Convergent Evolution of Pathogen Effectors toward Reactive Oxygen Species Signaling Networks in Plants

Abstract

Microbial pathogens have evolved protein effectors to promote virulence and cause disease in host plants. Pathogen effectors delivered into plant cells suppress plant immune responses and modulate host metabolism to support the infection processes of pathogens. Reactive oxygen species (ROS) act as cellular signaling molecules to trigger plant immune responses, such as pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity. In this review, we discuss recent insights into the molecular functions of pathogen effectors that target multiple steps in the ROS signaling pathway in plants. The perception of PAMPs by pattern recognition receptors leads to the rapid and strong production of ROS through activation of NADPH oxidase Respiratory Burst Oxidase Homologs (RBOHs) as well as peroxidases. Specific pathogen effectors directly or indirectly interact with plant nucleotide-binding leucine-rich repeat receptors to induce ROS production and the hypersensitive response in plant cells. By contrast, virulent pathogens possess effectors capable of suppressing plant ROS bursts in different ways during infection. PAMP-triggered ROS bursts are suppressed by pathogen effectors that target mitogen-activated protein kinase cascades. Moreover, pathogen effectors target vesicle trafficking or metabolic priming, leading to the suppression of ROS production. Secreted pathogen effectors block the metabolic coenzyme NADP-malic enzyme, inhibiting the transfer of electrons to the NADPH oxidases (RBOHs) responsible for ROS generation. Collectively, pathogen effectors may have evolved to converge on a common host protein network to suppress the common plant immune system, including the ROS burst and cell death response in plants.

Keywords: PAMP-triggered immunity; effector-triggered immunity; mitogen-activated protein kinase; pathogen effector; reactive oxygen species; respiratory burst oxidase homolog.

FIGURE 1

Proposed model for NADPH oxidase-dependent ROS generation via NADP-malic enzyme (ME)-mediated electron supply. NADP-malic enzyme (ME) serves as a source of NADPH and pyruvate in the cytosol of various plant tissues. It catalyzes the oxidative decarboxylation of L-malate to yield pyruvate, CO₂, and NADPH in the presence of a bivalent cation, such as Mg⁺⁺. NADPH oxidase, known as the Respiratory Burst Oxidase Homolog (RBOH), catalyzes the generation of superoxide (⋅O₂^-) by the one-electron reduction of molecular oxygen using NADPH as an electron donor. Superoxide can spontaneously form hydrogen peroxide (H₂O₂) that will undergo further reactions to generate reactive oxygen species (ROS).

FIGURE 2

Reactive oxygen species (ROS) production and signaling in plant-pathogen interactions. (A) Perception of microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs) or effectors by plants via membrane-bound pattern recognition receptors (PRRs) or intracellular nucleotide-binding leucine-rich repeat (NLR) receptors, respectively, activates NADPH oxidases as well as peroxidases, triggering ROS production and the ROS burst. (B) Perception of MAMPs or PAMPs by PRRs induces weak ROS bursts, leading to PAMP-triggered immunity (PTI)-dependent basal defense responses. Adapted pathogens secrete virulent effector proteins into plant cells to suppress the ROS burst and PTI, resulting in effector-triggered susceptibility (ETS) to cause disease in their respective host plants. Pathogen avirulence (Avr) effectors interact directly or indirectly with intracellular NLR proteins, leading to a strong ROS burst and HR cell death response, key components of effector-triggered immunity (ETI). ETI is an accelerated and amplified PTI response, resulting in disease resistance and, usually, a resistance (R) gene-mediated HR cell death response at the infection site in plants.

FIGURE 3

Convergent effector targeting of ROS signaling networks in plant cells. Plant pathogens secrete effectors into the apoplastic area and cytoplasm of plant cells. These apoplastic effectors interfere with PRR-mediated PAMP recognition, ultimately leading to the inactivation of plasma membrane-bound RBOHs. Cytoplasmic effectors target the kinase domains of PRRs, the receptor-like cytoplasmic kinase (RLCK) BIK1, MAPK cascades, and WRKY transcription factors, inhibiting the transcription of RBOHs and NADP-ME, both essential for robust ROS generation. A bacterial type III effector inhibits BIK1 downstream of the PRR (BAK1), enhancing virulence. Some cytoplasmic effectors target vesicle trafficking, suppressing the transport of ROS-producing RBOH enzymes to the plasma membrane during infection. Secreted pathogen effectors block the metabolic coenzyme NADP-ME, inhibiting the transfer of electrons to the NADPH oxidases (RBOHs) responsible for ROS generation. The biomembrane channels, aquaporins, mediate H₂O₂ transport across biological membranes. However, when specific pathogen effectors recognize intracellular NLR proteins, the effector no longer functions as a virulence factor; here, the effector-NLR complex leads to a strong apoplastic ROS burst and HR cell death response. The PRR-associated kinase BIK1 directly phosphorylates the NADPH oxidase RBOHD to enhance RBOHD-mediated ROS production. Rac1 directly interacts with both NLR and RBOH to activate RBOH. Apoplastic ROS are toxic to pathogens and also activate MAPK cascades and RBOH enzymes in their role as immune signal molecules.