ACC oxidase genes expressed in the wood-forming tissues of loblolly pine (Pinus taeda L.) include a pair of nearly identical paralogs (NIPs)

Abstract

1-Aminocyclopropane-1-carboxylate (ACC) oxidase catalyzes the final reaction of the ethylene biosynthetic pathway, converting the unusual cyclic amino acid, ACC, into ethylene. Past studies have shown a possible link between ethylene and compression wood formation in conifers, but the relationship has received no more than modest study at the gene expression level. In this study, a cDNA clone encoding a putative ACC oxidase, PtACO1, was isolated from a cDNA library produced using mRNA from lignifying xylem of loblolly pine (Pinus taeda) trunk wood. The cDNA clone comprised an open reading frame of 1461 bp encoding a protein of 333 amino acids. Using PCR amplification techniques, a genomic clone corresponding to PtACO1 was isolated and shown to contain three introns with typical GT/AG boundaries defining the splice junctions. The PtACO1 gene product shared 70% identity with an ACC oxidase from European white birch (Betula pendula), and phylogenetic analyses clearly placed the gene product in the ACC oxidase cluster of the Arabidopsis thaliana 2-oxoglutarate-dependent dioxygenase superfamily tree. The PtACO1 sequence was used to identify additional ACC oxidase clones from loblolly pine root cDNA libraries characterized as part of an expressed sequence tag (EST) discovery project. The PtACO1 sequence was also used to recover additional paralogous sequences from genomic DNA, one of which (PtACO2) turned out to be >98% identical to PtACO1 in the nucleotide coding sequence, leading to its classification as a "nearly identical paralog" (NIP). Quantitative PCR analyses showed that the expression level of PtACO1-like transcripts varied in different tissues, as well as in response to hormonal treatments and bending. Possible roles for PtACO1 in compression wood formation in loblolly pine and the discovery of its NIP are discussed in light of these results.