Xylem sap growth and biofilm formation of Xylella fastidiosa and its connection with host adaptation and virulence

Abstract

Xylella fastidiosa is a xylem-limited, insect-transmitted plant pathogen that causes severe diseases in many crops. Xylem sap is the only nutrient medium for bacteria to survive and multiply inside plants. This study examined how host xylem sap influences bacterial growth, biofilm formation and adaptation, and how these in vitro traits relate to disease outcomes in grapevine and blueberry. Diverse X. fastidiosa strains representing multiple subspecies and host origins were tested in culture media amended with grapevine or blueberry sap. In grape sap, strains virulent in grape showed strong increases in biofilm, while asymptomatic strains showed little response; similar host-specific patterns were observed with blueberry sap. Microfluidic assays confirmed that sap-induced biofilm traits are reproducible under xylem-like flow, and serial passage experiments indicated that high-biofilm forming strains adapt more effectively to sap environments. Greenhouse inoculations also revealed that strains with stronger biofilm responses generally caused more severe symptoms and reached higher bacterial populations in plants. Together, these results demonstrate that host sap composition and bacterial genotype jointly shape X. fastidiosa’s attachment and adaptation strategies, linking in vitro sap responses to virulence in planta. Sap-based phenotyping provides a rapid way to assess strain behavior before greenhouse or field trials, improving the ability to connect in vitro sap responses with host compatibility and disease development.