A similar induc tion of transcription factors and defence related genes was observed by Bonaventure and co workers. However, in contrast to the previously observed reaction of fou2 to wounding, further induction of these transcripts upon infestation was much weaker than observed in wt plants. A similar lack of stress responses resulting from prolonged Inhibitors,Modulators,Libraries high endogenous JA levels was observed in potato plants subjected to wounding and water stress. Inhibitors,Modulators,Libraries Although several of the genes involved in JA biosynthesis are induced by JA thereby creating a positive feedback loop, there exists also a negative regulatory feedback loop protecting the plants from the adverse effects of their own defence.

The constitutive Drug_discovery up regulation of the JA synthesis pathway in the fou2 mutant probably triggers this negative feedback loop, leading to desensitization of processes involved in the activation of the aphid induced defence. JAZ family proteins act to repress transcription of JA inducible genes and thus modulate JA mediated plant responses. The high induction of several JAZ genes in the fou2 mutant indicates activation of the desensitization mechanism and may explain the reduced responsiveness of fou2 plants challenged with B. brassicae. The negative regulation of JA responses is delayed and takes effect some time after the proceeding induction. The hyper activation of JA biosynthesis genes in fou2 plants shortly after mechanical wounding that was observed by Bonaventure and co workers was not observed by us after 72 h of sustained B. brassicae infestation.

This might be due to a stealthy manner Inhibitors,Modulators,Libraries of aphid feeding that causes only minimal tissue damage. The induction of the wound specific JA responses in aphid infested plants is therefore much weaker than in mechanically wounded plants. In addition, the high level of JAZ repressors may also tune the JA regulated tran scriptional changes in the aphid attacked fou2 plants after 72 h. Aphid fitness is comparable on wt and aos genotypes but reduced on fou2 Despite the reduced responsiveness Inhibitors,Modulators,Libraries of a wide range of defence linked genes in the aos mutant, we did not observe any improvement in aphid fitness in comparison to wt plants. This may seem surprising as JA signalling seems to be important for plant defence mechanisms induced upon infestation. In contrast to our results, Ellis and co workers observed increased growth of green peach aphid populations on the coi1 16 mutant that had defects in JA signalling. However the coi1 16 line carries an additional mutation that might have influenced M. persicae responses observed by Ellis and co workers. This mutation lies in the PENE TRATION2 gene encoding a glycoside hydrolase and renders the PEN2 protein with highly reduced stabi lity.