# Green Production and Biotechnological Applications of Cell Wall Lytic Enzymes
## 1 Introduction  
The plant cell wall is a complex and heterogeneous structure composed of polysaccharides and phenolic compounds assembled in two distinct layers called primary and secondary cell walls [1]. The primary cell wall is mainly composed of cellulose, hemicelluloses, and pectin and is typical of cells under active development and of young tissues. The secondary cell wall is deposited close to the primary one in mature cells and, besides cellulose and hemicellulose, is formed by lignin, while pectin is present only in small amounts. Clusters of secondary cell walls constitute lignocellulosic, a heterogeneous material mainly composed by lignin, hemicellulose, and cellulose; in addition, to provide structural support to the plant, it represents a powerful barrier against both biotic and abiotic stresses [2].  
In order to open a breach and depolymerize the cell wall polysaccharides into simple sugars, microbes have evolved specialized enzymes, referred to as cell wall lytic enzymes (CWLEs). In general,lignocellulolytic fungi and bacteria secrete different types of CWLEs at the microbe-plant interface in a sequential manner, mainly depending on the substrate they are dealing with. Many microbial species also evolved cellulosomes, which are multi-CWLE complexes constituted by distinct catalytic and binding domains carrying different degrading activities in a single polypeptide chain [3]. The high heterogeneity of CWLEs makes these enzymes highly valuable in different fields ranging from the agriculture to medicine. CWLEs are classified in the CAZy database ([[http://www.cazy.org/](http://www.cazy.org/)]([http://www.cazy.org/](http://www.cazy.org/))), where glycoside hydrolases, the most numerous class of carbohydrate active enzymes, are represented by more than 160 different families.  
Energy demand is constantly growing, also because of the unstoppable growth of the world population, and the extensive use of fossil fuels has negative effects on air pollution and, ultimately, on climate. To avoid this, clean forms of energy are required. Lignocellulose, the most abundant carbon source on Earth, may be a solution since it is composed of complex carbohydrates that, if efficiently hydrolyzed, can be used to produce biofuels. Degradation of lignocellulosic may be achieved by different methods including chemical, physical, and biological methods [4]. Efficient degradation of lignocellulosic by chemical methods or by combining physico-chemical treatments are expensive and, in some cases, highly polluting. Conversely, the conversion of lignocellulosic into fermentable sugars by using CWLEs may be considered as an eco-friendly alternative. Moreover, the broad substrate specificity of many CWLEs makes these enzymes highly versatile and, thus, exploitable in other important fields such as agriculture, food processing, and medicine. However, the industrial use of CWLEs is characterized by a high cost and low efficiency. Under this perspective, both the identification of novel CWLEs and the optimization of CWLE-expressing biofatories are crucial. In this review, recent advances in the heterologous expression of CWLEs as well as some of their possible biotechnological applications will be discussed. In order to support a sustainable biofactory of CWLEs, we will focus on the expression system in plants, since it is characterized by a high productivity/cost ratio and it consumes atmospheric CO\({}_{2}\) through photosynthesis, thus positively impacting global warming.