Differential colonization patterns of Xylella fastidiosa infecting citrus genotypes

Xylella fastidiosa is a phytopathogenic bacterium that causes disease in many different crops worldwide. In Brazil, X. fastidiosa subsp. pauca causes citrus variegated chlorosis (CVC), which is a disease responsible for economic losses in the citrus agribusiness. Variable host responses to bacterial colonization and disease development have been observed. This work studies the colonization processes of a pathogenic GFP-labelled X. fastidiosa citrus strain in sweet orange (susceptible) and tangor (resistant) parents and two resulting hybrids that exhibited contrasting responses to CVC. Xylella fastidiosa showed increased populations and movement in the susceptible genotypes, but slower compared to other hosts such as grapevine. Scanning electron microscopy revealed that the predominant pitted stem morphology in citrus makes the bacterial movement difficult. In susceptible genotypes X. fastidiosa can move from the primary to the secondary xylem, whilst it is confined to the primary xylem in resistant plants. Associated with this is an induction of lignification that occurs earlier in the resistant genotypes when in the presence of the pathogen, and represents a genetic mechanism that leads to formation of a physical barrier, impairing bacterial colonization. (AU)