Elicitor and Receptor Molecules: Orchestrators of Plant Defense and Immunity

Abstract

Pathogen-associated molecular patterns (PAMPs), microbe-associated molecular patterns (MAMPs), herbivore-associated molecular patterns (HAMPs), and damage-associated molecular patterns (DAMPs) are molecules produced by microorganisms and insects in the event of infection, microbial priming, and insect predation. These molecules are then recognized by receptor molecules on or within the plant, which activates the defense signaling pathways, resulting in plant's ability to overcome pathogenic invasion, induce systemic resistance, and protect against insect predation and damage. These small molecular motifs are conserved in all organisms. Fungi, bacteria, and insects have their own specific molecular patterns that induce defenses in plants. Most of the molecular patterns are either present as part of the pathogen's structure or exudates (in bacteria and fungi), or insect saliva and honeydew. Since biotic stresses such as pathogens and insects can impair crop yield and production, understanding the interaction between these organisms and the host via the elicitor-receptor interaction is essential to equip us with the knowledge necessary to design durable resistance in plants. In addition, it is also important to look into the role played by beneficial microbes and synthetic elicitors in activating plants' defense and protection against disease and predation. This review addresses receptors, elicitors, and the receptor-elicitor interactions where these components in fungi, bacteria, and insects will be elaborated, giving special emphasis to the molecules, responses, and mechanisms at play, variations between organisms where applicable, and applications and prospects.

Keywords: ISR; SAR; damage-associated molecular patterns (DAMPs); herbivory-associated molecular patterns (HAMPs); microbe-associated molecular patterns (MAMPs); pathogen-associated molecular patterns (PAMPs); plant defense systems; plant pattern recognition receptors (PRRs); receptor molecules.

Figure 1

Pathogen-associated molecular patterns (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) for different types of molecular patterns produced by pathogenic and non-pathogenic microorganisms and insects (microbe-associated molecular patterns (MAMPs), damage-associated molecular patterns (DAMPs), and herbivore-associated molecular patterns (HAMPs). The well-established elicitor–receptor model for pathogenic fungus and bacteria, insects and non-pathogenic microbes are chitin elicitor binding protein/chitin elicitor receptor kinase (CEBIP/CERK), chitin, FLAGELLIN SENSITIVE2/BRI1-associated kinase (FLS2/BAK1), flg22, PEP receptor 1/2 (PEPR1/2), peptides (Pep), Lysin motif protein ½ (LYM1/LYM2), LPS, and hydrophobin-like elicitor (SM1). Other examples of elicitors and receptors are provided in Table 1 and Table 2.