**3.1. Pathogen recognition**

Recognition of pathogen infection triggers an immunity system in the plant of two branches [13]. (i) In the first, common molecules are recognized in many classes of microorganisms, including those that are nonpathogenic and called immunity associated to pathogen-associated molecular patterns (PAMP)s or PAMP-triggered immunity (PTI). (ii) The second branch responds to factors of virulence and pathogenesis, so it is designated as effector-triggered immunity (ETI). Both mechanisms are described below (**Figure 2**).

PAMPs are considered as conserved elements by different classes of microorganisms and are essential for survival and pathogenicity [51]. Among the molecules identified as PAMPs for FOL, some components of their cell walls such as chitin, glucan, and glycoproteins are considered in this group due to their interaction with the host [52, 53].

The synthesis of chitin is regulated by the chsV gene and its expression depends on the Fmk1-MAPK signaling pathway. This was demonstrated with mutants of this gene, as they were unable to infect and colonize tomato plants or to grow invasively in tomato fruit tissue. In addition to this, hypersensitivity to defense compounds produced by the host was observed [54, 55].

A Molecular Vision of the Interaction of Tomato Plants and *Fusarium oxysporum* f. sp. *lycopersici* http://dx.doi.org/10.5772/intechopen.72127 85

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

**2.4. Lithic enzymes**

factor [44].

**3. Tomato plant**

observed [54, 55].

**3.1. Pathogen recognition**

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

84 Fusarium - Plant Diseases, Pathogen Diversity, Genetic Diversity, Resistance and Molecular Markers

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

Recognition of pathogen infection triggers an immunity system in the plant of two branches [13]. (i) In the first, common molecules are recognized in many classes of microorganisms, including those that are nonpathogenic and called immunity associated to pathogen-associated molecular patterns (PAMP)s or PAMP-triggered immunity (PTI). (ii) The second branch responds to factors of virulence and pathogenesis, so it is designated as effector-triggered

PAMPs are considered as conserved elements by different classes of microorganisms and are essential for survival and pathogenicity [51]. Among the molecules identified as PAMPs for FOL, some components of their cell walls such as chitin, glucan, and glycoproteins are consid-

The synthesis of chitin is regulated by the chsV gene and its expression depends on the Fmk1-MAPK signaling pathway. This was demonstrated with mutants of this gene, as they were unable to infect and colonize tomato plants or to grow invasively in tomato fruit tissue. In addition to this, hypersensitivity to defense compounds produced by the host was

perceived as the inability of the pathogen to colonize the roots [49, 50].

immunity (ETI). Both mechanisms are described below (**Figure 2**).

ered in this group due to their interaction with the host [52, 53].

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 tail [59–61].

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 fragments of the pathogen [65].
