UC Riverside

Xylella fastidiosa (Xf), a xylem-limited fastidious bacterium, is the causal agent of Pierce's Disease (PD) of grapevine. Xf has a very broad host range, including grapevine, citrus, almond, oleander, peach and maple. PD is a lethal disease of grapevine and understanding the disease from the perspective of molecular interactions between Xf and grapevine is very important. Pathogens often encounter reactive oxygen species (ROS) from a variety of sources during the infection process. These ROS can be toxic to the pathogen and correspondingly, the pathogen has evolved several tightly regulated mechanisms to cope with this stress. OxyR is a redox-responsive transcription factor that regulates expression of antioxidant enzymes. Interestingly. OxyR is the only known oxidative stress regulator in the Xf genome leading us to speculate that it plays a vital role in adaptation to the host environment. We constructed an oxyR mutant and found it was significantly more sensitive to H₂O₂ than wild type Xf. In addition, we found that the Xf oxyR mutant was reduced in surface attachment, cell-cell aggregation and mature biofilm formation. In planta tests indicated that the oxyR mutant was significantly compromised in the ability to colonize the host xylem, but, interestingly, no difference in virulence was observed when compared with wild type Xf.

The Type II secretion system (T2SS) is an important protein secretion system in plant pathogenic bacteria. The repertoire of proteins secreted by this system are largely involved in nutrition and include plant cell wall degrading enzymes (CWDEs). Xf employs CWDEs to degrade xylem pit membranes to facilitate systematic movement within the xylem. The majority of the demonstrated and putative CWDEs are predicted to be secreted by the T2SS. By knocking out a T2SS structure gene xpsE, which encodes a putative ATPase that provides the energy that drives the T2SS, I demonstrated that Xf requires XpsE for full virulence and establishment in the xylem vessels indicating that the T2SS is an important factor employed during the infection process.