## 4 Functional Properties of Chitosan and Derivatives
### Elicitation and Stimulation of Plants  
As largely described in the literature, CS and derivatives also have applications as elicitors of plant growth defensive and stimulant responses [141; 142; 143]. In general, the idea that plant cells could release chemicals substances during pathogen aggression was issued by the scientific community in the early 20 th century. It was commonly referred to as phytoalexins to designate these plant antibiotics inducing a defense response against phytopathogens [141; 142]. Later, these biomolecules resulting in the synthesis of phytoalexins were designated by the term \"elicitors\". Thus, oligosaccharidesadrides derived from plants (endogens oligosaccharidesadrides: oligoxyloglucan and oligogalacturonate) or fungi (exogens oligosaccharidesadrides: oligo-\(\beta\)-glucan and oligochitin/COS) were widely described as active biological elicitors on biological mechanisms such as growth, cell development, symbiosis, and defense reactions [143; 144]. During the aggression stage of a plant by a phytopathogen, different eliciting signals are emitted by both partners. First, in the early stages, oligogalacturonates, resulting from pectocellulosic wall degradation with fungal pectinase activities, set off acquired systemic resistance (ASR) in plants [145; 146; 147]. Several major components [147] can be distinguished to account for observed behaviors: _(i)_ interaction with pecto-cellulosic walls of the host, _(ii)_ induction of phytoalexins, _(iii)_ specificity, _(iv)_ hypersensitivity, _(v)_ the action of toxins, _(vi)_ the effect of ethylene, and _(vii)_ the induction of pathogenesis-related proteins. Thus, ASR begins when all the different signals are perceived by a specific plant cell membrane receptor. Consequently, the plant then activates its natural defenses, such as the production of specific enzymes like chitinases, deacetylases, and \(\beta\)-(1,3)-glucanases, which will degrade the parietal constituents of the fungus to generate oligochitins, COS, and oligo-\(\beta\)-(1,3)-glucans [148]. Apart from all these oligo-\(\beta\)-(1,3)-glucan, oligochitins (\(\beta\)-(1,4)-\(N\)-acetyl-oligoglucamines) and their deacetylated analogs (COS) are involved in the defense processes in many plant species, such as wheat (Tricicum) and rice (Oryza sativa) [149; 150]. The heptaoligochitin (DP 7) and octaoligochitin (DP 8) structures were found to be the most active elicitors [149; 150]. Some examples of CS and oligochitin/COS elicitors derivatives are summarized in Table 3.  
COS also exhibit activity on pea (_Pisum sativum_) and tomato (_Solanum lycopersicum_) leaves defenses, but at concentrations higher than those described for \(N\)-acetylated forms (oligochitins) [151; 152]. Some other COS fractions were described to induce: (i) the synthesis of callose, which is a \(\beta\)-(1,3)-glucans during the defense responses of plants such as parsley (Petroselinum crispum) and soybean (Glycine max) [153; 154] and (ii) lignin deposition and phenolic acid increasing in leaves of wheat [155]. More, CS and COS were also shown to highly stimulate positive plant effects on Potato (_Solanum tuberosum_ L.), strawberry (_Fragaria ananassa_ Dutch. ), basil (_Ocimum ciliatum_), rape (_Brassica rapa_ L.), maize (_Zea mays_ L.) and barley (_Hordeum vulgare_ L.) [156]. Generally speaking, these bioelicitor activities from oligochitins/COS seem to be essentially modulated by ionic interactions between