The senescence of carnation (Dianthus caryophyllus L.) flower petals is associated with increased synthesis of the phytohormone ethylene. This ethylene serves to initiate and regulate the processes of programmed cell death. We are using molecular approaches to study the regulation of ethylene biosynthesis in various floral organs during development and senescence of flowers. We have isolated and cloned mRNAs which encode the ethylene biosynthetic pathway enzymes s-adenosylmethionine (SAM) synthetase, 1-aminocyclopropane-1-carboxylate (ACC) synthase and the ethylene forming enzyme (EFE) from carnation flower petals. These cDNAs have been used as molecular probes to determine the steady-state mRNA levels of these transcripts in senescing flowers. The increase in ethylene associated with petal senescence is accompanied by a dramatic increase in the abundance of transcripts for both ACC synthase and EFE. In striking contrast, the level of SAM synthetase mRNA decreases significantly with the onset of petal senescence. Genomic DNA Southern blots reveal both ACC synthase and EFE are encoded by multigene families. We have recently isolated several genomic clones from carnation which represent different ACC synthase genes. The structure and organization of these gene will be presented.