Organ-specific and auxin-inducible expression of two tobacco par A-related genes in transgenic plants.

We have isolated four genomic DNA clones that contain the transcription initiation site of the parA gene(s) from a tobacco genomic library by hybridization with the 5' segment of the parA cDNA previously isolated. They were classified into two types on the basis of their nucleotide sequences. Southern blot analysis indicated that two types of clones were respectively derived from the two parental species of tobacco, Nicotiana tomentosiformis and Nicotiana sylvestris. The genes corresponding to these clones were designated as parAt and parAs, respectively, and the parA cDNA clone was shown to code for mRNA from parAt on the basis of its nucleotide sequence. The 5' regions about 400 nucleotides upstream from the transcription initiation sites of the parAt and parAs genomic clones were highly homologous to one another, but regions further upstream showed no significant similarity. The coding sequence of the GUS (beta-glucuronidase) reporter gene was linked to the 5'-upstream regions of parAt and parAs, and the sites of expression of these fusion genes were examined in transgenic tobacco plants. In the absence of auxins, both fusion genes were expressed in capsules at a late stage of seed development, mature seeds, a root apex and a root-hair zone whereas no significant expression was seen in other organs. Their expression was enhanced by 2,4-dichlorophenoxyacetic acid in most of the organs of tobacco. The results show that expression of these genes is regulated by both organ-specific and auxin-inducible mechanisms.