**2.4. Lithic enzymes**

The initiation of FOL infection requires degradation of the host cell wall through the action of a complex of enzymes with lytic activity such as xylanases, cellulases, pectinases, and polygalacturonases, which contribute to the penetration and colonization of the plant [42]. The genes xyl2 and xyl3 are responsible for the coding of xylanases, which degrade xylan. The xyl2 gene is expressed during the final stages of the disease, while xyl3 is present throughout the cycle [43]. The XlnR gene is known to be the main transcriptional activator of the xylanase genes. However, it was demonstrated that it is not determinant in the virulence of FOL, perhaps due to the presence of other xylanase genes whose expression is independent of this transcription factor [44].

The PG1 and PG5 genes are responsible for the expression of extracellular endopolygalacturonases, the latter expressed mostly during the early stages of infection [45, 46]. On the other hand, the characterization of several enzymes with lytic activity, such as PG1, exo-polygalacturonases (PG2 and PG3), an endoxylanase (XYL1), and an endopectate lyase (PL1), has been reported. Coded by genes pg1, pgx4, pg5, xyl2, xyl3, prt1, and pl1, these are expressed during different stages of interaction with the host plant indicating a possible role in the pathogenesis [47, 48]. While the absence of the Fpr1 protein (F-box protein, required for pathogenicity) results in the lack of expression of some enzymes involved in cell wall degradation, this is perceived as the inability of the pathogen to colonize the roots [49, 50].

Glucans are also considered as elicitor molecules, although glucans may be present in the cell wall of both plants and fungi, *β*-1,6-glucan is specific to the cell wall of fungi, resulting in a potential PAMP [56]. Glycoproteins present in the cell wall of FOL, whose function is the adhesion of hyphae to plant tissue, are encoded by the Fem1 gene and are considered within this group [57]. Each of these elements can be recognized by the plant defense system since it has pattern recognition receptors (PRR) [58]. Plant PRRs are located on the surface of the plasma membrane and can be two types. The receptor-like kinases (RLK) contain a ligand-binding ectodomain, a single-pass transmembrane domain, and an intracellular kinase domain. Or they may be receptor-like proteins (RLPs) typically consisting of a repetitive domain rich in extracellular leucine, a transmembrane domain, and a short cytoplasmic

A Molecular Vision of the Interaction of Tomato Plants and *Fusarium oxysporum* f. sp. *lycopersici*

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**Figure 2.** Scheme of activation of the defense system by PTI and ETI in tomato plant.

To date, chitin has been the most extensively studied PAMPs; the presence of a receptor for chitin in rice cells has been detailed [62], while in *Arabidopsis thaliana*, an RLK-type lysin motif receptor-like protein (LysM RLK1) with an extracellular domain containing three predicted LysM motifs has been detailed. These studies have shown that the binding between the receptor and chitin is specific and direct. However, this interaction is not a simple ligandbinding reaction but could be accompanied by a conformational change of the receptor protein. This allows it to participate in signaling leading to gene induction and defense responses against pathogenic fungi [63, 64]. On the other hand, little is known about the mechanism of recognition of glucans as PAMPs in tomato plants. In soybeans, a glucanelicitor–binding protein (GEBP) harbored a glucanase domain and a high-affinity glucan binding motif, which makes this protein a powerful tool to release and detect elicitor frag-

tail [59–61].

ments of the pathogen [65].
