Plant transcription factors (TFs) are sequence-specific DNA-binding proteins that play role in defense responses against pathogen attack. Transcription factor has four domain (Liu et al., 1999), a DNA binding domain, an oligomerization site, a transcription-regulation domain and a nuclear localization signal (NLS). The DNA- binding domain is responsible for binding of the TFs to specific cis-regulatory DNA sequences in the promoters of genes that they regulate. The NLS are short peptide motifs that mediate the nuclear import of protein by binding to their receptor known as importins (Kosugi et al., 2009). Some plant TFs may lack NLS but can be imported in the nucleus by dimerizing with proteins that possess these signals. The DNA-binding basic domain of helix-loop-helix TFs may serve as NLS (Goldfarb and Lewandowska 1994).
There are two mechanism of transmission of signals from cell-surface receptors to the nucleus and both involve protein phosphorylation (Karin and Hunter 1995):
- First: A regulated translocation of activated protein kinases from the cytoplasm into the nucleus where they phosphorylate target transcription factors
- Second: The TFs are inactive in the cytoplasm but upon activation are translocated into the nucleus.
Upon receiving a signal from the cell membrane signal transduction, TFs are activated and then translocated from the cytoplasm into the nucleus where they interact with the corresponding DNA frame (cis-acting elements) (Liu et al., 2018). Phosphorylation has the ability to modulate nuclear translocation.
Transcription Factor families are as follows:
- APETALA2/ETHYLENE RESPONSIVE FACTOR (AP2/ERF)
- BASIC-DOMAIN LEUCINE-ZIPPER (bZIP)
- BASIC HELIX-LOOP-HELIX (bHLH)
- NAM/ATAF/CUC (NAC)
WRKY TFs are regulatory component of plant responses to pathogen. WRKY33 play a role in plant defense against necrotrophic pathogens (Zheng et al., 2006). The expression of WRKY70 is activated by SA and repressed by JA. WRKY70 can function either as a positive or as a negative regulator of PR gene expression (Li et al., 2004).
- APETALA2 (AP2)/ ETHYLENE RESPONSIVE FACTOR (ERF):
Transcription factors AP2/ERFs regulates plant developmental processes and their response to biotic and abiotic stresses (Saleh and Pages 2003).
AP2/ERFs contain four subfamilies (Sakuma et al., 2002):
- APETALA2 (AP2)
- RELATED TO ABSCISIC ACID INSENSITIVE3/VIVIPAROUS1 (RAV)
- DEHYDRATION RESPONSIVE ELEMENT BINDING proteins (DREBs) (subgroup A1-A6)
- ETHYLENE RESPONSIVE FACTORS (ERFs) (subgroup V-X)
Ethylene Responsive Factor (ERF1) and (ERF6) in Arabidopsis regulates defense gene expression and resistance to the necrotrophic fungal pathogen Botrytis cinerea (Berrocal-Lobo et al., 2002; Meng et al., 2013).
- NAC :
The NAC (NAM, ATAF and CUC) family is plant specific group of TFs. The NAC acronym is derived from three genes that initially contain a particular domain (the NAC domain): NAM (for no apical meristem), ATAF1 and -2 and CUC (for cup-shaped cotyledon). Some NAC TFs can act as a positive or negative regulators of the plant defense responses (Yuan et al., 2019). Kaneda et al. (2009) reported the over expression of Os NAC4 (rice NAC4) that leads to hypersensitive response (HR) cell death accompanied by the loss of plasma membrane integrity, nuclear DNA fragmentation and typical morphological changes.
- BASIC LEUCINE ZIPPER DOMAIN (bZIP):
Basic Leucine Zipper Domain TF have ability to regulate genes associated with PAMP-triggered immunity, effector-triggered immunity and hormonal signaling network (Noman et al., 2017). The best known bZIP involved in plant defense belong to the TGA family. TGA2 and NON EXPRESSER OF PR GENE1 (NPR1) are activator of systemic acquired resistance (SAR). During SAR TGA2 recruits NPR1 as part of enhanceosome (Boyle et al., 2009). Nuclear localization of NPR1 is essential for induction of PR genes (Kinkema et al., 2000).
- BASIC HELIX-LOOP-HELIX (bHLH):
The bHLH transcription factor MYC2 acts as a master regulator of JA signaling pathway and can control the crosstalk between JA and other hormone signaling pathway. MYC2 can physically interact with other key regulatory proteins to form heterodimers with other TFs, it also has ability to activate or repress gene expression in response to multiple signals (Kazan and Manners 2013). Fernandez-Calvo et al. (2011) reported that MYC3 and MYC4 are activators of JA-regulated program that function along with MYC2 to regulate different subset of the JA-dependent transcriptional response.
In plant MYB TFs regulate biotic and abiotic stresses. Segarra et al. (2009) reported MYB72 functions as a node of convergence in the signalling pathway that are induced by the different beneficial microorganisms. MYB96 is a molecular link that mediates ABA-SA crosstalks. Observation of Seo and Park (2010) indicate that MYB96 – mediated abscisic acid (ABA) signals enhance plant disease resistance by inducing SA biosynthesis. Raffaele et al. (2006) demonstrated that Arabidopsis thaliana MYB30 (AtMYB30) expression in response to an HR-inducing bacterial pathogen is dependent on SA accumulation but NPR1 independent.
Plants have survival strategy relying on gene regulation by transcription factor.
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