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 inAgrobacterium tumefaciens, 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. Bacterialvirgenes andsghAare differentially transcribed at early and later infection stages, respectively. Plant metabolite sucrose is a signal ligand that inactivates SghR and consequently inducessghAexpression. Disruption ofsghAleads to increasedvirexpressionin plantaand enhances tumor formation whereas mutation ofsghRdecreasesvirexpression and tumor formation. These results depict a remarkable mechanism by whichA. tumefacienstaps on the reserved pool of plant signal SA to reprogram its virulence upon establishment of infection.