Characterisation and detection of viruses (Cucumovirus, Potyvirus) infecting vanilla in Réunion Island and Polynesian Islands
Natural vanilla (Vanilla planifolia, V. tahitensis) is economically important for a number of producing countries, but viral diseases are prejudicial to its successful cultivation. Techniques are required to detect viruses in vanilla plants in order to establish quarantine procedures and sources of virus-free planting material. This study contributed to the progress in managing viral diseases of vanilla by firstly identifying Cucumber mosaic virus (CMV, cucumovirus) as causing severe distortion, stunting, sterility and sometimes death of vanilla plants. Vanilla CMV isolates from French Polynesia (Pacific Ocean) and Réunion Island (Indian Ocean) were classified in CMV subgroup IB, adding to the short list of subgroup IB isolates detected outside of Asia. Two isolates were putatively classified in subgroup IA, suggesting that CMV in vanilla is still evolving and/or that vanilla is infected from different sources. Subgroup II CMV from New Zealand was also experimentally infectious to V. planifolia, showing that vanilla crops should be protected from all potential sources of CMV inoculum. Existing serological and molecular detection rests were performant for the detection of CMV directly from vanilla tissue. Secondly, this study provided the first coat protein sequence information for Vanilla mosaic virus (VanMV, Potyvirus). A Cook Islands isolate (VanMV-CI) and a French Polynesia isolate (VanMV-FP) had distinctly different coat proteins. The VanMV-FP CP N-terminus contained a stretch of amino-acid repeats (GTN) typical of natively unfolded proteins. This GTN stretch was located downstream of a DVG motif (which replaced the more common aphid transmission DAG motif), suggesting a role in improving aphid transmission, or regulating formation of the HC-virus complex. CP core nucleotide sequence identities indicated VanMV-CI and VanMV-FP were strains of Dasheen mosaic virus (DsMV). In contrast, CP amino-acid sequence homologies between VanMV- CI and DsMV were intermediate between strains and species, and CP amino-acid homologies between VanMV-FP and DsMV were typical of distinct species. In addition, VanMv-CI and VanMv-FP had characteristic 3'NTR sequences and NIb/CP cleavage sites, and only infected vanilla. Hence, it is proposed that VanMV-CI and VanMV-FP are considered new Potyvirus species and named Vanilla mosaic Cook Islands virus and Vanilla mosaic French Polynesia virus. Alternatively, the two isolates may be grouped under the name Dasheen mosaic virus-Vanilla (DsMV-V) and distinguished from Dasheen mosaic virus-Dasheen(DsMV-D). Primers to VanMV-CI and VanMV-FP were designed and permitted RT-PCR detection of the viruses directly from vanilla tissue. VanMV-Cl and VanMV-FP could be differentiated from DsMV and Watermelon mosaic virus (WMV-Tonga), and differentiated from each other by comparison of amplicon size. Long-term specific potyvirus diagnosis is however expected to be difficult due to potyviral variability in vanilla. Future research should concentrate on techniques such as microarrays to permit simultaneous detection combined with specific identification of potyvirus species. Such techniques would be beneficial to viral disease management in vanilla and many other crops.
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