Abstract
Our previous study demonstrated that the expression of GhNAC4, a NAC transcription factor from cotton, was induced by abiotic stresses and abscisic acid (ABA). In the present study, we investigated the molecular mechanisms underlying ABA and stress response of GhNAC4. Overexpression of GhNAC4 in transgenic tobacco conferred tolerance to salinity and drought treatments with associated enhanced expression of several stress-responsive marker genes. GhNAC4 is a protein that is translocated to the nucleus where it exhibits transcriptional activation property and also forms homo-dimers. In this study, we also investigated the domains essential for the biochemical functions of GhNAC4. We developed transgenic tobacco plants overexpressing the GhNAC4 NAC-domain and the transcriptional regulatory (TR) domain separately. NAC-domain transgenics showed hypersensitivity to exogenous ABA while TR-domain transgenics exhibited reduced sensitivity. Abiotic stress assays indicated that transgenic plants expressing both the domains separately were more tolerant than wild type plants with the NAC-domain transgenics showing increased tolerance as compared to TR-domain transgenics. Expression analysis revealed that various stress-responsive genes were upregulated in both NAC-domain and TR-domain transgenics under salinity and drought treatments. These results suggest that the stress tolerance ability of GhNAC4 is associated with both the component domains while the ABA responsiveness is largely associated with N-terminal NAC-domain.
Highlights
Transcription factors (TFs) are key modulators of gene expression patterns leading to the regulation of growth, development and environmental stress responses
The findings suggested that the abscisic acid (ABA)-responsiveness of GhNAC4 is largely associated with the NAC-domain while the abiotic stress tolerance functionality of GhNAC4 is associated with both NAC- and transcriptional regulatory (TR) domains
We demonstrated that the NAC-domain is largely responsible for the ABA responsiveness of GhNAC4
Summary
Transcription factors (TFs) are key modulators of gene expression patterns leading to the regulation of growth, development and environmental stress responses. The earliest reports of the modular nature of TFs came from the ‘domain-swap’ experiment with the fusion of DNA binding domain of LexA ( Escherichia coli repressor protein) and the activation domain of GAL4 TF ( Saccharomyces cerevisiae transcriptional activator). This resulted in a chimeric transcriptional activator that identified LexA binding site (Brent and Ptashne 1985). This remarkable modular nature of TFs has been confirmed in many other systems (Graham et al 1999; Hollenberg and Evans 1988; Porsch et al 2005). The exact position of these domains/modules within the chimeric proteins is highly flexible suggesting that each domain represents an independent structural module with an independent function (Frankel and Kim 1991)
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