Interspecies Expression of an EF-HAND CALCIUM-BINDING PROTEIN in Xanthomonas perforans Leads to Reduced Virulence and Decreased Immune Evasion in Tomato Plants.

Many phytopathogenic bacteria require a type III secretion system (TTSS) to activate effector-triggered immunity (ETI). We identified a calcium-binding protein, EfhX_Xfa, in the citrus pathogen Xanthomonas citri subsp. aurantifolii that does not require a TTSS to activate reactive oxygen species (ROS) and elicit a hypersensitive reaction (HR) in tomato leaves following infection. Purified, recombinant EfhX_Xfa was shown to bind two moles of calcium per mole of protein, whereas mutation of the first of two EF-hands did not bind calcium. EfhX_Xfa expression was determined to be inducible in hrp-inducing medium. Additionally, growth of X. perforans transconjugants with and without the efhX_Xfa gene in hrp-inducing medium differed in intracellular calcium concentration; the transconjugant without efhX_Xfa yielded higher cell pellet masses and higher increased intracellular calcium concentrations relative to cells expressing EfhX_Xfa. An EfhX_Xfa homolog, EfhX_Xe, present in the pepper pathogen X. euvesicatoria, triggered ROS production and an HR in tomato leaves when expressed in the tomato pathogen X. perforans and is a host-limiting factor. Interestingly, all tested X. perforans and X. euvesicatoria strains pathogenic on tomato contain a stop codon immediately upstream of the first EF-hand domain in the efhX_Xe gene, whereas most X. euvesicatoria strains pathogenic on pepper do not. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.