Abstract
Pathogens interaction with a host plant starts a set of immune responses that result in complex changes in gene expression and plant physiology. Light is an important modulator of plant defense response and recent studies have evidenced the novel influence of this environmental stimulus in the virulence of several bacterial pathogens. Xanthomonas citri subsp. citri is the bacterium responsible for citrus canker disease, which affects most citrus cultivars. The ability of this bacterium to colonize host plants is influenced by bacterial blue-light sensing through a LOV-domain protein and disease symptoms are considerably altered upon deletion of this protein. In this work we aimed to unravel the role of this photoreceptor during the bacterial counteraction of plant immune responses leading to citrus canker development. We performed a transcriptomic analysis in Citrus sinensis leaves inoculated with the wild type X. citri subsp. citri and with a mutant strain lacking the LOV protein by a cDNA microarray and evaluated the differentially regulated genes corresponding to specific biological processes. A down-regulation of photosynthesis-related genes (together with a corresponding decrease in photosynthesis rates) was observed upon bacterial infection, this effect being more pronounced in plants infected with the lov-mutant bacterial strain. Infection with this strain was also accompanied with the up-regulation of several secondary metabolism- and defense response-related genes. Moreover, we found that relevant plant physiological alterations triggered by pathogen attack such as cell wall fortification and tissue disruption were amplified during the lov-mutant strain infection. These results suggest the participation of the LOV-domain protein from X. citri subsp. citri in the bacterial counteraction of host plant defense response, contributing in this way to disease development.
Highlights
During plant-pathogen interactions, plants first recognize pathogen-associated molecular patterns (PAMPs) which are slowly evolving molecular structures unique to microbes, such as bacterial flagellin or fungal chitin [1]
We evaluated physiological changes in plants inoculated with both strains of X. citri subsp. citri and observed that the Xcc-LOV protein mutant strain produced a stronger decrease in host photosynthesis, as well as higher tissue disruption in the site of infection than the wild type (WT) strain
We described the construction of a mutant X. citri subsp. citri strain called Δlov, lacking a LOV-type blue-light photoreceptor (Xcc-LOV protein)
Summary
During plant-pathogen interactions, plants first recognize pathogen-associated molecular patterns (PAMPs) which are slowly evolving molecular structures unique to microbes, such as bacterial flagellin or fungal chitin [1]. Plants undergo physical changes such as cell wall thickening or callose deposition; biochemical modifications such as production of reactive oxygen species (ROS) and signaling compounds (salicylic acid, jasmonic acid, abscisic acid and ethylene) and synthesis of defense-related proteins and secondary metabolites such as phytoalexins, which prevent pathogen growth [4,5]. This complex set of responses involves a massive reprogramming of gene expression in the infected plant [6]. The reallocation of carbon metabolism favors the production of secondary compounds with antimicrobial activity, prioritizing plant defense responses, while down-regulation of photosynthesis restricts carbon source availability for the pathogen [7]
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