Resistance breaking Root-knot nematodes carry a fitness cost associated with defective feeding site development.

Root-knot nematodes (RKNs) cause an estimated 157 billion dollars in annual yield losses worldwide. In tomato, resistance to RKNs is conferred by the single dominant resistance gene Mi-1, which has been widely integrated into commercial cultivars. Prolonged and widespread use of Mi-1 has led to the emergence of resistance-breaking populations in tomato fields worldwide; however, the consequences of resistance breaking for nematode performance on susceptible hosts remain poorly understood. Here, we compared infection outcomes of two closely related strains of Meloidogyne javanica, VW4 (Mi-1-avirulent; wild-type) and VW5 (Mi-1-virulent; resistance-breaking), on three susceptible hosts: tomato, cucumber, and rice. Across all hosts, VW5 produced significantly fewer eggs than VW4, revealing a fitness cost associated with Mi-1 virulence. Light and transmission electron microscopy of tomato and cucumber galls revealed impaired feeding site establishment by VW5. Consistent with these observations, transcriptomic profiling of nematode-infected roots showed that VW5 infection induced weaker host transcriptional reprogramming than VW4 and lacked host gene expression signatures associated with effective suppression of plant defense responses. Together, these findings demonstrate that adaptation to Mi-1-mediated resistance is accompanied by a fitness cost on susceptible plants and impaired feeding site formation and altered host reprogramming. Furthermore, these results establish VW4 and VW5 as a powerful resource for understanding host genes and pathways required for successful parasitism and feeding site development.