# S seaweed Polysaccharides in Agriculture: A Next Step towards Sustainability
## 1 Introduction  
Seaweed extracts include a diverse range of bioactive compounds that stimulate and directly boost plant growth and its defensive responses to pathogens [1]. Some studies have shown that seaweed-based biostimulants can achieve better results in plant growth parameters when compared to commercial fertilizers [2]. Plants grown in soils treated with seaweed extracts, either applied to the soil directly or sprayed on the foliage, show a wide range of responses. When applied to the soil these extracts can stimulate soil microflora and cause soil water retention and remediation. Seaweed extracts alleviate the nutrient deficit in plants and can have a positive impact on the phytohormone balance [2]. Plants serve as biosensors for detecting the presence of bioactive compounds provided by seaweeds, testing them and even evaluating their effects, in general. This is an easy way to guarantee that seaweed extracts have a consistent level of bioactivity [3]. Consequently, seaweed extracts can operate as elicitors in plants by stimulating their defenses, including salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) pathways [4]. These phytoelicitors can improve crop yield [5] and plant vigor [5], increase the uptake of soil nutrients [6], provide longer shelf life of the fruits [5], improve seed germination [7], and protect plants against several abiotic and biotic stresses such as salinity, drought, temperature [2], and pathogens [8]. The active organic compounds responsible for the bioactivity may change depending on the seaweed class and species, as well as the extraction method used. They often include a variety of organic and inorganic bioactive compounds such as polysaccharides (alginate, agar, and carrageenan), polyphenols, phytohormones (auxins, cytokins, and gibberellins), phytohormone-like (betaine), minerals (potassium, phosphorus, calcium, and some trace elements), and other different components (lipids, peptides, glycoproteins, and proteins) [4]. Searweed extracts are mainly composed of polysaccharides, sugars known for improving plant growth in a similar way to hormones [9; 10] and whose characteristics depend on the species and ecological conditions of the seaweed [1; 11]. The extraction procedures have a significant impact on the content of seaweed extracts. During the extraction process, complex compounds such as polysaccharides are often transformed into oligomers that are extremely bioactive in plants. In a study related to the influence of oligo-alginates and oligo-carrageenan in the resistance of tobacco plants against tobacco mosaic virus (TMV) [12], it was reported that these seaweed oligosaccharides could stimulate the plant's growth and defense against TMV by activating the antioxidant enzyme ascorbate peroxidase (AP), which modulates the level of the antioxidant compound ascorbate (ASC) and docosahexaenoic acid (DHA). In addition, the activation of the defense enzyme phenylalanine ammonia-lyase (PAL) led to the activation of the phenylpropanolid pathway and to the synthesis of secondary metabolites with an antiviral activity. However, there are many challenges in obtaining seaweed extracts without compromising the biochemical integrity of the bioactive compounds and ensuring the efficacy of their biostimulant potential. Some of the methods used are water-based extraction, acid hydrolysis-based extraction, alkaline hydrolysis-based extraction, microwave-assisted extraction, ultrasound-assisted extraction, pressurized liquid extraction, enzyme-assisted extraction, and super-critical fluid extraction [2]. It is hypothesized that seaweed polysaccharides' bioactivities are affected by the presence and position of sulfate groups in the molecular chain of the polymers. The degree of sulfation, their concentration, and oxidation, all together, have an impact on these bioactivities [13]. Typically, alginophytes show the lowest sulfate group content, whereas carrageenanophytes, the highest [14].