## 4 Discussion
In this case, the responses were systemic and seemed to be related to the salicylic acid (SA). This form of defense reaction, known also as systemic acquired resistance (SAR), is a type of induced resistance wherein the SA signaling pathway is involved [66]. A previous study has shown systemic effects after syringe infiltration of polysaccharides and oligosaccharides from the green seaweed _Ulva lactuca_ in tomato plants. These responses were accompanied by the accumulation of salicylic acid in the leaves located above and below the elicitation site [5]. The application of these elicitors to plant foliage is a practical method in the field and could be used to protect against foliar, stem, and vascular diseases, since the responses are expressed in the shoot and root systems of the plant. Furthermore, the foliar application is more prompt and target-oriented than other modes of application such as soil application because the molecules are directly in contact with the plant without dilution or degradation that can happen in soil. Comparing the elicitor activities of the crude (AGB) and purified (AGP) extracts from C. _decorticatum_, the results have not shown a considerable difference in responses. Both fractions presented the same monosaccharide composition (Gal, Glc, Ara, Xyl, and Rha) and the same M\({}_{w}\) (\(>\)to 2000 kDa), as well as an important amount of proteins and sulfates in both fractions AGB (\(>\)to 20% (\(w/w\))) and AGP (\(<\) to 14% (\(w/w\))). The carbohydrates, the molecular weight, and the presence of proteins and sulfates groups in this polymer seemed to be the main factors influencing the eliciting responses. To further understand this, additional studies are required. Several studies have demonstrated that the sulfate groups of some polysaccharides and oligosaccharides have the main role in various biochemical and biological activities. In tobacco and _Arabidopsis thaliana_, the treatment with sulfated laminarin induced an SA signaling pathway, leading to resistance against Tobacco mosaic virus infection compared to the native unsulfated laminarin [67]. In tomato seedlings, the desulfation of ulvans extracted from _Ulva lactuca_ reduced the PAL activity in the leaves of treated plants. Compared to glucuronan, an unsulfated polysaccharide, the sulfated ulvans exhibited a more efficient elicitor effect, this difference in responses was related to the sequences of aldobiuronic acids and particularly the sulfate substituent [5]. In contrast, [PERSON] et al. have investigated the effects of sulfation degree variation of oligoglucuronanson H\({}_{2}\)O\({}_{2}\) production and defense-related genes induction in grapevine cells; they found that the sulfated oligoglucuronans did not possess any significant induction of the oxidative burst whatever their degrees of polymerization [68]. On the other side, [PERSON] et al. have demonstrated the importance of carbohydrate moiety in the elicitor capacity. After the digestion of the glycoprotein from _Pyricularia oryzae_ with pronase, the activity was not affected; however, when the molecule was altered with sodium periodate, the elicitor capacity was totally suppressed, indicating that the responses were associated with the saccharide part of the polymer [69]. As well as this, the elicitor responses of a high molecular weight glycoprotein isolated from _Colletotrichum falcatum_, were found to reside in the carbohydrate moiety [70]. Otherwise, others have shown that in some glycoprotein elicitors, only the protein moiety is responsible for the induction of the natural defenses of plants [60; 71]. This difference could be attributed to many factors, such as plant species, monosaccharides, amino acid compositions, or the molecular weight of the molecules. As discussed above, elicitation by the crude and the purified extract of _C. decorticatum_ manifested globally similar responses in tomato seedlings. These results showed the importance of crude extracts in the stimulation of plant immunity.