The front line of defence: a meta-analysis of apoplastic proteases in plant immunity

Abstract

Secreted proteases act at the front line of defence and play pivotal roles in disease resistance. However, the criteria for apoplastic immune proteases are not always defined and followed. Here, we critically reviewed 46 apoplastic proteases that function in plant defence. We found that most apoplastic immune proteases are induced upon infection and seventeen proteases are genetically required for the immune response. Proteolytic activity has been confirmed for most of the proteases but is rarely shown to be required for biological function and the apoplastic location of proteases can be subjective and dynamic. Pathogen-derived inhibitors have only been described for cysteine and serine proteases and the selection pressure acting on immune proteases is rarely investigated. We discuss six different mechanisms by which these proteases mediate plant immunity and summarise the challenges for future research.

Differing Responses to Phytophthora cinnamomi Infection in Susceptible and Partially Resistant Persea americana (Mill.) Rootstocks: A Case for the Role of Receptor-Like Kinases and Apoplastic Proteases

“…In an elegant study, Zimmermann et al ( 2016 ) showed that P69B promoted cell death in tomato, although the overexpression of P69B alone could not. Thus, control over PCD is an undoubtedly important aspect of defense responses, and apoplastic subtilases form an integral part of its control, although additional proteases also participate (Godson and van der Hoorn, 2021 ). Investigations aimed at determining whether these subtilases display caspase-like activity would be highly informative.…”

“…Markedly, SCLPs are known to contain signal peptides and are quite often found in the apoplast (Fraser et al, 2005 ; Sueldo et al, 2014 ; Grosse-Holz et al, 2018 ). Similarly, MMPs from the M10 family in plants have all been described as containing signal peptides and are the only metalloproteases in plants that are classed as apoplastic (Godson and van der Hoorn, 2021 ). Therefore, it is interesting to note that all but one DE avocado apoplastic metalloprotease coding gene was predicted to belong to the M10 MMP family.…”

“…Notably, most PRRs are either transmembrane receptor-like kinases (RLKs) or receptor-like proteins (RLPs) and, as such, constitute an essential part of the pathogen-sensing machinery in plants (Böhm et al, 2014 ; Couto and Zipfel, 2016 ; Yu et al, 2017 ). Here, plant proteases have important roles in the signaling process, both prior to and following the recognition of PAMPs or host-derived damage-associated molecular patterns (DAMPs; Hou et al, 2018 ; Godson and van der Hoorn, 2021 ). Intriguingly, pathogens have been known to evade recognition by PRRs through the actions of effector proteins, some of which seem to trick the host plant into activating RLKs, which are involved in growth and development (Dou and Zhou, 2012 ).…”

A Comprehensive Assessment of the Secretome Responsible for Host Adaptation of the Legume Root Pathogen Aphanomyces euteiches

“…Secreted proteases have been proposed to be involved in the digestion of the host barrier, such as the crayfish cuticle in the case of Aphanomyces astaci [98], or the human epidermis in the case of Pythium insidiosum [99]. However, it is assumed that proteolysis of plant substrates is a strategy employed by plant pathogens during the infection process [100,101], meaning most of the proteases predicted in Phytophthora sp. contribute to pathogen virulence [102,103].…”

The involvement of AtMKK1 and AtMKK3 in plant-deleterious microbial volatile compounds-induced innate immunity.

“…The tissue defense responses include stomatal closure and callose deposition (Zhou et al 2013). The molecular defenses include the secretion of antimicrobial substrates such as antimicrobial peptides, phytoalexins and apoplastic proteases (Ebel 1986;Ahuja et al 2012;Campos et al 2018;Godson and Hoorn 2021).…”