Natural competence and host adaptation in plant pathogenic bacterium Xylella fastidiosa

Abstract

The plant pathogen Xylella fastidiosa is a Gram-negative, fastidious, xylem-limited, vector- transmitted bacterium, causing severe diseases on numerous economically important plants including grapes, citrus, coffee, olive and almond, among others. The outbreak of X. fastidiosa in new geographic regions and the new host-pathogen genotype association present a significant agricultural challenge. Extensive evidence indicates that homologous recombination (HR) may be one of the major forces contributing to the appearance of new genotypes, host adaptation and host shifts in X. fastidiosa. Natural competence has been considered a key mechanism facilitating HR in this pathogen. However, natural competence has been experimentally confirmed in only a few strains, and its distribution across diverse X. fastidiosa strains, as well as the underlying regulatory mechanisms, remains poorly understood. Moreover, while natural competence has been demonstrated under laboratory conditions, its occurrence in planta and potential metabolic impact have not yet been confirmed. Additionally, limited information exists regarding the molecular mechanisms that contribute to plant adaptation in X. fastidiosa. To address these gaps, we characterized natural competence in 133 X. fastidiosa strains from diverse origins and observed substantial variation in their ability to acquire foreign DNA. We identified XadA2 as an important factor influencing natural competence and potentially contributing to its variation among strains. Furthermore, we assessed the occurrence of natural competence in planta by co-inoculating strains with differing virulence levels and found that the addition of extracellular DNA slightly altered disease severity. Pool-seq analysis provided further evidence that genetic exchange via natural competence likely occurs in planta, contributing to changes in virulence. Additionally, we investigated the genetic basis of sap adaptation in X. fastidiosa by constructing a partial mutant 2 library using Random-Barcoded Transposon Sequencing (RB-TnSeq) techniques. Several candidate genes associated with sap adaptation were identified. Interestingly, mutations in type IV pilus (TIVP) genes significantly increased bacterial fitness, suggesting that TIVP function imposes a substantial metabolic cost. Similarly, our in vitro growth studies indicate that the DNA uptake process associated with natural competence may also be metabolically costly. These findings suggest that natural competence may play distinct roles across different X. fastidiosa populations, with a potential trade-off between its adaptive benefits and metabolic burden.