Analysis by DC-EPG of the resistance to Bemisia tabaci on an Mi-tomato line

The tomato Mi gene confers resistance to nematodes, Meloidogyne spp., and t o the potato aphid, Macrosiphum euphorbiae (Thomas). Previous greenhouse ch oice assays with Bemisia tabaci (Gennadius) showed that tomato commercial v arieties carrying this gene had significantly lower values of host suitabil ity and whitefly reproduction than varieties lacking Mi. This indicated tha t Mi, or another gene in its region, could regulate partial resistance. In order to characterise this resistance, probing and feeding behaviour of Bem isia tabaci B-biotype was studied with DC Electrical Penetration Graph (EPG ) technique on the near-isogenic tomato lines Moneymaker (without Mi) and M otelle (carrying Mi). Significant differences (P < 0.05) between tomato lin es were found in EPG parameters related to epidermis and/or mesophyll tissu es. On Motelle, a lower percentage of whiteflies achieved phloem phase and they made more probes before attaining first phloem phase, had a higher rat io (number of probes before first phloem phase)/(total number of probes), h ad a longer total duration of non-probing time, and a longer time before ma king the first intracellular puncture and before making the first phloem ph ase. In contrast, most of the parameters related to phloem phase were found not to differ significantly between these near-isogenic lines. The behavio ural data strongly suggest that the partial resistance in the variety Motel le is due to factors in the epidermis and/or mesophyll that inhibit the whi teflies from reaching phloem sieve elements. However, once the stylets reac h a sieve element, whitefly behaviour did not differ between the two variet ies. Thus, phloem sap of the two varieties appears to be equally acceptable to the whiteflies. Further studies are necessary to provide a better under standing of these mechanisms of resistance to whiteflies in tomatoes.