Root-specific expression of a western white pine PR10 gene is mediated by different promoter regions in transgenic tobacco

We report here the isolation and characterization of a novel PR10 gene, PmPR10-1.14, from western white pine ( Pinus monticola Dougl. ex. D. Don). The PmPR10-1.14 gene encodes a polypeptide exhibiting high similarity with other members of the PR10 family and corresponds to one of six isoforms immunodetected in the roots of western white pine. Northern blot and western immunoblot analyses showed that expression of the PR10 gene family, including PmPR10-1.14, was detected in vegetative tissues constitutively, but not in developing reproductive organs. RT-PCR with gene-specific primers showed that the transcript of PmPR10-1.14 gene was found only in lateral roots and needles during growth. To study PR10 gene regulation at the cellular level, PmPR10-1.14 promoter was fused to the beta-glucuronidase (GUS) report gene, and analyzed for transient and stable gene expression. The transient expression assays in agroinfiltrated tobacco leaves indicated that the core promoter of PmPR10-1.14 gene resided in the sequence from -101 to +69 relative to the first nucleotide of PR10 cDNA. Furthermore, the promoter region from -311 to -101 acted as an enhancer, and the region from -506 to -311 as a silencer. Fluorometric GUS assays of transgenic tobacco plants demonstrated that the longest promoter of 1675 bp directed GUS expression constitutively at high levels in the roots of mature plants, but expression levels were too low to be detectable in other organs in histochemical assays. Histochemical localization analysis showed that PmPR10-1.14 promoter directed a tissue-specific expression exclusively during the initiation and development of the lateral roots. The distal 5' deletion of the promoter to -311 did not decrease the expression level significantly in the roots, suggesting that the cis-regulatory elements necessary for a high level of gene expression reside in the proximal fragment from -311 to +69. As one striking feature, PmPR10-1.14 promoter contains two copies of direct repeated sequences as long as 281 bp at its distal 5' region. Deletion of one copy (-1326 to -1045) or both copies (-1675 to -1045) of the repeated sequences increased gene expression significantly in leaves and stems, which was regulated developmentally. Further deletion to -820 erased the increased gene expression in leaves and stems. These experiments revealed that the root-specific expression of PmPR10-1.14 gene is mediated by different promoter regions with both negative and positive regulatory mechanisms in transgenic tobacco plants.