PLANT DEFENSE: TRANSCRIPTION PLAYS THE CONTROL POINT FOR REGULATION OF GENE EXPRESSION

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):

  1. DNA Binding region (recognizes specific DNA sequences in promoters)
  2. Transcription –regulation domain   
  3. Oligomerization  site 
  4. Nuclear localization domain

TF families play role in plant defense responses (Seo and Choi 2015; Dubos et al., 2010; Hong 2016):  

  1.  WRKY
  2. APETALA2/ethylene responsive factor (AP2/ERF)
  3. Basic-domain leucine-zipper (bZIP)
  4. Basic helix-loop-helix (bHLH)
  5. NAM/ATAF/CUC (NAC)
  6. MYB

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):

  1. Transcription (co)factors must have the ability to rapidly perceive signal relayed by signaling hormone   and translate it into functional response.
  2. 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.
  3. 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.
  4. 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.

References:

Despres, C., Chubak, C., Rochon, A., Clark, R., Bethune, T., Desveaux, D. and Fobert, P. R. 2003 The Arabidopsis NPR1 Disease Resistance Protein is a Novel Cofactor that Confers Redox Regulation of DNA Binding Activity to the basic domain/Leucine Zipper Transcription Factor TGA1. Plant Cell 15(9): 2181 – 2191

doi:10.1105/tpc.012849

Dubos, C., Stracke, R., Grotewold, E., Weisshaar, B., Martin, C. and Lepiniec, L. 2010 MYB Transcription Factors in Arabidopsis. Trends in Plant Sci. 15(10): 573 – 581

doi.org/10.1016/j.tplants.2010.06.005

Guilfoyle, T. J. and Dietrich, M. A. 1987 Plant RNA Polymerases: Structures, Regulation and Genes. In: Tailoring Genes for Crop Improvement.  Bruening G., Harada, J., Kosuge, T., Hollaender, A., Kuny, G. and Wilson, C. M. (eds.). Basic Life Sciences.  Springer, Boston, MA. Vol. 41:  87 – 100

doi.org/10.1007/978-1-4684-5329-4_8

Hong, J. C. 2016 General Aspects of Plant Transcription Factor Families. In “Plant Transcription Factors Evolutionary, Structural and Functional Aspects”. Gonzalez, D. H (ed.). Academic Press.  Chapter 3 pp:  35 – 56

doi.org/10.1016/B978-0-12-800854-6.00003-8

Johnson, C., Boden, E. and Arias, J. 2003 Salicylic Acid and NPR1 Induce the Recruitment of Transactivating  TGA Factors to a Defense Gene Promoter in Arabidopsis. Plant Cell 15(8): 1846 – 1858

doi:10.1105/tpc.012211

Kinkema, M., Fan, W. and Dong, X. 2000 Nuclear Localization of NPR1 is Required for Activation of PR Gene Expression. Plant Cell 12: 2339 – 2350

doi.org/10.1105/tpc.12.12.2339

Lecourieux, D., Ranjeva, R. and Pugin, A. 2006 Calcium in Plant Defence-Signaling Pathways. New Phytol. 171(2) 249 -269

doi: 10.1111/j.1469-8137.2006.01777.x

Liu, L., White, M. J. and MacRae, T. H. 1999 Transcription Factors and their Genes in Higher Plants Functional Domains, Evolution and Regulation. Eur. J. Biochem. 262: 247 – 257

doi.org/10.1046/j.1432-1327.1999.00349.x

Ma, W. and Berkowitz, G. A. 2007 The Grateful Dead: Calcium and Cell Death in Plant Innate Immunity.  Cell Microbiol. 9(11): 2571 – 2585

doi:10.1111/j.1462-5822.2007.01031.x

Mou, Z., Fan, W. and Dong, X. 2003 Inducers of Plant Systemic Acquired Resistance Regulate NPR1 Function through Redox Changes. Cell 113(7): 935 – 944

doi.org/10.1016/S0092-8674(03)00429-X

Meshi, T. and Iwabuchi, M. 1995 Plant Transcription Factors. Plant Cell Physiol. 36(8): 1405 – 1420

doi.org/10.1093/oxfordjournals.pcp.a078903

Moore, J. W., Loake, G. J. and Spoel, S. H. 2011 Transcription Dynamics in Plant Immunity. Plant Cell 23(8): 2809 – 2820

doi:10.1105/tpc.111.087346

Pieterse, C. M. J., Leon-Reyes, A., Van der Ent, S. and Van Wees, S. C. M.  2009 Networking by Small-Molecule Hormone in Plant Immunity. Nat. Chem. Biol. 5: 308 – 316

doi:10.1038/nchembio.164

Seo, E. and Choi, D. 2015 Functional Studies of Transcription Factors Involved in Defenses in the Genomics Era. Brief.  Funct. Genomics 14(4): 260 – 267

doi.org/10.1093/bfgp/elv011

Spoel, S. H., Mou, Z., Tada, Y., Spivey, N. W., Genschik, P. and Dong, X. 2009 Proteosome-Mediated Turnover of the Transcription Coactivator NPR1 Plays Dual Roles in Regulating Plant Immunity. Cell 137(5): 860 – 872

doi: 10.1016/j.cell.2009.03.038

Wang, D., Amornsiripanitch, N. and Dong, X. 2006 A Genomic Approach to Identify Regulatory Nodes in the Transcriptional Network of Systemic Acquired Resistance in Plants. PLoS Pathog. 2(11): e123

doi: 10.1371/journal.ppat.0020123

Yanagisawa, S. 1998 Transcription Factors in Plants: Physiological Function and Regulation of Expression. J. Plant Res. 111(3): 363 – 371

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Zhang, Y., Fan, W., Kinkema, M., Li, X. and Dong, X. 1999 Interaction of NPR1  with Basic Leucine Zipper Protein Transcription Factors that Bind Sequences Required for Salicylic Acid Induction of the PR-1 Gene. Proc. Natl. Acad. Sci. USA 96(11): 6523 – 6528

doi: 10.1073/pnas.96.11.6523

Zhou, J. M., Trifa, Y., Silva, H., Pontier, D., Lam, E., Shah, J. and Klessig, D. F. 2000 NPR1 differentially Interacts with Members of the TGA/OBF Family of Transcription Factors that Bind an Element of the PR-1 Gene Required for Induction by Salicylic Acid. Mol. Plant Microbe Interact. 13: 191 – 202

doi: 10.1094/MPMI.2000.13.2.191

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