Bioengineering and Molecular Manipulation of Mitogen-Activated Kinases to Activate Plant Innate Immunity for Crop Disease Management

Abstract

Plants have innate immune system to protect them against wide range of pathogens. However, the plant immune system is a sleeping system in unstressed healthy plants. Specific signaling systems have to be manipulated to trigger the expression of defense-related genes for effective management of crop diseases. The important components in the plant immune signal transduction system include calcium, reactive oxygen species, nitric oxide, salicylic acid, jasmonic acid, and ethylene—dependent signaling systems. The MAPKs transduce extracellular stimuli into intracellular transcription factors through activation of these signaling systems. A typical MAPK signaling module consists of three interconnected protein kinases: a MAP kinase kinase kinase (MAPKKK), a MAP kinase kinase (MAPKK), and a MAP kinase (MAPK). MAPKs act downstream of plant pattern recognition receptors (PRRs) and transduce extracellular stimuli into intracellular responses in plants. Although there are many MAPKs reported in plants, only a few of them are involved in plant immune signaling system and even among the few MAPKs, some regulate plant defense responses positively, while others regulate the defense responses negatively. The immunity-activating MAPKs also differ in activating the complex signaling systems. Technologies have been developed to utilize appropriate MAPK genes for developing disease-resistant plants. The MAPKs modulate phosphorylation of transcription factors to trigger transcription of defense genes. Bioengineering specific MAPK genes has been shown to induce disease resistance by triggering phosphorylation of transcription factors. BWMK1, the rice MAPK, phosphorylates the rice transcription factor OsEREBP1. EREBPs are known to bind to the GCC box DNA motif (AGCCGCC) that is located in the promoter of several PR genes. Transgenic tobacco plants expressing the rice BWMK1 gene show enhanced resistance against bacterial and oomycete pathogens. Some MAPK genes have been shown to regulate SA-mediated systemic acquired resistance (SAR). The cotton MAPK gene GhMPK7 and the maize MAPK gene ZmSIMK1 activate SA signaling system and transgenic plants overexpressing these genes show enhanced disease resistance. Some MAPK genes (BnMPK4 and MK1) trigger the JA-mediated signaling system and these genes have been exploited to develop transgenic plants expressing enhanced resistance against necrotrophic pathogens. The cotton MAPK genes GhMPK16 and GHMPK2 activate SA, JA and ET signaling complex and these genes have been engineered to develop disease-resistant transgenic plants. Some mitogen-activated protein kinase kinase (MAPKK) genes have also been exploited to activate immune responses for crop disease management. The potato StMEK1DD gene was used for engineering for disease resistance. Transgenic potato plants carrying the StMEK1DD allele expressed from the pathogen-inducible potato vetispiradiene synthase (PVS) promoter showed resistance to various pathogens. Another MAPKK gene isolated from cotton, GhMKK5, has been utilised for developing disease-resistant Nicotiana benthamiana plants. The MAPKK gene MKK7 triggers accumulation of SA in Arabidopsis and the activation-tagged bud1 mutant, in which the expression of MKK7 is increased, shows enhanced resistance to pathogens. Some MAPK genes negatively regulate the defense responses and these genes also have been exploited to develop disease-resistant plants by knocking-out these MAPK genes. OsMPK6 knock-out plants showed SA-dependent systemic acquired resistance (SAR) against pathogens. MAPKs are negatively regulated by dephosphorylation through MAPK phosphatases (MKPs) Knockout of negative regulators of defense responses also could be a promising way for the production of disease-resistant plants. MKPs may nullify the function of the MAPKs in inducing resistance against necrotrophic pathogens. To overcome the negative function of the MKPs, MKP-suppressed plants have been developed. Tobacco plants in which NtMKP1 was silenced were produced by introducing an NtMKP1 antisense construct. These plants show enhanced resistance against the necrotrophic fungal pathogen. OsEDR1, a MAPKKK gene, negatively regulates disease resistance. Transgenic rice plants showing suppression of OsEDR1 expression were developed using RNAi strategy. The OsEDR1-RNAi plants show enhanced resistance against the rice bacterial blight pathogen. In another approach, OsEDR1-knockout rice plants were developed. A rice mutant with T-DNA inserted in the fourth exon of OsEDR1 has been identified as OsEDR1-knockout plant. The transgenic OsEDR1-knockout plants show enhanced resistance against the bacterial pathogen. A kinase-deficient form of EDR1 gene has also been used to develop disease resistant plants. Overexpression of the kinase-deficient full-length EDR1 gene in wild-type Arabidopsis thaliana plants caused a dominant negative phenotype, conferring resistance to diseases. MAPK gene can also be manipulated to trigger the immune responses by using the biocontrol agent Trichoderma asperellum. A MAPK, designated as Trichoderma-induced MAPK (TIPK), has been identified and characterized in the Trichoderma-induced disease-resistant cucumber plants. The TIPK gene has been cloned and cucumber plants overexpressing the TIPK gene show resistance against pathogen. MAPK genes appear to be potential tools for developing disease resistant plants using various bioengineering technologies.