Molecular Manipulation of Transcription Factors, the Master Regulators of PAMP-Triggered Signaling Systems

Abstract

Transcription factors (TFs) are the ‘master switches’, which switch on the gene transcription processes. They play important role in initiating transcription of defense genes regulating expression of defense genes in plant innate immune systems. Many TFs have been shown to be involved in the defense signaling system and they confer resistance against several bacterial, oomycete, fungal, and viral pathogens. Induction of expression of genes encoding TFs appears to be critical in inducing defense-related genes and conferring disease resistance. Pathogen-associated molecular patterns (PAMPs) switch on the expression of various transcription factor genes. Messages generated by PAMP-PRR signaling complex are sensed by calcium ion, which acts as a signal carrier. Calcium signatures are recognized by different calcium sensors to transduce calcium-mediated signals into downstream events. Calmodulins (CaMs) are the important Ca2+ sensors identified in plants. Several transcription factors are involved in activation of calmodulin and they bind with calmodulins and decode the message from calmodulin proteins to activate defense responses in plants. PAMP triggers rapid and transient production of H2O2, which has been shown to regulate expression of several transcription factors in plants. Activation of these transcription factors may result in activation of transcription of several defense genes. Several mitogen-activated protein kinases (MAPKs) modulate phosphorylation of transcription factors to trigger transcription of defense genes. The MAPKs transduce extracellular stimuli into intracellular transcription factors, which enhance expression of defense-related genes. Some transcription factors trigger biosynthesis of salicylic acid. SA induces enhanced expression of several WRKY, ERF, and TGA transcription factors to activate transcription of defense genes. Transcription factors play an important role in triggering SA-dependent systemic acquired resistance (SAR). Some transcription factors are involved in activation of JA biosynthesis pathway. JA may trigger expression of several transcription factors. JA may also downregulate expression of some transcription factors. Transcription factors also regulate ethylene signaling system in plant innate immunity. Technologies have been developed to bioengineer the transcription factor genes (OsWRKY13, OsWRKY22, OsWRKY30, OsWRKY31, OsWRKY42, OsWRKY45, OsWRKY47, OsWRKY53, OsWRKY71, OsWRKY89, TaWRKY45, VvWRKY1, VvWRKY2, VpWRKY3, MdWRKY1, GhWRKY15, AtMYB44, RF2a and RF2b, CabZIP2, Pti5, Pti4, GbERF2, NtERF5, Tsi1, OsBIERF3, CaPF1, OPBP1, HvRAF, ERF1, OsEREBP1, OsNAC111, OsNAC6, TaNAC1 and CaATL1) for management of viral, bacterial, fungal, and oomycete diseases in rice, wheat, barley, tomato, grapevine, pepper, amd tobacco. Several transcription factors trigger “priming” of defense responses. The priming results in a faster and stronger induction of defense mechanisms after pathogen attack. Some plant defense activators, including benzothiadiazole (BTH) and β-aminobutyric acid (BABA), and some rhizobacteria such as Pseudomonas fluorescens WCS417r trigger priming of the transcription factors and defense responses. The primed genes may be poised for enhanced activation of gene expression by histone modification. The histone modifications might create a memory of the primary infection that is associated with an amplified reaction to a second stress conditions. The priming can be inherited epigenetically from disease-exposed plants and descendants of primed plants exhibit next-generation systemic acquired resistance. DNA methylation may also play important role in transgenerational SAR. These plant defense activators and rhizobacteria, which modulate the priming of transcription factors, are potential tools for management of crop diseases.