DIFFERENTIAL ACTIVATION OF 2 ACC OXIDASE GENE PROMOTERS FROM MELON DURING PLANT DEVELOPMENT AND IN RESPONSE TO PATHOGEN ATTACK

ACC (1-aminocyclopropane-1-carboxylate) oxidase genes are differential ly expressed in melon during development and in response to various st resses. We investigated the molecular basis of their transcription by analysing the 5' untranslated regions of the ACC oxidase genes CM-AC01 and CM-AC03. In order to determine how their temporal and spatial exp ression patterns were established, we fused the promoter regions of CM -AC01 (726 bp) and CM-AC03 (2260 bp) to the beta-glucuronidase (GUS) r eporter gene and examined their regulation in transgenic tobacco plant s. The CM-AC01 promoter was able to drive GUS expression in response t o wounding, and to treatment with ethylene or copper sulfate. It was a lso rapidly induced (8-12 h postinoculation) in tobacco leaves inocula ted with the hypersensitive response (HR)-inducing bacterium Ralstonia solanacearum. Expression was also observed during compatible interact ions but was delayed. In contrast, the CM-AC03 promoter was not expres sed in response to infection, but was up-regulated during flower devel opment. Both promoters were regulated during leaf senescence but in di fferent patterns. The CM-AC01-driven GUS activity increased sharply co ncomitantly with the onset of chlorophyll breakdown, while the CM-AC03 promoter drove strong GUS expression in green, fully expanded leaves and this declined at the onset of senescence. This result is consisten t with the expression patterns of these two genes in senescent melon l eaves. These data suggest that the regulation of expression of CM-AC01 is related preferentially to stress responses, whereas CM-AC03 seems to be associated with developmental processes. The possible role of et hylene is discussed, particularly in the regulation of the CM-AC01 gen e in response to stress and during senescence.