ed from 4 dpi onwards, which plays an important role in the transduction of calcium signaling and could be involved in the resistant Brefeldin A ATPase inhibitor response and a catalase that may protect the pathogen from the strong oxidative burst associated with resistance. Additional information about the resistance response could be found by analyzing genes that are modulated in both interactions but with a peculiar pat tern, or those showing a reverse modulation of gene expression in one or the other interactions. An interesting finding is that many of the genes modu lated earlier or more strongly during resistance are clas sified as signal transduction related genes. These include several transcripts involved in cal cium signaling, transcription factors, kinases and a homolog of NDR1.

This last gene is induced at 2 and 4 dpi following infection with race 1 and only at 8 dpi following infection with race 1,2. NDR1 was originally identified in Arabidopsis as a factor required for resistance to both bacterial and fungal pathogens and it is known to mediate resistance controlled by R genes of the nucleotide binding site leu cine rich repeats class, which is distinct from the Toll interleukin receptor class. Fom 2 in Charentais Fom 2 plants is indeed a non TIR R gene, although with a peculiar structure that lacks the typical N terminal coiled coil domain. Therefore, it seems plausible that its action might require NDR1. Other genes modulated earlier in the establishment of resistance include two adenosylhomocysteinases, an aspartokinase homoserine dehydrogenase and a serine carboxypeptidase.

An Arabidopsis adenosylhomocystei nase encoded by the gene HOG1 is required for DNA methylation gene silencing. The involvement of RNA silencing machinery in plant innate immunity has recently been demonstrated not only against viruses but also bacterial and fungal pathogens, including Verticillium in Arabi dopsis. The same transcript increases in aphid infested sorghum AV-951 plants. The potential involvement of these candidate genes in Charentais Fom 2 controlled resistance could be the object of future investigations. Most genes in Cluster D are not differentially modu lated in the incompatible and compatible interactions. Interesting examples include TDFs related to ACC oxi dases. These enzymes participate in the last step of ethy lene biosynthesis and are involved in the response to stress and to pathogens, but are also implicated in senes cence, necrosis and disease development.

Ethylene has been associated with both wilting and resistance against vascular diseases. We detected four transcripts with similarity to ACC oxidases. These showed variable expression profiles, but there was no difference between compatible and incompatible interactions, which research use only suggests that ethylene might be involved in both susceptibility and resistance. In melon, different ACC oxidase genes are induced differentially during development and pathogen infection. The same variable modulation has been detected for ot