Herrera, R (reprint author), Univ Talca, Inst Biol Vegetal & Biotecnol, 2 Norte 685, Talca, Chile.Loss of verticality in conifers affects the normal wood development by inducing changes and chemical modifications in tree stems called compression wood. It is known that ethylene influences the response during this abnormal wood-forming process. The expression pattern of genes involved in the ethylene biosynthesis pathway during gravitropic response in gymnosperms has been identified in adult trees. Young seedlings of radiata pine were inclined to reveal the expression pattern of these genes by the quantitative real-time PCR (qRT-PCR) technique. The seedlings were exposed to gravitropic stimuli and harvested after 2.5 and 10 h (early responses) of inclination, and after 24 h (late response). Sampling includes transverse cuts at three heights of the whole stem of inclined seedlings. Our data revealed that genes encoding for 1-aminocyclopropane-1-carboxylate oxidase (ACO) and 1-aminocyclopropane-1-carboxylate synthase (ACS) were differentially expressed during the time of leaning, and, interestingly, at the basal portion of radiata pine stems. Additionally, transcriptional analysis in maritime pine showed a conserved profile of gene activation in conifers, and in mature compression wood, ACO gene transcription was strongly upregulated. These results indicate that the concerted activation of genes involved in ethylene biosynthesis could be responding to leaning signals in young radiata and maritime pine seedlings.