Differential responses of the promoters from nearly identical paralogs of loblolly pine (Pinus taeda L.) ACC oxidase to biotic and abiotic stresses in transgenic Arabidopsis thaliana

Promoters from an ACC oxidase gene (PtACO1) and its nearly identical paralog (NIP) (PtACO2) of loblolly pine (Pinus taeda L.) were recovered from genomic DNA using PCR amplification. Transgenic Arabidopsis plants harboring genetic constructs from which β-glucuronidase (GUS) expression was driven by the full-length (pACO1:GUS, pACO2:GUS) or truncated (pACO1-1.2:GUS, pACO2-1.2:GUS) loblolly pine ACC oxidase gene promoters displayed distinctive patterns of expression for the different promoter constructs. Both full-length promoter constructs, but not those using truncated promoters, responded to indole-3-acetic acid (IAA) and wounding. Both pACO1:GUS and pACO1-1.2:GUS responded to pathogen attack, while neither version of the pACO2 promoter responded to infection. In the inflorescence stalks, the full-length pACO1 promoter construct, but not the truncated pACO1-1.2:GUS or either pACO2 construct, responded to bending stress. When flowering transgenic Arabidopsis plants were placed in a horizontal position for 48 h, expression from pACO2:GUS, but not the other constructs, was induced on the underside of shoots undergoing gravitropic reorientation. The expression pattern for the pACO2:GUS construct in transgenic Arabidopsis was consistent with what might be expected for a gene promoter involved in the compression wood response in loblolly pine. Although near complete sequence identity between PtACO1 and PtACO2 transcripts prevented quantitation of specific gene products, the promoter expression analyses presented in this study provide strong evidence that the two ACC oxidase genes are likely differentially expressed and responded to different external stimuli in pine. These results are discussed with respect to the potential functional differences between these two genes in loblolly pine.