SIGNALING MECHANISMS AND PLANT DEFENSE RESPONSES

The plants have diverse immune receptors that can recognize invading pathogens. The  pathogen or damage associated molecular patterns (PAMPs/DAMPs) can be detected by the  surface-localized  pattern recognition receptors (PRRs)  such as receptor-like kinases (RLKs) and  receptor-like proteins (RLPs). The RLKs and RLPs can perceive signals for regulation of plant growth and immunity (Boller and Felix 2009; Li et al., 2016; Tang et al., 2017). Some plants have developed intracellular disease resistance (R) protein to counter the virulence effector of pathogens (He et al., 2007). Plant pathogens secrete effectors into the plant apoplast or cytoplasm to enhance virulence, whereas, the plants have PRRs that can recognize apoplastic effector and intracellular immune receptor that can recognize cytoplasmic effector. Plant intracellular immune receptor contain nucleotide-binding leucine-rich repeat domain   (Li et al., 2016).  

During plant defense responses there is an elevation of calcium (Ca+) in plant cell (Zhang et al., 2014) and changes in protein phosphorylation in response to elicitor depends on the presence of Ca+ (Dietrich et al., 1990). Plant transcription factors (TFs) play role in plant immunity by regulating genes related to pathogen-associated molecular pattern-triggered immunity, effector triggered immunity, hormone signaling pathway and phytoalexin synthesis (Seo et al., 2015). Calcium as a second messenger is involved in both systemic acquired resistance (SAR) and/or hypersensitive response (HR) depending on the calcium signature (Lecourieux et al., 2005; Ma and Berkowitz 2007). SAR requires single molecule salicylic acid (SA) and is associated with the accumulation of pathogenesis related (PR) protein which contributes to resistance (Durrant and Dong 2004).  Calcium influx across the plasma membrane is a part of signaling chain leading to resistance against pathogen (Zimmermann et al., 1997). The elevated level of  Ca+  in response to PAMPs perception or R gene interactions may occur due to phosphorylation, G-protein signaling and/or an increase in cyclic nucleotides (Ma and Berkowitz 2007).  The concentration of calcium in cytosol is mobilized from Ca+ stores like vacuole, endoplasmic reticulum, mitochondria and chloroplast (Johnson et al., 1995, Xiong et al., 2006). Changes in concentration of Ca+ ion so called Ca+ signals are decoded by different types of calcium –dependent protein kinases and mitogen-activated protein kinases (Wurzinger et al., 2011).  This change in the intracellular Ca+ concentration is correlated with defense responses leading to production of hydrogen peroxide (H2O2),   nitric oxide (NO), phytoalexin formation as well as induced expression of PR genes (Nurnberger et al., 1994; Hahlbrock et al., 1995; Zhang et al., 2014).

Protein phosphorylation/dephosphorylation control many biological processes in plants.  Phosphorylation acts as a molecular switch that can control protein activity in signal transduction, metabolism, cell division etc. (Sickmann and Meyer 2001).  Post-translational modifications of proteins affect their conformation, activity, stability and localization. Protein phosphorylation is a post-translational modification that regulates plant growth and immunity (Ghelis 2011; Park et al., 2012). Phosphorylation implies to addition of a phosphate group to an amino acid.  Autophosphorylation on specific serine/threonine residue in the cytoplasmic domain is critical for activation of leucine-rich repeat receptor-like kinases which have role in disease resistance (Mitra et al., 2015). PRRs activation induces rapid autophosphorylation, leading to phosphorylation of many other proteins (Park et al., 2012). Phosphorylation occurs on the hydroxyl group of hydroxyamino acids such as serine, threonine and tyrosine and  is executed by protein kinases that transfers a phosphoryl group from ATP to the hydroxyl group attached to the polar group (R) of the amino acid, resulting in phosphoester (R-O-PO3) bond (Stulemeijer and Joosten 2008). This phosphorylated proteins can be dephosphorylated by protein phosphatases.  Protein phosphatases hydrolyse the phosphoester bond releasing the phosphoryl group and restoring the hydroxyamino acid into its unphosphorylated state. Protein dephosphorylation may also be involved in defense signaling. Conrath et al. (1997) demonstrated by using protein kinase and phosphatase inhibitors that PR-1  gene induction can be mediated by dephosphorylation of serine/threonine residues of two or more unidentified phosphoproteins. Protein phosphorylation and dephosphorylation regulate transcription factor (TF) function including cellular localization, protein stability, protein-protein interactions and DNA binding (Whitmarsh and Davis 2000).

Transcriptional regulation of gene expression is mediated by TF which can activate or repress transcription (Schwechheimer and Bevan 1998). Transcription of immune response in the nucleus is regulated by TFs.  Transcription factor may contain a DNA-binding domain, a transcription regulation domain, dimerization site and nuclear localization domain (Liu et al., 2001).  After receiving   the signal from the cell surface receptor, transcription factors 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).  As TF has DNA binding domain it can regulate gene expression. Transcription factor phosphorylation plays a role in defense responses.

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