Tomato leaf curl disease, caused by rapidly evolving begomoviruses, poses a major threat to tomato production worldwide. Although resistance genes like Ty-1/Ty-3 have effectively managed key viruses such as tomato yellow leaf curl virus (TYLCV), emerging new viral variants continuously challenge this control. In this study, we identified and characterized a resistance-breaking isolate of tomato leaf curl Karnataka virus (ToLCKV, MZ578458, species: Begomovirussolanumkarnatakaense) from symptomatic tomato genotypes carrying the Ty-gene(s). Agro-infectious construct of this new isolate of ToLCKV was found to be capable of overcoming homozygous Ty-3 mediated resistance in tomato cultivars, which otherwise showed resistance when inoculated with agro-infectious constructs of another predominant begomovirus, tomato leaf curl New Delhi virus (ToLCNDV). Compared with other ToLCKV isolates, key genetic variations were detected in ORF C1, the 5′ region of C2, and the intergenic region, while the viral suppressors of RNA silencing and other pathogenicity-associated genes showed evidence of adaptive evolution. Host range assays confirmed systemic infectivity in Solanaceae hosts and asymptomatic replication of the virus in multiple Cucurbitaceae hosts. Elevated temperature (35 °C) significantly enhanced viral accumulation and hastened symptom development in both susceptible (cv. Pusa Ruby) and Ty-3-based resistant cultivars (cv. Kashi Adarsh, cv. Kashi Aman), compared to ambient conditions (25 °C). Notably, no direct correlation between symptom progression and viral titres was found, suggesting that resistance breakdown under heat stress involves complex host–virus dynamics beyond viral load alone. To our knowledge, this is the first report documenting the breakdown of Ty-3-mediated resistance by a begomovirus, together with evidence that elevated temperature exacerbates this breakdown under controlled environmental conditions. The findings of our study highlight the synergistic threat of viral evolution and climate change and emphasize the need for future research into the molecular basis of temperature-mediated resistance failure to guide the development of climate-resilient tomato cultivars.
Mondal et al. (Wed,) studied this question.