citrus (1). Xanthomonas citri subsp. 54 citri (X. citri) causes citrus canker disease worldwide in most commercial varieties of citrus (2), 55 for this reason there was an increase in the number of works focusing on the identification of 56 genes involved with the mechanism of pathogenicity and plant-pathogen interaction aiming the 57 development of strategies to control X. citri (3-8) X. citri is disperse by wind-driven rain and 58 then it adheres to the plant leaf surfaces (9). The stable adhesion to the host is the first step 59 towards biofilm formation (10), which enhances the epiphytic persistence of the bacteria on host 60 leaves, plays an important role in the early stages of infection (11), and confers resistance to 61 different environmental stresses,...