Problems Encountered with the Selection of Cucumber Mosaic Virus (CMV) Isolates for Resistance Breeding Programs

Abstract

Among the Chili breeding lines from the Asian Vegetable Research Center, two were chosen for the screening of a larger selection of Cucumber mosaic virus (CMV) isolates, mainly from Asian countries. The chili line (VC246) showed a resistance against several CMV-isolates and was compared with chili line VC27a that was susceptible to CMV infection. Among the 28 CMV isolates, five were identified as resistance breaking (AN-like) and non-resistance breaking (P3613-like) for the line VC246, whereas all isolates could establish a systemic infection on VC27a. However, further testing revealed that resistance in VC246 was also dependent on the way of inoculation and the inoculums itself. Graft inoculation could overcome the resistance, and the inoculation with isolated viral RNA resulted in no infection at all on the resistant chili line, independent of the virus isolate. Using a pseudo-recombinant approach, we identified RNA2 of resistance breaking isolates as responsible for systemic infection and confined the area within RNA2 to the 3′ terminal part including the ORF 2b. Sequence alignments of that area revealed eight distinct mutations on amino acid level, which was present either in resistance or non-resistance breaking isolates. A reversion from the P3613-like to the AN-like sequence of two of these mutations induced no effect on Capsicum sp., but induced symptoms on several tobacco species distinct from those induced by the wild-type virus. However, pseudorecombinants, each generated from sets of two different AN-like isolates, which were expected to infect VC246 systemically, did not indicating that probably RNA2 must be in a specific context to have the effect. In this case, a generalized attribution of functions to single amino acid exchanges might be impossible or at least extremely difficult.