Methyl jasmonate induced expression of the tobacco putrescine N-methyltransferase genes requires both G-box and GCC-motif elements

Abstract

Putrescine N-methyltransferase (PMT) catalyzes the first committed step of nicotine biosynthesis, converting putrescine into N-methylputrescine. A variety of chemical, environmental, and developmental cues have been implicated in its regulation. Here we have examined the differential expression of beta-glucuronidase (GUS) transgenes under the control of the transcriptional regulatory sequences of four distinct members of the NtPMT gene family from tobacco (Nicotiana tabacum L.). BY-2 cell cultures expressing various NtPMT promoter-GUS constructs were examined for their response to treatment with various combinations of methyl jasmonate (MeJA), auxin (AUX), and ethylene (ETH). All four NtPMT gene promoters examined were inducible by MeJA, although the extent of the induction varied dramatically, with the NtPMT1a promoter being the most responsive. High AUX levels in the cell growth media repressed NtPMT::GUS transgene expression and inhibited their MeJA-induced transcription. Treatment of BY-2 cells with ETH alone did not result in a significant alteration in NtPMT::GUS expression. However, similar to AUX, ETH treatment led to the suppression of MeJA-induced transcription. Detailed deletion analysis of the NtPMT1a gene promoter showed that as little as 111 bp upstream of the transcriptional start site were sufficient to confer MeJA-responsiveness. Deletion of a conserved G-box element (GCACGTTG) at -103 to -96 bp completely abolished MeJA-responsiveness. Further mutagenesis studies revealed that in addition to a functional G-box, MeJA-responsiveness of the NtPMT1a promoter also required a TA-rich region and a GCC-motif (TGCGCCC) located at -80 to -69 bp and -62 to -56 bp relative to the start site, respectively. A synthetic G-box tetramer (4 X syn G-box) fused to a -83 bp fragment from the NtPMT1a promoter (containing the TA-rich region, GCC-box, and TATA-box) displayed a 30-fold induction by MeJA treatment, whereas when the 4 X syn G-box was fused to a minimal (-46 bp) promoter fragment derived from the CaMV 35S gene, no induction by MeJA treatment was detected. Our results indicate that multiple intersecting signal transduction pathways and different transcriptional regulatory factors are involved in mediating JA-responsiveness of NtPMT expression in tobacco.