Breeding for Tomato Brown Rugose Fruit Virus Resistance

Tomato brown rugose fruit virus (ToBRFV) has had major impacts on tomato production worldwide since its emergence in ~2014. The most critical factor behind the global pandemic of ToBRFV is the escape of the long-standing tomato tobamovirus resistance gene, Tm-22. ToBRFV has spurred research and brought renewed interest to genetic mechanisms of resistance against tobamoviruses for use in commercial tomato breeding. Many new potential sources of resistance to ToBRFV have been reported in peer-reviewed publications and commercial patents. However, the identity of genes responsible for resistance traits remains mostly elusive, and breeding these traits from wild relatives into commercial elite tomato varieties has negatively impacted tomato production by affecting fruit quality and development. Tm-1 has been the focus of multiple publications investigating the molecular basis in preventing ToBRFV replication, often linked to unknown resistance genes on other chromosomes. The role of Tm-1 includes complex genetic interactions, expression and gene dosage effects, gene duplications, orthologous variants, and particularly resistance escape mechanisms which can all affect the strength of Tm-1-mediated resistance. Other novel resistance traits are also increasingly vulnerable to resistance-breaking isolates. Here, we review recent major developments relating to ToBRFV resistance and breeding of commercial varieties.