Abstract
The plant cell wall (CW) is a complex structure that acts as a mechanical barrier, restricting the access to most microbes. Phytopathogenic microorganisms can deploy an arsenal of CW-degrading enzymes (CWDEs) that are required for virulence. In turn, plants have evolved proteins able to inhibit the activity of specific microbial CWDEs, reducing CW damage and favoring the accumulation of CW-derived fragments that act as damage-associated molecular patterns (DAMPs) and trigger an immune response in the host. CW-derived DAMPs might be a component of the complex system of surveillance of CW integrity (CWI), that plants have evolved to detect changes in CW properties. Microbial CWDEs can activate the plant CWI maintenance system and induce compensatory responses to reinforce CWs during infection. Recent evidence indicates that the CWI surveillance system interacts in a complex way with the innate immune system to fine-tune downstream responses and strike a balance between defense and growth.
Highlights
These results indicate that OGOXs might fine-tune resistance to pathogens through multiple mechanisms that involve dampening of elicitor activity, generation of reactive oxygen species (ROS), and modification of substrates for microbial CW-degrading enzymes (CWDEs)
In the past four decades, we have accumulated an overwhelming amount of evidence indicating that a key feature of most plant–pathogen interactions is the deconstruction of the host cell wall (CW), where a significant portion of the evolutionary arms race between plants and their pathogens takes place (Figure 1)
Several questions are still open, as highlighted in a recent review [10]; among these, the most relevant for the field of plant–pathogen interactions are probably: (1) How many CW-derived elicitors exist and what role do they play in immunity? (2) What is the relationship between CW integrity (CWI) maintenance, development, and immunity? It is expected that the adoption of novel tools and techniques to probe and image in vivo specific CW epitopes, and to analyze CW composition on a microscopic scale [10] will reveal so far undetected details of the complex events occurring in the plant CW during pathogen penetration and invasion
Summary
The major load-bearing component of plant CWs is cellulose, which provides tensile strength; non-cellulosic polysaccharides, structural proteins, and other non-saccharide components, like lignin, all contribute to the specific mechanical and biochemical properties of the CW in different cell types [2,3]. Increasing evidence indicates the existence of multiple mechanisms that plant cells employ to detect in a timely manner changes in CW integrity (CWI), and to mount responses that compensate for the damage inflicted by the pathogen, stiffening the CW and making it more recalcitrant to deconstruction [8,9,10,11]. Some pathogens can secrete decoy proteins that bind to CWDE inhibitors
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