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INVESTIGATOR: Kung, S.

PERFORMING INSTITUTION:
UNIVERSITY OF CALIFORNIA, BERKELEY
BERKELEY, CALIFORNIA 94720

HOMOLOGOUS RECOMBINATION AND THE EMERGENCE OF NOVEL PATHOGENS

NON-TECHNICAL SUMMARY: Emerging infectious plant diseases continue to affect agricultural production in both the United States and abroad. However, little research has been conducted on the evolutionary forces driving pathogen diversification and emergence. Xylella fastidiosa is a vector-borne xylem-limited bacterium that causes disease of economic importance in several crops such as grape, almond, and peach. The number of new diseases caused by X. fastidiosa has increased dramatically in recent years; however, the factors driving the emergence of these diseases and the mechanisms that allow the pathogen to adapt to new host environments are not understood. We recently found that X. fastidiosa is naturally competent and can recombine DNA acquired from the environment into its genome. We propose to characterize the biology of competency and determine what factors control its induction. We will investigate target genes identified by BLAST searches and previous microarray work for their role in transformation and recombination through molecular and biological assays. In addition, we will study the role of recombination in the evolution of X. fastidiosa's pathogenicity and determine what role if any its insect vector plays in promoting recombination events. We hypothesize that homologous recombination in X. fastidiosa occurs at high rates and that a constant reshuffle of genes throughout the genome results in the emergence of pathogenicity and new strains, which may drive epidemics. Understanding the role of horizontal gene transfer in the emergence of new diseases may help prevent future crop failure and famine resulting from pathogenic infections.

OBJECTIVES: Xylella fastidiosa is an economically important plant pathogen in the US with a wide host range. It causes disease in several agriculturally relevant crops such as grape, almond, peach and plum, in addition to trees and ornamental plants. The introduction of a strain causing disease in citrus is a major threat to the US industry. Although some X. fastidiosa diseases have been known for decades, many have emerged recently. Notably, several new diseases have been reported in California after the introduction of a generalist insect vector. We hypothesize that this polyphagous insect is responsible for transporting different strains of the pathogen into new hosts, increasing the frequency of recombination events and the emergence of new pathogens. This hypothesis is supported by both a dramatic increase in emerging X. fastidiosa diseases in California and a growing number of studies demonstrating that homologous recombination is a major contributor to this bacterium's genome plasticity. The role of homologous recombination on pathogen evolution has only recently been appreciated, primarily in the last decade after multi-locus sequence typing approaches have shown that recombination rates in many bacteria are much higher than mutation rates. However, experimental tests of the role of homologous recombination on pathogen adaptation, host range, virulence and other relevant processes have been extremely limited. We propose to use X. fastidiosa as a model system to ask such questions because it is naturally competent, has a wide host range, and virulence seems to be a complex trait derived from factors associated with pathogen multiplication and movement within the host's xylem vessel network. Understanding the biology and evolutionary role of homologous recombination in X. fastidiosa may lead to insights into why this pathogen has such a wide host range and new diseases have emerged after the introduction of a new vector into California. In addition, it will provide evidence to the role of this process in plant pathogen evolution and adaptation. Lastly, understanding the biology of competency in detail may generate useful information for research on other plant pathogenic bacteria. As bacterial plant pathogens are a major contributing factor to crop loss in both the United States and abroad, understanding the potential mechanisms through which new diseases emerge could play a vital role in mitigating crop losses, keeping American agriculture competitive and helping to end world hunger. We propose to address the following hypotheses: i) Low nutrient conditions and low cell densities induce natural competence and recombination. ii) Insect vectors provide ideal conditions for genetic exchange to occur between pathogen strains. iii) Homologous recombination events facilitate the adaptation of pathogens to new environments. Outputs include presentation of research at scientific meetings and grower groups impacted by X. fastidiosa diseases, publication of peer-reviewed research manuscripts, and sharing of data with research collaborators and other scientists as requested.