Characterization of a 1-aminocyclopropane-1-carboxylate synthase gene from loblolly pine (Pinus taeda L.)

1-Aminocyclopropane-1-carboxylate (ACC) synthase catalyzes what is typically the rate-limiting step in the biosynthesis of ethylene, a gaseous plant growth regulator that plays numerous roles in the growth and development of higher plants. Although ACC synthase genes have been characterized from a wide variety of angiosperm plant species, no ACC synthase genes have been described previously for gymnosperms. Evidence suggests that ethylene helps to regulate wood formation in trees, and may also signal for the metabolic shifts that lead to compression wood formation on the undersides of branches and leaning stems in gymnosperm trees. Since compression wood is an inferior feedstock for the manufacturing of most wood products, a better understanding of the factors influencing its formation could lead to substantial economic benefits. This study describes the isolation and characterization of a putative ACC synthase gene, PtaACS1, from loblolly pine (Pinus taeda L.), an important commercial forest tree species. Also described is an apparent splice variant of PtaACS1 (PtaACS1s) that is missing 138 bp from the 5′ end of the transcript, including bases that encode a conserved amino acid residue considered critical for ACC synthase activity. The two sequences share interesting homologies with a group of plant aminotransferases, in addition to ACC synthases, but structural models and the conservation of critical catalytic amino acid residues strongly support PtaACS1 as encoding an active ACC synthase. The two transcripts were differentially expressed in various tissues of loblolly pine, as well as in response to perturbations of pine seedling stems. Transcript levels of this ACC synthase gene increased rapidly in response to bending stress but returned to near starting levels within 30 min. It remains unclear to what extent bending-induced expression of this gene product plays a role in compression wood formation.