MELON RESISTANCE TO THE APHID APHIS-GOSSYPII - BEHAVIORAL-ANALYSIS AND CHEMICAL CORRELATIONS WITH NITROGENOUS COMPOUNDS

In the melon, the Vat (monogenic, dominant) resistance gene governs bo th an antixenotic reaction to the melon aphid Aphis gossypii Clover (H omoptera, Aphididae) and a resistance to non-persistent virus transmis sion, restricted to this vector species. We investigated the behaviour al features and tissue localisation of the antixenosis resistance by t he electrical penetration graph technique (EPG, DC system). We also co mpared the chemical composition in amino compounds and proteins of the phloem sap collected from two isogenic lines of melon (Cucumis melo L .), carrying the Vat gene or not. All behavioural and chemical data in dicated that this resistance is constitutive. EPG analysis clearly sho wed that access to phloem, although delayed by alterations in pathway activities, was not impaired in terms of frequency of access or initia tion of feeding. The most striking feature was, however, a very reduce d duration of ingestion from phloem of resistant plants, making this c ompartment one of the tissues where the effects of the Vat gene are un ambiguously expressed. This was confirmed by clear differential activi ty of phloem extracts in artificial no-choice bioassays. Chemical anal yses have shown that phloem saps from the two isogenic lines were extr emely similar in profiles of ninhydrin positive compounds: and contain ed a low total amount of free amino acids (less than 10 mM). Out of mo re than 40 distinguishable peaks in the chromatograms (protein and non -protein amino acids, as well as small peptides), only five differenti ated the two genotypes. Two of them were increased in the resistant ge notype: glutamic acid and a major unknown peak, probably a non-protein amino acid (different from beta pyrazolyl-alanine, a Cucumis-specific amino acid). The three others were depressed in resistant plants, and included the sulphur amino acid cystine and a peptide peak partly com posed of the cysteine-containing peptide glutathione (reduced form). S ap collection also showed that phloem exudation rates, as well as tota l protein and glutathione levels, were depressed in phloem sap from re sistant plants. Such data are all indicative of a modified phloem-seal ing physiology, linked to sulfhydryl oxidation processes, in plants ca rrying the Vat gene. The originality of the mechanism of Vat resistanc e to aphids is discussed.