## 4 Discussion
The results showed that both extracts induced the natural defense mechanisms by enhancing the PAL activity, the phenolic compounds, and the lignin contents in the upper leaves (n and n - 1) and the roots depending on the incubation period. PAL is the key enzyme associated with the natural defenses of plants; it is one of the most studied enzymes in plant secondary metabolisms, and it catalyzes the first reaction of the phenylpropanol pathway through the deamination of phenylalanine to _trans_-cinnamic acid, which is an essential precursor to the biosynthesis of the majority of polyphenols and lignin [52]. Lignin is a complex polymer derived from the polymerization of monolignols in the presence of peroxididases and lacases. Lignin is involved in plant resistance to pathogens by providing rigidity to the cell walls to avoid permeability to phytopathogens [53]. To the best of our knowledge, this is the first report indicating the ability of sulfated arabinadicans-rich protein isolated from the green seaweed _C. decorticatum_ to act as a potent elicitor of the natural defenses of plants. It has been previously reported that aqueous (hot/cold) extracts from _C. decorticatum_, harvested from the Karachi coast, were found to be active elicitors capable of inducing browning and secondary metabolites in chickpea tissues [54; 55]. The extracted polysaccharides were characterized by the presence of glucose, rhamn, arabinose, xylose, fucose, and glucuronic and galacturonic acids as the main monosaccharides, with a low degree of sulfates (<4 to 8%) and a negligible amount of proteins (0.5%) [55]. These features are completely different from those found in this work using the same species, which illustrates the influence of the geographic location and the extraction processes, as discussed above. At present, alginates, fucoidans, laminarins, carrageenanans, glucuronans, and ulvans are the main polysaccharides, isolated from marine macroalgae, that have been reported as bio-elicitors of defense mechanisms in diverse plants [3; 5; 7; 8; 40; 41; 56]. Currently, the only polysaccharide from the macroalgae released in the market as an elicitor is laminarin from the brown algae _Laminaria digitata_, known as Iodus 40 [57]. This product is solely used to control the powdery mildew on wheat in the field [58]. As has been previously reported in the literature, glycoproteins (GP) isolated from several microorganisms are known as elicitors and are capable of stimulating plant defense responses. A peptide-galactoglucomanan isolated from the cell wall of the fungal plant pathogen _Cladosporium fulvum_ has enhanced the accumulation of phytoalexins in treated tomato leaves and fruits. The monosaccharide composition was mannose, galactose, and traces of glucose, and the protein moiety was rich in alanine, asparagine/asparatic acid, glutamine/glutamic acid, proline, serine, and threonine [59]. GPs extracted from other fungi, namely _Verticillium dahliae, Phytophthora palmivora, Phytophthora megasperma, Botrytis cinerea_, and _Alternaria tenuissima_, have been found to induce phytoalexin formation in cotton cell suspension cultures [60], expression of defense genes, and accumulations of H\({}_{2}\)O\({}_{2}\), SA, scopoletin (Scp), and abscisic acid (ABA) in rubber, tobacco, and tomato leaves [61; 62; 63; 64]. On the other hand, GPs derived from bacteria have also exhibited plant immunity-inducing effects [65]. In this study, the tomato seedlings were treated by two different modes of application: the injection in the midstem, in which the molecules are penetrated directly into the plant system, and the foliar spray, in which elicitors are uptaken by both the stomata and cuticle of the leaves. Whatever the elicitor application, the responses followed globally similar patterns and were mobilized throughout the plant. The PAL activity, the phenolic compounds, and the lignin contents were promoted in roots after foliar application and in both roots and leaves after infiltration of elicitors in tomato plants, which were not directly treated.