Source: School of Life Sciences
Written by: School of Life Sciences
Edited by: Wang Dongmei

Plants can produce animal cytokine-like immune peptides to activate immunity and improve survival against pathogen attacks. Peps are a class of immune peptides widely distributed in the plant kingdom, and are thought to be generated from the C-termini of their precursors (PROPEPs) by proteolysis. Like the bacterial flagellin elicitor flg22, exogenously applied Peps can stimulate plant defense responses after being perceived by the cognate PEPR receptors. Interestingly, Pep signaling also seems to play roles in salinity responses, ethylene signaling, and dark-induced leaf senescence in plants. Although Peps have been long considered as proteolytic products of PROPEPs, the identity and action mechanism of the proteolytic enzyme responsible for PROPEP processing has remained enigmatic.
 
Recently, Prof. Jian-Feng Li’s group in the School of Life Sciences at Sun Yat-sen University has published a research article, entitled "Type-II metacaspases mediate the processing of plant elicitor peptides in Arabidopsis”, in the prestigious journal Molecular Plant (Impact factor: 10.8). This work reports the type-II metacaspases as the proteolytic enzymes to mediate the processing of plant elicitor peptides (Peps) in plant innate immunity in the plant model organism Arabidopsis. Prof. Jian-Feng Li is the corresponding author, and Dr. Wenzhong Shen (Post-Doc) and Ms. Jiuer Liu (Master Student) are the co-first authors.
      
On March 22, 2019, a research article from Prof. Simon Stael’s group at the Ghent University, Belgium, entitled “Damage on plants activates Ca²⁺-dependent metacaspases for release of immunomodulatory peptides”, was published in the journal Science. In the article, Prof. Stael and colleagues reported their findings in Arabidopsis that wounding can induce the activation of Ca²⁺-dependent MC4 to process PROPEP1 into Pep1. These two studies approach the same conclusion from different angles and with different emphasis. Of note, Stael et al. proposed that MC4 is exclusively responsible for PROPEP1 processing in the leaf tissue. In contrast, we found that multiple type-II MCs are able to mediate PROPEP1 processing in leaves. Taken together, the findings of these two studies will lay the foundation for better understanding the regulation of Pep signaling in plant immunity and development.


Based on the findings from this study and others, we envision an overall picture of MC-mediated Pep1 signaling in plant immunity as follows. The perception of bacterial flagellin (flg22) by the receptor FLS2 and co-receptor BAK1 initiates defense signaling, which on one hand upregulates PROPEP1 expression, leading to accumulated PROPEP1 proteins that are sequestered on the cytosolic face of the tonoplast. On the other hand, flg22 perception also stimulates the cytosolic Ca2+ spike that can activate the type-II MCs in the cytosol through self-cleavage. Activated MCs will in turn cleave PROPEP1, and the processed Pep1 will enter the cytosol, from where it can somehow move to the apoplast and be perceived by the PEPR receptors. The activated Pep signaling will eventually amplify plant immunity. 

This work was supported by the Foundation of Guangzhou Science and Technology Key Project (201904020041) and the National Natural Science Foundation of China (31770295) to Prof. Jian-Feng Li.
 
Link to the paper: www.cell.com/molecular-plant/fulltext/S1674-2052(19)30264-3