# Green Production and Biotechnological Applications of Cell Wall Lytic Enzymes
## 5 Bio-Applications of CWLEs
### Use of CWLEs to Improve Plant Resistance Against Pathogens  
Plant expression of CWLEs can trigger the host defense, rendering their endogenous accumulation as self-deleterious for plants. Under this perspective, plant expression of CWLEs resembles a double-edged sword: on one side, activation of defense responses may confer increased resistance against pathogens but, on the other side, may strongly affect plant fitness and productivity, likely because of the well-known growth-defense trade-off [155; 156]. A valid strategy to overcome this issue may reside in a balanced activation of defense responses, capable of guaranteeing protection against microbes by concomitantly avoiding exaggerated and deleterious immune reactions. Notably, plant expression of CWLEs capable of triggering defense responses can be used as an eco-friendly alternative to pesticides, preventing the spreading of disease in plants and lowering the costs for managing pests. CWLEs themselves and/or their activity can trigger the activation of the first layer of plant immunity, generally known as pattern-triggered immunity (PTI) [157]. PTI is driven by sensing of both nonself and altered-self molecules (i.e., PAMPs and DAMPs, respectively), which are recognized by PRRs [157; 158]. The recognition of PAMPs/DAMPs leads to the activation of different kinds of intracellular defense responses, such as reactive oxygen species (ROS) production, transcriptional reprogramming of defense-related genes, and callose deposition [159]. Also, activation of defense machinery leads to a \"primed\" state of the plant, which is prone to respond faster and harder to a hypothetical pathogen attack. For example, pre-treatment with bacterial flagellin or with oligogalacturonides (OGs) makes plant more resistant to the necrotrophic fungus _Botrytis cinerea_[10; 160]. However, treatment with PAMPs/DAMPs is not easily obtainable in the field. On the contrary, immunity-induced protection against pathogens can be achieved in plants by expressing specific CWLEs capable of stimulating plant defenses. Inducible expression of a chimeric protein, constituted by the polygalacturonase from _Fusarium phillophilium_ (FpPG) and the polygalacturonase-inhibiting protein 2 (PvPGIP2) from _P. vulgaris_, confers resistance to transgenic _A. thaliana_ plants against pathogens characterized by different lifestyles such as _B. cinerea_, _Pectobacterium caroroporum_, and _Pseudomonas syringae_[90]. In particular, the chimeric protein, named as the OG-machine, releases elicitor-active OGs and allows the activation of a broad range of defense responses on command. By expressing the gene encoding the OG-machine under the control of a truncated version of the _PR1_ promoter (mainly induced by pathogens), a plant with potential field application was successfully obtained (patent ID: US10385347B2 [161]). Interestingly, transgenic plant crops expressing PvPGIP2 alone [162; 163; 164] showed a lower level of protection compared to OG-machine plants, pointing to the activity of a CWLE (i.e., the FpPG) as pivotal for achieving a high level of protection against a broad range of pathogens. CWLEs and their activity are mostly known as activators of plant immunity, since most of them are exploited by phytopathogens as \"molecular\" weapons to break plant cell wall components [83]. Recently, identification of proteins with a novel \"counteracting activity\" towards CWLEs opened a new perspective for improving plant resistance. Transgenic microbes from _Thricoderma_ species (commonly used as biocontrol agents) expressing a glucose oxidase from _Aspergillus niger_ (goxA) showed an improved ability both to inhibit phytopathogens and to induce systemic resistance in plants [165]. Interestingly, five berberine-bridge enzyme-like proteins (BBEIs) from _Arabidopsis thaliana_ are capable of oxidizing oligosaccharides released from the plant cell wall by microbial CWLEs [87; 88].