Identification by Tn-seq of Dickeya dadantii genes required for survival in chicory plants.

Identification of the virulence factors of plant pathogenic bacteria has relied on the test of individual mutants on plants, a time-consuming process. Tn-seq is a very powerful method for identifying those genes required for bacterial growth in their host. We used this method in a soft-rot pathogenic bacterium to identify the genes required for the multiplication of Dickeya dadantii in chicory. About 100 genes were identified showing decreased or increased fitness in the plant. Most of them had no previously attributed role in plant-bacteria interaction. Following our screening, in planta competition assays confirmed that the uridine monophosphate biosynthesis pathway and the purine biosynthesis pathway are essential to the survival of Dickeya dadantii in the plant since the mutants ∆carA, ∆purF, ∆purL, ∆guaB and ∆pyrE are unable to survive in the plant in contrast to the WT bacterium. This study also demonstrates that the biosynthetic pathways of leucine, cysteine and lysine are essential for bacterial survival in the plant and that RsmC and GcpA are important in regulating the infection process since the mutants ∆rsmC and ∆gcpA are hypervirulent. Finally, our study shows that D. dadantii flagellin is glycosylated and that this modification confers fitness to the bacteria during plant infection. Assay by this method of large collections of environmental pathogenic strains now available will allow an easy and rapid identification of new virulence factors. This article is protected by copyright. All rights reserved.