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Aphis gossypii is the main virus vector in muskmelon crops. The melon gene Vat confers resistance to non-persistent virus transmission by this aphid. The mechanism of this resistance is not well understood, but no relationship has been detected between resistance and the probing behaviour of aphids on resistant plants. Results presented here suggest that temporary blockage of aphid stylet tips preventing virus particle release may explain the resistance conferred by Vat gene. We performed experiments in which viruliferous aphids were allowed to probe different sequences of resistant (Vatbearing) and/or susceptible melon plants. The results demonstrated that A. gossypii inoculates Cucumber mosaic virus (CMV) efficiently in susceptible plants having previously probed resistant plants, showing that the resistance mechanism is reversible. Furthermore, the infection rate obtained for susceptible plants was the same (25%) regardless of whether the transmitting aphid had come directly from the CMV source or had subsequently probed on resistant plants. This result suggests that virus is not lost from stylet to plant during probing of resistant plants, supporting the temporary blockage hypothesis. We also found that the ability of Myzus persicae to transmit CMV is noticeably reduced after probing on resistant plants, providing evidence that this aphid species also responds to the presence of the Vat gene. Finally, we also found that in probes immediately after virus acquisition M. persicae inoculates resistant plants with CMV more efficiently than susceptible plants, perhaps because the Vat gene product induces increased salivation by this aphid.

The Vat gene of melon confers resistance to Aphis gossypii through both antixenotic and antibiotic mechanisms. This article reports several experiments carried out to detect this gene in melon lines from a melon breeding program. These included antixenosis, antibiosis and virus transmission trials. Results showed that, for Vat detection, antibiosis trials were not as discriminating as antixenosis trials. The antixenosis trials discriminated more clearly between resistant and susceptible lines after 72 h than after 24 h. We additionally developed a rapid and simple choice-test method to assess antixenotic effect. This test discriminated rapidly and effectively between resistant and susceptible lines. The aphids showed significant rejection of resistant lines after only 1.5 h of exposure. Thus compared to the conventional antixenosis trials this test has several advantages, including rapidity, ease of use, and non-destructiveness (allowing replicate testing of a single plant, or subsequent obtention of seed).

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