Ultrastructure of phloem-based resistance in melon to the cotton aphid
Garzo E.*, Fernandez-Pascual, M., Morcillo, C., Fereres, A., Gomez-Guillamon, M.L. and Tjallingii, W.F.
Instituto de Ciencias Agrarias, CSIC
Resistance of the melon line TGR-1551 to the aphid Aphis gossypii (Glover) is based on preventing aphids from ingesting phloem sap. In electrical penetration graphs (EPGs), this resistance has been characterized with A. gossypii showing unusually long phloem salivation periods (waveform E1) mostly followed by pathway activities (waveform C) or if followed by phloem ingestion (waveform E2), ingestion was not sustained for more than 10 minutes. Stylectomy with aphids on susceptible and resistant plants was performed during EPG recording while the stylet tips were phloem inserted. This was followed by dissection of the penetrated leaf section, plant tissue fixation, resin embedding, and ultrathin sectioning for transmission electron microscopic (TEM) observation in order to study the resistance mechanism in the TGR melon. The most obvious aspects appeared to be the coagulation of phloem proteins in the stylet punctured in sieve elements. Stylets of five aphids per genotype were amputated during the EPG waveforms E1, sieve element (SE) salivation and E2, SE ingestion. Cross-sections of stylet bundles in susceptible melon plants showed that the contents of the stylet canals were totally clear and also, no coagulated phloem proteins occurred in their punctured sieve elements. In contrast, electron-dense coagulations were found in both locations in the resistant plants. Due to calcium binding, aphid saliva has been hypothesized to play an essential role in preventing/suppressing such coagulations that cause occlusion of sieves plate and in the food canal of the aphid’s stylets. Doubts about this role of E1 salivation are discussed on the basis of our results.