Role of a lov protein from Xantomonas citri subsp. citri in bacterial strategies to counteract plant inmune responses?

KRAISELBURD I, DAURELIO LD, CORTADI AA TALÓN M, TADEO F, ORELLANO EG

Rosario

Congreso; IX Congreso Argentino de Microbiologia General; 2013

Plants are constantly exposed to pathogen attack and they possess a complex set of defense responses to prevent the successful invasion and spreading of pathogenic microorganisms. The onset of these responses involves a massive reprogramming of gene expression in the infected plant. Virulent pathogen strains modulate host tissues processes in order to delay the defense responses until they will not be effective anymore. Most living organisms have the ability to sense and respond to light through photoreceptors such as LOV (Light, Oxygen, Voltage)-domain proteins, which are sensitive for the blue light region of the visible spectrum. Xanthomonas citri subsp. citri (Xcc) is a gram negative bacterium responsible for citrus canker, a severe disease that affects the Citrus genus. This bacterium presents a LOV-domain protein that we previously characterized as a functional photoreceptor. We demonstrated that this protein participates in the regulation of bacterial features directly associated with Xcc ability to colonize host plants. On the other hand, the symptoms developed in host plants infected with a mutant strain of Xcc lacking the LOV protein (Δlov) are considerably different than those developed in plants infected with the wild type strain. In a previous work we presented the preliminary data obtained by a transcriptomic evaluation of Citrus sinensis leaves infected with the wild type Xcc and the Δlov-mutant strain. In this work, we performed the analysis of the transcriptome variations in order to detect the significant over-represented biological processes differentially affected between treatments. These processes included photosynthesis, sucrose catabolism, biotic stress and secondary metabolism. We present the differential expression ratios of genes corresponding to proteins involved in plant defense response that resulted up-regulated in C. sinensis leaves infected with the Δlov-mutant Xcc strain. The transcriptome of these leaves also showed the up-regulation of genes corresponding to enzymes involved in lignin biosynthesis and cell membrane and wall degradation. We confirmed the transcriptomic variations at physiological level by lignin detection and integrity evaluation of bacterial infected C. sinensis tissues. We found that lignin accumulation was higher in leaves inoculated with the Δlov-mutant strain and that tissue integrity was more severely affected compared to wild type Xcc-inoculated leaves. The high representation of up-regulated genes related to plant defense mechanisms against pathogens in tissues inoculated with the Δlov-mutant strain of Xcc, together with the enhanced biochemical and structural host tissue alterations suggest a stronger host response upon infection with this strain and represent a novel participation of a light-sensing bacterial protein in the counteraction of plant defense responses.