Plant immunity is accomplished by the action of transcriptional regulator. Signaling pathways cross-communicate in an antagonistic or synergistic manner enabling plant to regulate its immune response (Pieterse et al., 2009). Transcription play a central role in regulation of expression of gene and it appears to be controlled by numerous transcription factors (TFs) that mediates the effect of inter or extracellular signals (Yanagisawa 1998).
Transcription factors are DNA-binding proteins that play a role in regulation of gene expression by binding to specific DNA sequences called cis-acting element in the promoter region of the target gene. The activities of TFs are controlled by post-translational modification such as phosphorylation, glycosylation as well as nuclear transport and oligomerization (Meshi and Iwabuchi 1995). In nucleus, genomic DNA is transcribed by RNA polymerase II (RNAPII). RNA polymerase II produces precursors to mRNA and some small nuclear RNAs (Guilfoyle and Dietrich 1987).The mRNA transcription is the control point for the regulation of the gene expression which requires a complex interaction of several proteins involved in formation of preinitiation complex at the core of promoter region located close to the transcription site (Hong 2016).
A typical plant transcription factor has four domain (Liu et al., 1999):
- DNA Binding region (recognizes specific DNA sequences in promoters)
- Transcription –regulation domain
- Oligomerization site
- Nuclear localization domain
TF families play role in plant defense responses (Seo and Choi 2015; Dubos et al., 2010; Hong 2016):
- APETALA2/ethylene responsive factor (AP2/ERF)
- Basic-domain leucine-zipper (bZIP)
- Basic helix-loop-helix (bHLH)
- NAM/ATAF/CUC (NAC)
The plants transcriptional regulators consists of DNA binding transcription factors and a cofactors. TFs function as activator and repressor and cofactors that do not bind to the DNA directly can coactivate or corepress transcription through interaction with DNA binding TFs.
As an executioner of gene expression, transcription (co)factors must have following requirement (Moore et al., 2011):
- Transcription (co)factors must have the ability to rapidly perceive signal relayed by signaling hormone and translate it into functional response.
- Transcriptional factor should rapidly locate their cognate DNA binding motif, whereas, cofactors that do not directly bind to DNA must recognize the correct chromatin site at which they are required.
- Transcription (co)repressor must suppress the recruitment of RNA Polymerase II (RNAPII) whereas, transcription (co)activator must recruit RNAPII to target promoter in a regulated manner. The RNAPII should be recruited and executed number of times in order to produce abundant mRNA according to the intensity of the signal.
- Transcription (co)factors function within large network in a synergistic or antagonistic manner to regulate the expression of gene network that contain feedforward and feedback loop (Moore et al., 2011).
The mechanism by which the transcription (co)factors perceive cellular signal are diverse. Plant cell signal transcription (co)activators switch from inactive state to active state by:
i Rapid Ca2+ accumulation upon activation of plant defense response (Lecourieux et al., 2006; Ma and Berkowitz 2007).
ii Cellular redox also regulate TGA transcription factors that interacts with NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) to form transcription transactivating complex (Zhang et al., 1999; Zhou et al., 2000).
In plants, immune (co)activators sequester, both in cytoplasm as well as in nucleus. In unchallenged cell the NPR1 is sequestered in cytoplasm as an oligomeric complex (Mou et al., 2003) whereas, in the nucleus the NPR1 monomer is continuously cleared by the proteasome which restricts its transcription coactivator activity to prevent untimely activation of SAR (Spoel et al., 2009).
Signaling hormone salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) mobilize the activity of transcriptional regulator. SA signal molecule induces defense gene expression via activation of NPR1 (Mou et al., 2003). Accumulation of SA triggers a change in cellular reduction potential resulting change in NPR1 from oligomeric to monomeric state through reduction of disulphide bonds. The NPR1 monomer moves to the nucleus where it functions as a co-activator of gene transcription (Kinkema et al., 2000). Nuclear translocation of NPR1 is a regulatory step. The NPR1 may affect both the DNA binding capacity and the activity of TGA transcription factor (Despres et al., 2003). In the nucleus the NPR1 monomer interacts with TGA TF to induce expression of PR genes thus activating SAR (Johnson et al., 2003). NPR1 also directly activates the expression of several WRKY transcription factors having both the activator and suppressor activities (Mou et al., 2003; Wang et al., 2006). For a successful transcription (co) activators must overcome (co) repressor activities.
As soon as the transcription (co) activator finds its location on the chromatin they initiate transcription.
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