A Highly-Conserved Single-Stranded DNA-Binding Protein in <em>Xanthomonas</em> Functions as a Harpin-Like Protein to Trigger Plant Immunity

<div><p>Harpins are produced by Gram-negative phytopathogenic bacteria and typically elicit hypersensitive response (HR) in non-host plants. The characterization of harpins in <em>Xanthomonas</em> species is largely unexplored. Here we demonstrate that <em>Xanthomonas</em> produce a highly conserved single-stranded DNA-binding protein (SSB_X) that elicits HR in tobacco as by harpin Hpa1. SSB_X, like Hpa1, is an acidic, glycine-rich, heat-stable protein that lacks cysteine residues. SSB_X-triggered HR in tobacco, as by Hpa1, is characterized by the oxidative burst, the expression of HR markers (<em>HIN1</em>, <em>HSR203J</em>), pathogenesis-related genes, and callose deposition. Both SSB_X- and Hpa1-induced HRs can be inhibited by general metabolism inhibitors actinomycin D, cycloheximide, and lanthanum chloride. Furthermore, those HRs activate the expression of <em>BAK1</em> and <em>BIK1</em> genes that are essential for induction of mitogen-activated protein kinase (MAPK) and salicylic acid pathways. Once applied to plants, SSB_X induces resistance to the fungal pathogen <em>Alternaria alternata</em> and enhances plant growth. When ssb_X<em>was deleted in</em> X. oryzae <em>pv.</em> oryzicola, <em>the causal agent of bacterial leaf streak in rice, the resulting</em> ssb_Xoc<em>mutant was reduced in virulence and bacterial growth</em> in planta, <em>but retained its ability to trigger HR in tobacco. Interestingly,</em> ssb_Xoc<em>contains an imperfect PIP-box (plant-inducible promoter) and the expression of</em> ssb_Xoc<em>is regulated by</em> HrpX, which belongs to the AraC family of transcriptional activators. Immunoblotting evidence showed that SSB_x secretion requires a functional type-III secretion system as Hpa1 does. This is the first report demonstrating that <em>Xanthomonas</em> produce a highly-conserved SSB_X that functions as a harpin-like protein for plant immunity.</p>