Isolation and characterization of salt inducible asparagine synthetase gene promoter from the halophyte Pandanus odorifer (Forssk.) Kuntze

The asparagine synthetase (ASN) gene plays a vital role in plant growth and development and in dealing with abiotic and biotic stresses. In the present study, we identified two isoforms of ASN genes (PoASN1 and PoASN2) from the leaf transcriptome of 1 M NaCl-treated Pandanus odorifer and confirmed their salt-inducible expression by RT-qPCR. PoASN1 showed 12.4 fold upregulation in response to salt stress, although PoASN2 was slightly (1.8 fold) upregulated compared with the control. To understand salt inducible nature, isolation and analysis of the promoter region of the ASN genes were performed. We employed a genome-skimming approach to isolate and characterize the regulatory genomic counterparts of these genes. This study provides insights into gene structure, cis-regulatory elements, transcription factor binding sites (TFbs), and CpG islands in the promoters. The gene structure analysis showed the presence of two extra exons and introns in PoASN2. A difference was also observed in the gene size of PoASN1 (4,221 bp) and PoASN2 (25,158 bp). The presence of additional putative cis-regulatory elements such as ABRE, ABRE2, ABRE3a, ABRE4 motifs, and DRE-core elements might be one of the reasons for the inducible expression of PoASN1 during salinity. Furthermore, the occurrence of different TFbs suggests that the expression of PoASN genes may be regulated by an intricate network of transcription factors. Since P. odorifer is a halophyte, this promoter information can be utilized drive expression of the candidate genes under salt stress to engineer salt tolerance in glycophytes.