Host Detection at Plant Wound Sites

At the outset of infection, the VirA and VirG proteins, members of the family of "two-component"regulators, perceive a combination of phenolic compounds, monosaccharides, and acidity that are released from wound sites. VirA is a transmembrane sensory protein kinase that phosphorylates VirG. VirG activates transcription of approximately 10 operons of genes, most of which are required to transfer DNA from the Ti plasmid to the nuclei of infected host cells. VirA is an especially complex four domain protein, and we are using a variety of genetic techniques to understand the role of each of these domains in host perception. (see J. Bacteriol. 178: 4710).

Recognition of wound released chemical signals by VirA and VirG.
Recognition of wound released chemical signals by VirA and VirG.

Several VirG-regulated genes encode products that are NOT required for tumorigenesis, indicating that wound-released chemical signals elicit multiple responses in the bacterium. We are currently identifying several new genes of this class and are attempting to find roles for their products. We recently have found that the virH2 gene encodes a protein that O-demethylates some but not all phenolic inducers, converting them to noninducers. For example, VirH2 O-demethylates ferulic acid, a strong vir gene inducer, creating caffeic acid, which is inactive as an inducer. The goal could be to provide a negative feedback loop, or governor, on virgene induction. Furthermore, ferulic acid is far more toxic than caffeic acid, particularly for a virH mutant (Kalogeraki, Zhu, and Winans, Mol. Microbiol. in press).