Novel pathways converge with quorum sensing to regulate plant and insect host-specific factors in Erwinia carotovora

Abstract

Erwinia carotovora Ecc15 is a vector-borne phytopathogen that relies on insects to be transmitted between plant hosts. To interact with its hosts, this bacterium depends on host-specific bacterial traits. Plant tissue maceration depends on production of plant cell wall degrading enzymes (PCWDE), while survival in the digestive tract of the insect requires the Erwinia virulence factor (evf). Evf expression is responsible for the cost of Ecc15 infection in Drosophila melanogaster and overexpression is lethal to the insect host. Therefore, its expression must be well controlled. Expression of evf and PCWDEs is co-regulated by quorum sensing via the transcriptional regulator Hor. Since virulence factors are often controlled by multiple signals, we asked which additional factors regulate evf expression. Using a genetic screen, we identified the sensor histidine kinase arcB and a new TetR-like regulator (named herein as lvtR, after Low Virulence Transcriptional Repressor), as novel regulators not only of evf, but also of pelA, which encodes a major PCWDE. We further demonstrate that arcB and lvtR mutants have reduced plant tissue maceration and reduced development delay and lethality in Drosophila melanogaster, compared to wild-type bacteria. Thus showing the importance of these regulators in the establishment of Erwinia-host-vector interactions. We also found that ArcB and LvtR regulation converges on Hor, independently of quorum sensing, to co-regulate expression of both plant and insect bacterial interaction factors during plant infection. Taken together, our results reveal a novel regulatory hub that enables Ecc15 to integrate quorum sensing responses and environmental cues to co-regulate traits required for infection of both the plant and the insect vector. Moreover, we show that ArcB regulation of bacteria-host interaction processes is conserved in other bacteria. Author SummaryVector-borne pathogens depend on continuous cycles of replication and transmission between hosts and vectors, requiring multiple factors to interact with each of the hosts. The expression of these multiple interaction factors can be very costly, so it is expected that regulation of virulence has been evolutionarily tuned to produce expressions patterns that minimize the cost of establishing the infection while maximizing transmission efficiency of the pathogen. Here, we investigate the tripartite interaction between Ecc15, a plant and an insect, and show that quorum sensing, a widely conserved sensory regulator arcB and a regulator of previously unknown function, lvtR, converge to simultaneously co-regulate the expression of bacterial factors required for these interactions. Gene expression regulation is channeled through the conserved regulator Hor, which serves as a molecular hub for the integration of these multiple signals. Our data suggest that integration of multiple signals to co-regulate plant and insect associated factors ensure fine-tune titration of gene expression and maximization of bacterial energetic resources.