Melon resistance to the aphid Aphis gossypii: behavioural analysis and chemical correlations with nitrogenous compounds

Abstract

In the melon, the Vat (monogenic, dominant) resistance gene governs both an antixenotic reaction to the melon aphid Aphis gossypii Glover (Homoptera, Aphididae) and a resistance to non-persistent virus transmission, restricted to this vector species. We investigated the behavioural features and tissue localisation of the antixenosis resistance by the electrical penetration graph technique (EPG, DC system). We also compared 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 indicated that this resistance is constitutive. EPG analysis clearly showed that access to phloem, although delayed by alterations in pathway activities, was not impaired in terms of frequency of access or initiation of feeding. The most striking feature was, however, a very reduced duration of ingestion from phloem of resistant plants, making this compartment one of the tissues where the effects of the Vat gene are unambiguously expressed. This was confirmed by clear differential activity of phloem extracts in artificial no-choice bioassays. Chemical analyses have shown that phloem saps from the two isogenic lines were extremely similar in profiles of ninhydrin positive compounds, and contained a low total amount of free amino acids (less than 10 mM). Out of more than 40 distinguishable peaks in the chromatograms (protein and non-protein amino acids, as well as small peptides), only five differentiated the two genotypes. Two of them were increased in the resistant genotype: glutamic acid and a major unknown peak, probably a non-protein amino acid (different from 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 composed of the cysteine-containing peptide glutathione (reduced form). Sap collection also showed that phloem exudation rates, as well as total protein and glutathione levels, were depressed in phloem sap from resistant plants. Such data are all indicative of a modified phloem-sealing physiology, linked to sulfhydryl oxidation processes, in plants carrying the Vat gene. The originality of the mechanism of Vat resistance to aphids is discussed.

Aphid Resistance inMedicago truncatulaInvolves Antixenosis and Phloem-Specific, Inducible Antibiosis, and Maps to a Single Locus Flanked by NBS-LRR Resistance Gene Analogs

“…Unlike our study, however, preinfestation of a susceptible cultivar (unrelated to the resistant cultivar) caused an increase in sap ingestion, suggesting that aphids induced changes in the host plant's physiology leading to enhanced susceptibility. Our results and those of Sauge et al contrast with those of Chen et al (1997), who observed no effect of prior infestation on the feeding behavior of A. gossypii on melon near-isogenic lines, with or without the Vat gene.…”

“…Interestingly, they found no preference for either melon genotype at 4 h after release, but a highly significant preference for the susceptible genotype at 24 and 48 h after release. Like AKR-mediated resistance, the resistance mechanism in melon was later shown to be phloem localized (Kennedy et al, 1978;Chen et al, 1997;Klingler et al, 1998), suggesting that time was spent probing phloem tissue before alatae chose between alternative hosts.…”

“…Plant properties outside of the phloem were reported to affect feeding behavior of another phloem feeder, sweetpotato whitefly (B. tabaci), when the insect was exposed to Mi-containing tomato plants (Jiang et al, 2001). However, phloem-specific resistance similar to that of AKR has been reported for several aphid-plant interactions, some of which are mediated by a single dominant gene (van Helden and Tjallingii, 1993;Chen et al, 1997;Klingler et al, 1998;Kaloshian et al, 2000). Sauge et al (2002) tested the effect of prior infestation on feeding behavior of Myzus persicae on a resistant genotype of peach (Prunus persicae), in which resistance is also conditioned by a single, dominant gene .…”

Transcriptional responses of tolerant and susceptible soybeans to soybean aphid (Aphis glycines Matsumura) herbivory

“…No significant differences were found at day 5 in the tolerant genotype. In plants, all of the aphid resistance genes, reported so far, encode nucleotide binding site-LRR proteins (Crute and Dunn 1980;Chen et al 1997;Rossi et al 1998;Milligan et al 1998;Nombela et al 2003;Wroblewski et al 2007). These large, and often abundant, proteins aid in the detection of diverse pathogens including bacteria, viruses, fungi, insects, and nematodes.…”

Molecular sabotage of plant defense by aphid saliva

“…When aphids penetrate a SE with their stylet tips, they start injecting watery saliva into the SE (20), and, if the penetration is successful, after a period of secretion of watery saliva, the aphid then begins ingesting sap from the SE. When feeding on resistant plants, the transition from salivation to ingestion often fails to occur or, if ingestion does occur, the aphid often reverts back to salivation after a brief ingestion period (21,22).…”