## 2 Virulence and Occurrence of Secretion Systems in _Xanthomonas_ spp. _Xanthomonas_ species encode six different protein secretion systems (types 1 to 6) and abundance, phylogenetic lineages, and the number of copies of each protein secretion system varies among species or pathotypes, and occurrence of all six secretion systems in a single species is common [2, 4]. Type 6 and type 4 secretion systems (T4 SS and T6 SS) of various lineages are found in many species and have been suggested to play a role in the persistence of the bacteria in the environment. Chromosome-encoded T4 SSof _Xanthomonas citri_ subsp. _citri_ (_Xcci_) was reported to provide the capacity of contact-dependent bacterial killing through the delivery of toxins to cells of neighboring bacteria [4], whereas T6 SS was identified to prevent predation by the soil amoeba through an unknown mechanism [5]. Most _Xanthomonas_ strains harbor two type 2 secretion systems (T2 SS) classified as _xcs_ and _xps_. The contribution of the _xcs_ T2 SS to pathogenicity on plants or persistence in the environment is poorly understood, and the system is absent in several _Xanthomonas_ species such as _Xanthomonas oryzae_ pv. _oryaze_ (_Xoo_) [2]. The _xps_ T2 SS is conserved in the genomes of all sequenced _Xanthomonas_ species and is required for full virulence of _Xoo_, _Xanthomonas eurescatoria_ (_Xeu_), and _Xcci_[6; 7; 8]. The _xps_ T2 SS enables the secretion of numerous secreted hydrolases [9]. Secreted hydrolases such as xylanase, cellulases, esterases, and pectinases function as virulence factors in many _Xanthomonas_ species and are required for full virulence on host plants [9; 10; 11; 12]. However, cell wall hydrolysis by these hydrolases can also elicit immune responses through host recognition of damage-associated molecular patterns [13; 14]. Flagellar and injectivome type 3 secretion systems are abundant among _Xanthomonas_ species [3]. The flagellar T3 SS (FT3 SS) is required for motility, biofilm formation, and chemotactic movement [15; 16; 17]. Disruption of the FT3 SS affects entry and infectivity of the bacteria and is mostly associated with the epiphytic stage of _Xanthomonas_ life cycle [1; 17]. Injectivome type 3 secretion system (T3 SS) enables the translocation of effector proteins from bacteria to eukaryotic cells which affect host signaling and metabolism [18]. Many bacteria utilize this system to establish pathogenic or symbiotic interactions with their eukaryotic hosts. A hypersensitive reaction and pathogenicity 2 (_ltrp_2) family injectivome T3 SS [19; 20] is found in the majority of pathogenic _Xanthomonas_ strains and is considered as one of the main pathogenicity factors of the bacteria [3; 21]. Almost all pathogenic group 2 _Xanthomonas_ species harbor T3 SS genes that are localized in a \(\sim\)30 kb chromosomal pathogenicity island and composed of approximately 20 core structural component coding genes in six transcriptional units (A to F), while additional T3 SS accessory and effector coding genes within the pathogenicity island vary between species or pathotypes [22] (Figure 1). Disruption of any key structural component of the T3 SS results in the inability to cause disease, colonize host plant, and induce hypersensitive response (HR) in resistant host varieties [3]. In addition, _hrp_-dependent pathogenic _Xanthomonas_ strains encode on average 25-35 T3 SS effectors (T3 Es) that were reported to function in manipulating host immune signaling, liquid accumulation in the intercellular spaces, and nutrient accessibility [2; 3; 21]. The biochemical properties, host targets, and functions of T3 Es were previously reviewed [2; 3; 21]. Several group 2 xanthomomads, such as _X. maliensis_, _X. cannabis_, and multiple _X. arboricola_ strains that lack T3 SS or harbor a reduced arsenal of T3 Es were identified in the past decade [23; 24; 25]. These bacteria display reduced growth _in planta_, do not cause symptoms in host plants, and are considered as commensal strains.