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Damage on plants activates Ca2+-dependent metacaspases for release of immunomodulatory peptides

First author: Tim Hander; Affiliations: University of Basel (巴塞尔大学): Basel, Switzerland

Corresponding author: Simon Stael

Introduction    Cellular damage caused by wounding triggers signals to alert the surrounding tissue. As a universal process in all multicellular organisms, these signals activate the immune system to prevent infection and promote tissue regeneration, eventually leading to wound healing, but they have to be secured because aberrant immune stimulation can negatively affect health and growth. Plants, as sessile organisms, are regularly subject to chewing or sucking insects and physical damage inflicted by metazoans or exposure to the environment. In plants, short protein fragments or peptides can have immunomodulatory functions, such as those derived from the plant elicitor peptide (Pep) gene family. Peps are part of precursor proteins and have been proposed to act as wound signals that bind and activate the extracellular Pep receptors (PEPRs) to initiate an immune-like response. How Peps are produced and released upon wounding and how far this response extends from the harm site remain unclear.


Rationale    Proteases can generate peptides from precursor proteins, but the genome of the model plant, Arabidopsis thaliana, encodes approximately 600 different proteases. To understand the molecular mechanisms that control the immune-activating signals, it is essential to identify the immune response-contributing proteases. Furthermore, wound formation has to be observed with great speed and precision to delineate the extent of the overall process. Pep1 was studied here as a representative member of the gene family.


Results    Pep1 generation was detected within 30 s after the injury, peaking at 5 min and lasting up to 1 hour in two wound model experiments on Arabidopsis seedlings: pinching with forceps or mechanical disruption of tissue integrity through grinding. Screening with various protease inhibitors revealed the involvement of metacaspases in Pep1 formation. Metacaspases are cysteine proteases, conserved in plants, fungi, protists, and bacteria that were historically named after caspases because of a certain degree of structural homology. However, they differ in activity and substrate specificity. Metacaspases require low-millimolar amounts of calcium (Ca2+) for in vitro activity and cleave their substrate proteins after the amino acids arginine and lysine. Physical damage activated the abundant METACASPASE4 (MC4) in both wound experiments. MC4 released Pep1 from its protein precursor PRECURSOR OF PEP1 (PROPEP1) by cleaving it behind a conserved arginine. A mutant lacking MC4 was unable to produce Pep1 in leaf tissue, whereas certain redundancy occurred with other metacaspase (or metacaspase-like) activities in root tissue. Inside the plant cell, PROPEP1 is attached to the cytosolic side of the vacuolar membrane. When undisturbed, cytosolic Ca2+ concentrations ([Ca2+]cyt) are too low to activate metacaspases that need unusually high [Ca2+] to function. When Arabidopsis root epidermis cells were damaged by means of multiphoton laser ablation, Pep1 release from the vacuolar membrane occurred only in the cytosol of directly hit cells, as observed with confocal microscopy. Loss of plasma membrane integrity in these cells led to a high and prolonged influx of extracellular [Ca2+] into the cytosol, sufficient to activate metacaspases, leading to the cleavage and release of Pep1 from PROPEP1. Application of exogenous PROPEP1 protein fragments, longer than the native Pep1, reduced root growth—a known negative effect of Pep1 overload—irrespective of MC4 cleavage. Accordingly, PROPEP1 cleavage to overcome retention of Pep1 at the vacuolar membrane seems more important than obtaining the mature Pep1 size.


Plants exploit highly conserved mechanisms—such as Ca2+-partitioning across intact membranes and maturation of immunomodulatory peptides by proteases—to rapidly trigger defense responses against tissue damage. Pep1 is released by activation of MC4 upon prolonged high levels of [Ca2+]cyt that occur only in directly damaged cells, and this response is safeguarded by subcellular retention of PROPEP1 at the vacuolar membrane in the absence of damage. Metacaspases, together with Peps and PEPRs, now emerge as potential targets for breeding and improving crop immunity.


通讯Simon Staelhttps://plantandmicrobiology.berkeley.edu/profile/luan


doi: 10.1126/science.aar7486

Journal: Science

Published date: March 22, 2019


上一篇:Plant Cell:拟南芥植物激发子肽Pep作用于根的生长抑制


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