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The potyvirus Turnip mosaic virus (TuMV) is a major constraint on the cultivation of a wide range of plant species worldwide. It causes significant economic losses in brassica species such as Chinese cabbage (Brassica rapa), which is one the most important vegetable crops in the world. The B. rapa line RLR22 has broad-spectrum resistance to TuMV, which is undefeated. Many recessive resistances against plant viruses in the Potyvirus genus are based on mutations in plant eukaryotic translation initiation factor 4E (eIF4E), or its isoform eIF(iso)4E . B. rapa has three eIF4E genes and three eIF(iso)4E genes. Segregation following a cross between RLR22 and the TuMV-susceptible R-o-18 line of the closely related B. rapa ssp. trilocularis revealed the resistance was due to a recessive gene, retr01 that was epistatic to a dominant gene, ConTR01. My research revealed that retr01 is BraA.eIF(iso)4E.a and that ConTR01 is probably BraA.eIF(iso)4E.c. It also showed that the highly sought after broad-spectrum resistance to TuMV is due to a novel, recessive, natural mechanism, based on the mis-splicing of BraA.eIF(iso)4E.a in B. rapa. This results in a range of eIF(iso)4E splice variants, the most common of which retained the whole of intron 1 and appears to be non-functional for the virus. As the susceptible parent in the original cross, R-o-18, was a different sub-species to RLR22 (B. rapa var. pekinensis, Chinese cabbage), the genetic inheritance of resistance was also investigated in crosses with Chinese cabbage lines; F2 segregation ratios were consistent with those predicted for the single recessive gene (retr01 ). Yeast two-hybrid interactions …