Switching on Plant Immune Signaling Systems Using Pathogen-Induced Molecular Patterns/Host-Associated Molecular Patterns

Abstract

Plant immune system is a sleeping giant and on activation, it can trigger defense responses against a wide range of bacterial, fungal, oomycete, and viral pathogens. Specific signals are required to switch on the innate immune system. Pathogen-associated molecular patterns (PAMPs) are the alarm signal molecules triggering the plant immune system. The microbe-derived elicitors and invading pathogens release pathogen-induced molecular patterns (PIMPs)/host-associated molecular patterns (HAMPs or endogenous elicitors), which are involved in amplifying the PAMP signals to activate host defense responses. The well-characterized host-derived elicitors include oligogalacturonides (OGAs) and Pep proteins (Plant Elicitor Peptides). The OGAs activate cyclic nucleotide gated channels (CNGCs) and glutamate receptors (GLRs) and elicit a rapid elevation in cytosolic Ca2+. OGAs trigger a robust oxidative burst mediated by NADPH oxidase. OGAs trigger NO production by activating both nitrate reductase and NO synthase pathways. OGAs also induce the enzymes involved in MAPK signaling cascade. OGAs trigger the expression of genes involved in SA, JA, ET, and ABA signaling systems. Both the degrees of substitution (methylesterification and/or acetylation) and polymerization determine the efficacy of OGAs in triggering defense responses. Degree of methyl esterification modulates the elicitor activity of OGAs and OGAs with different degrees of polymerization differ in triggering defense responses. Ability of OGAs to trigger defense responses also depends on their level of acetylation. Methyl esterification, degree of polymerization, and level of acetylation are modulated by pectin methylesterases (PMEs), polygalacturonases (PGs), and pectin acetylesterases (PAEs), respectively. PME activity is tightly regulated by an inhibitor protein called pectin methylesterase inhibitor protein (PMEI). Transgenic plants overexpressing genes encoding PME inhibitor proteins show enhanced disease resistance. Transgenic plants expressing an attenuated version of a fungal PG show enhanced resistance against diseases. Polygalacturonase inhibitor proteins (PGIPs) belong to the super family of leucine-rich repeat (LRR) proteins and they play important role in switching on plant defense signaling pathways. Several studies conducted both under greenhouse and field conditions clearly show that PGIP gene is a potential tool for engineering disease resistance against various fungal, oomycete and bacterial pathogens. The expression of PGIP genes did not affect the agronomic characters of the transformed plants. The expression of PG inhibitor protein neither alters the physiological performances nor exhibits detrimental effects on the growth of transgenic plants. Pep proteins accumulate at the site of infection and switch on the immune signaling systems upon binding to plasma membrane-localized Pep Receptors (PEPRs). Peps activate CNGC2-dependent plasma membrane inwardly conducting Ca2+ permeable channels in mesophyll cells, resulting in elevation of cytosolic Ca2+ ([Ca2+]cyt) level. This activity is dependent on the Pep Receptors. BAK1 may function as a coreceptor with PEPRs and contribute to Pep immune signaling leading to defense gene expression. PEPR1 has been shown to interact with receptor-like cytoplasmic kinases botrytis-induced kinase 1 (BIK1) and PBS1-like 1 (PBL1) to mediate Pep1-induced defense responses. Downstream from the early Ca2+ signal, Ca2+-dependent protein kinases (CDPKs) are involved in decoding the Ca2+ signal. CDPK-dependent phosphorylation has been shown to be involved in the Pep signaling cascade, Pep proteins trigger both ROS and NO generation. Pep proteins mediate JA, ET, and SA signaling systems and induce the expression of defense genes. Pep proteins can be developed as potential tools for disease management. Application of Pep proteins induce resistance against fungal and bacterial pathogens. Transgenic plants overexpressing PROPEP1 and PROPEP2 genes show enhanced resistance against pathogens. PAMP signals also induce secretion of several peptides called secreted peptides and the secreted peptide precursors are called “prePIPs” (precursors of PAMP-induced Peptides). The prePIPs are cleaved close to the C terminus to form PIPs. PIP1 induces the precursor of the PIMP/HAMP PEP1, PROPEP1, while PEP1 induces the precursor of PIP1, prePIP1. Transgenic plants overexpressing prePIP1 or prePIP2 show enhanced resistance against fungal and bacterial diseases. Systemin is a damage/wound induced bioactive peptide and it triggers defense responses against various pathogens and insect pests. Systemin plays a key role in the JA biosynthesis pathway. The transgenic plants overexpressing prosystemin show enhanced resistance against necrotrophic fungal pathogens. PIMPs/HAMPs appear to be powerful tools to engineer disease resistance in field crops.