The Development of Control Methods for Xylella fastidiosa by Manipulating the Grapevine Microbiome, Host Defense System, and its Abiotic Environment

Xylella fastidiosa is a xylem-limited, Gram-negative, gammaproteobacterium, and the causal agent of Pierce’s disease (PD) of grapevine and several other diseases in many economically important crops. In the PD patho-system in California, it is natively vectored by the blue green sharpshooter (Graphocephala atropunctata) and vectored by the invasive glassy-winged sharpshooter (Homalodisca vitripennis). When transmitted into a healthy vine, X. fastidiosa colonizes the xylem leading to blockages both directly through biofilm formation and EPS build up and indirectly through the induction of tyloses and pectin gels. These blockages are believed to contribute to symptom development which includes marginal leaf necrosis, abscission of the leaf blade from the petiole, green islands in the periderm, desiccation of the berries, and eventually vine death. While there is no current cure, common management practices include severe pruning of diseased tissue and insecticide sprays to control vector populations. However, new management strategies are needed to create cost effective and sustainable solutions to protect these culturally valuable crops.As of late, biological control agents, the enhancement of existing treatments, and the development of transgenic disease-resistant or disease-tolerant plants have been implemented in an attempt to cope with phytopathogenic microbes. First, the concept of a biological control agent or biocontrol can be defined as the use of a third organism to act antagonistically against a pathogen and lessening disease symptoms in the host. This antagonism can occur via antibiotic production, direct predation, or nutrient competition and exclusion. We have identified two endophytic bacterial isolates that impart varying levels of protection to the grapevine from X. fastidiosa and have speculated on the mechanisms by which they do so. Second, the transgenesis of plants has been proven to be effective at controlling pathogens. Transcriptomic data can be used to identify genes that are upregulated in a successful defense response against a pathogen and these genes can be transgenically engineered to be overexpressed and confer resistance or tolerance. Here, we have overexpressed a class III peroxidase and found that it conferred tolerance to the grapevines against X. fastidiosa. Third, metal ions have recently been found to break down the resistance of some pathogenic bacteria to certain antibiotics. We investigated the in vitro activities of copper, a common metal used in vineyards, on X. fastidiosa. We found that it affects several characteristics such as cell surface hydrophobicity and cell attachment to a surface. We found that it also increases the susceptibility of X. fastidiosa cells in a biofilms to antibiotics. Collectively, this research has set the framework for the development of novel PD management strategies and can potentially serve as a model for other X. fastidiosa patho-systems affecting other economically important crops