# S seaweed Polysaccharides in Agriculture: A Next Step towards Sustainability
## 3 Seaweed Poly- and Oligosaccharides Bioactivities on Plants
### Carrageenanans and Oligo-Carrageenanans  
Carrageenanans and their oligomers, extracted from various red seaweeds, present a significant source of bioactive substances that activate plants' defense mechanisms and offer resistance against abiotic and biotic stresses. This can be achieved by modulating various physiological and biochemical processes [52]. Additionally, carrageenanans control several metabolic activities in plants, including cell division, purine and pyrimidine synthesis, assimilation of nitrogen and sulfur, and photosynthesis [14,53]. [PERSON] et al. [54] demonstrated that oligosaccharides (of varying molecular weights, but less than 500 Da) of \(\lambda\)-carrageenanan act as inducers of embryogenesis. It should be noted that, in this study, both alginate and agar oligosaccharides were also tested, but with less significant results than those obtained by carrageenanans. In general, the oligosaccharides obtained from carrageenanan promote an increased plant height, greater leaf biomass, and better carbon fixation, as well as superior nitrogen assimilation and greater overall plant growth [53], in addition to promoting plant defenses as elicitors and activating their defense mechanisms against pathogens [55]. Tobacco plants [56] and eucalyptus trees [57] treated with commercially available \(\kappa\)-, \(\iota\)-, and \(\lambda\)-carrageenanans showed positive results in their growth. The oligo-carrageenanans enhanced photosynthesis, basal metabolism, and the synthesis of secondary metabolites such as essential oils and polyphenolic compounds (Table 2). In addition, \(\kappa\)-, \(\iota\)-, and \(\lambda\)-carrageenanans were reported (Table 2) to induce long-term protection against viral, bacterial, and fungal infections at a systemic level by activating the phenylalanine ammonia-lyase (PAL) enzyme and enhancing the accumulation of phenylpropanoids with potential antimicrobial activities [58; 59]. As said previously, the level of sulfation of the polymer is suggested to influence their bioactivity and, therefore, their targeted applications for plant defenses [13]. The sulfate group content differs depending on the type of carrageenanan: \(\kappa\)-carrageenanan has 20-30% of sulfate group content, \(\iota\)-carrageenanan has 28-35%, and \(\lambda\)-carrageenanan has 32-39% [59; 60]. Among the three carrageenanans, \(\lambda\)-carrageenanan was considered the most potent elicitor due to its high sulfur content, inducing systemic resistance in plants. Plants treated with \(\lambda\)-carrageenan (Table 2), either through leaf infiltration or foliar spray, showed resistance against several pathogens by inducing SA-, JA-, and ET-dependent defense pathways [59; 61; 62; 63; 64]. I-carrageenanan, sprayed on leaves, was reported (Table 2) to stimulate the growth of tobacco plants by enhancing photosynthesis, basal metabolism, and cell cycle, as well as ascorbate (ASC) levels and ascorbate peroxidase (AP) enzyme activity [65]. This oligocarrageenan can elicit resistance against the moth _Trichplusa ni_ in _Arabidopsis thaliana_ by inducing various defense mechanisms, including JA- and SA-dependent pathways, proteinase inhibitors, and an alteration of the products of glycoslate hydrolysis [66]. K-carrageenanan, used in leaf spray treatment, was reported (Table 2) to stimulate the growth of chickpea plants, maize plants [67], and pine trees [68] by enhancing the basal metabolism and the production of secondary metabolites. Additionally, \(\kappa\)-carrageenanan, applied through leaf infiltration, showed resistance against several pathogens by inducing SA-, JA-, and ET-dependent defense pathways [69; 70; 71; 72]. When compared to \(\lambda\)-carrageenanan, \(\kappa\)-and \(\iota\)-carrageenanan showed better results in the growth of the roots and leaves in kale by inducing the production of indole-3-acetic acid (IAA), responsible for the plant's development [14]. In sum, carrageenanans and oligo-carrageenanans can be employed as naturally occurring growth-enhancing, anti-fungal, and anti-viral agents.