Agrobacteria reprogram virulence gene expression by controlled release of host-conjugated signals.

Abstract

It is highly intriguing how bacterial pathogens can quickly shut down energy-costly infection machinery once successful infection is established. This study depicts that mutation of repressor SghR increases the expression of hydrolase SghA in <i>Agrobacterium tumefaciens</i>, which releases plant defense signal salicylic acid (SA) from its storage form SA β-glucoside (SAG). Addition of SA substantially reduces gene expression of bacterial virulence. Bacterial <i>vir</i> genes and <i>sghA</i> are differentially transcribed at early and later infection stages, respectively. Plant metabolite sucrose is a signal ligand that inactivates SghR and consequently induces <i>sghA</i> expression. Disruption of <i>sghA</i> leads to increased <i>vir</i> expression <i>in planta</i> and enhances tumor formation whereas mutation of <i>sghR</i> decreases <i>vir</i> expression and tumor formation. These results depict a remarkable mechanism by which <i>A. tumefaciens</i> taps on the reserved pool of plant signal SA to reprogram its virulence upon establishment of infection.