**2.** *F. oxysporum* **f. sp.** *lycopersici* **(FOL)**

#### **2.1. Host recognition by FOL**

It is known that FOL can produce three types of asexual spores: (i) microconidia, (ii) macroconidia, and (iii) chlamydospores; while the sexual or teleomorphic phase is unknown. FOL can survive saprophytically in soil and organic waste in the absence of a host, either as mycelium or in all types of spores mentioned [14]. Chlamydospores are resistance structures capable of remaining viable in the soil for several years according to environmental conditions, and this allows this pathogen to be dispersed rapidly with the movement of water, soil, or air [15, 16].

The presence of these reproductive structures of FOL in the development medium of tomato plant allows the plant-pathogen interaction to be initiated with a preinfection state, where the host recognition is carried out, and subsequently the germination of the spores, which will continue with the tissue infection [17].

In this stage, the identification of the host is vital for the initiation of the infection process, and this is done through the release of exudates from the host roots, as these compounds represent a carbon source for the fungus [18]. Its composition includes sugars, polysaccharides, amino acids, aliphatic, aromatic and fatty acids, sterols, phenolic compounds, enzymes, vitamins, plant growth regulators, and other secondary metabolites [19].

The specific compounds that FOL recognizes in its host have not been characterized. To initiate spore germination, it is known that exudates from the root of tomato plants stimulate the germination of FOL microconidia. In addition, a relationship was found between the stimulation of germination and the age of plants. The highest stimulation of germination was observed when the plants were 70 to 90 days old. Changes were also observed in root exudates such as the concentration of phenolic compounds and flavonoids or induced changes in the exudates by the degree of colonization of the arbuscular mycorrhizal fungus Glomus mosseae, modifying the spore germination and the degree of colonization [20, 21].
