Mi-mediated resistance against the potato aphid Macrosiphum euphorbiae (Hemiptera : Aphididae) limits sieve element ingestion

The effect of the tomato (Lycopersicon esculentum Miller) resistance gene, Mi, on the feeding behavior of the potato aphid, Macrosiphum euphorbiae (Th omas), was studied using an AC electronic monitoring system. Aphids were mo nitored for at least 16 h on nearly isogenic tomato lines, and we observed waveforms corresponding to the stylet pathway phase, sieve element and nonp hloem phase. The time to the first probe was not significantly different be tween resistant and susceptible tomato lines, indicating that leaf surface components were not involved in the resistance Also, time to the first siev e element contact, as indicated by the sieve element phase waveform, was no t significantly different, indicating that aphids were able to locate the s ieve element equally well on resistant and susceptible lines. However, the duration of the sieve element phase was 7- to 10-fold longer on susceptible plants than on resistant plants. The insect also made more frequent and br iefer probes on resistant tomato than on susceptible tomato. These results indicate that the mechanism of resistance mediated by Mi involves limitatio n of the duration of the sieve element phase, and salivation and ingestion of phloem fluids associated with it.