*Article* **Chemotactic Responses of** *Xanthomonas* **with Different Host Ranges**

**Marta Sena-Vélez 1,2 , Elisa Ferragud <sup>2</sup> , Cristina Redondo <sup>2</sup> , James H. Graham <sup>3</sup> and Jaime Cubero 2,\***


**Abstract:** *Xanthomonas citri* pv. *citri* (*Xcc*) (*X. citri* subsp. *citri*) type A is the causal agent of citrus bacterial canker (CBC) on most *Citrus* spp. and close relatives. Two narrow-host-range strains of *Xcc*, A<sup>w</sup> and A\*, from Florida and Southwest Asia, respectively, infect only Mexican lime (*Citrus aurantifolia*) and alemow (*C. macrophylla*). In the initial stage of infection, these xanthomonads enter via stomata to reach the apoplast. Herein, we investigated the differences in chemotactic responses for wide and narrow-host-range strains of *Xcc* A, *X. euvesicatoria* pv. *citrumelonis* (*X. alfalfae* subsp. *citrumelonis*), the causal agent of citrus bacterial spot, and *X. campestris* pv. *campestris*, the crucifer black rot pathogen. These strains of *Xanthomonas* were compared for carbon source use, the chemotactic responses toward carbon compounds, chemotaxis sensor content, and responses to apoplastic fluids from *Citrus* spp. and Chinese cabbage (*Brassica pekinensis*). Different chemotactic responses occurred for carbon sources and apoplastic fluids, depending on the *Xanthomonas* strain and the host plant from which the apoplastic fluid was derived. Differential chemotactic responses to carbon sources and citrus apoplasts suggest that these *Xanthomonas* strains sense host-specific signals that facilitate their location and entry of stomatal openings or wounds.

**Keywords:** *Citrus*; xanthomonads; bacterial motility; MCPs; methyl-accepting chemotaxis proteins
