Mechanism of Plant Apoplast Protein Triggering Plant Immunity

In the early stages of pathogenic bacteria invading plant tissues, the apoplast is the first area that pathogenic microorganisms encounter outside the cell membrane. In order to resist pathogenic bacteria, plants have evolved complex surveillance mechanisms that can sense dangerous events on the cell surface and quickly initiate immunity. This process is also called apoplast immunity. However, to mount a strong and effective defense response, plants must finely regulate the activation of immune pathways. Studies have found that some endogenous proteins and enzymes are synthesized in the form of inactive precursors, and they undergo post-translational processing to trigger apoplast immunity.

On April 30, 2024, Vincenzo Lionettid's team at Sapienza University of Rome published a review titled "Pull the fuzes: Processing protein precursors to generate apoplastic danger signals for triggering plant immunity" in Plant Communications. The article focuses on the precursors of plant cytokines, cell wall remodeling enzymes and proteases, as well as the physiological process of converting inactive precursors into immunomodulatory active peptides or enzymes. Additionally, synergistic interactions between plant cytokines, cell wall damage-associated molecular patterns and remodeling are explored, emphasizing their role in improving extracellular immunity and enhancing defense against pests.

As proteins secreted into the apoplast space, these immune polypeptides usually have three regions in their protein structure: signal peptide, processing region and active domain. Signal peptides guide protein secretion to the apoplast. When a danger signal occurs, the processed region is cleaved by proteases, releasing the active domain. The active domain is recognized by receptor-like kinases on the cell membrane, thereby activating downstream plant immune pathways. For example, the pro-PEP1 precursor in Arabidopsis is cleaved by the metacaspase 4 (MC4) to generate PEP1, which is then recognized by PEPR1/2 and activates plant immunity.

The secretion pathways of apoplastic proteins are mainly divided into two categories: the conventional secretion pathway and the unconventional secretion pathway. In the conventional pathway, proteins synthesized in the endoplasmic reticulum are processed by the Golgi apparatus and transported out of the cell through transport vesicles. The largest number of precursor proteins pass through this pathway, including pre-pro-PSY1, pre-pro-PSK1/4 and pre-pro-PIP1/2. In the unconventional secretion pathway, protein is released through multivacuolar bodies and vacuole-plasma membrane pathways. Proteins that pass through the unconventional secretion pathway include pro-ZIP1, pro-PEP1 and pro-Systemin.

When these immunoactive protein precursors are secreted into the apoplast space, they are processed under conditions of pathogenic bacterial infection and converted into active forms. In this review, the authors provide a detailed summary of apoplast immune-activating proteins in plants in response to different pathogenic bacteria and insects. For example, proteins that respond to pathogenic fungi include PEP1, PSY1, PIP1/2 and ZIP1. Proteins such as RALF17, IDA, PME17 and BXL4 respond to pathogenic bacteria. In addition to being used to fight pathogenic bacteria, some immune peptides can also enhance defense responses against insects, such as Systemin and HypSys. After being recognized by receptor-like kinases, these active peptides and proteins activate the expression of downstream PTI-responsive genes and plant hormone-responsive genes through MAPK and other pathways.

In summary, this review systematically summarizes various aspects of immune peptides and proteins involved in plant apoplast immunity. Apoplast immunity occurs at the earliest stages of plant defense against pathogenic bacteria. Understanding and applying these immune pathways will provide useful tools for developing broad-spectrum disease-resistant crops.

Reference

1. Del Corpo, D., et al. Pull the fuzes: Processing protein precursors to generate apoplastic danger signals for triggering plant immunity. Plant Commun. 2024 Apr 30: 100931.

Plant Proteins

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