## 1 Introduction  
The olive tree (_Olea europaea_ L.) plays a key socio-economic and ecological role in Mediterranean countries. It is one of the most cultivated crop species and the only one of the _Oleaceae_ family that produces comestible fruits processed for oil and table olive production [1]. In the European Union (EU), the olive-growing area was about 4.59 million hectares (ha) in 2017. Three-quarters of the utilized area were concentrated in Spain, with 2.5 million ha (55%), followed by Italy with 1.07 million ha (23%), and Greece with 15% of the total EU area under olive trees [2]. Spain is the biggest producer of olive oil, providing 59% of total EU production, followed by Italy, Portugal, and Greece with 24.8%, 8.4%, and 7.3%, respectively [3]. Morocco, after Spain, Italy, and Greece, has the fourth-largest olive tree cultivation, covering 78,400 ha and producing 1.48 million tons of olives per year [4]. It is the 2 nd largest producer of table olives and the 6 th of olive oil [5]. Unfortunately, the olive tree is subjected to several biotic stresses, influencing the health, yield, and quality of the oil and fruits [6; 7; 8]. _Verticillium dahlia_ (Verticillium wilt) and _Spilaceae oleagina_ (olive leaf spot) are the major pathogenic fungi of the olive tree in the world [7; 9]. Anthracnose, caused by _Colletotrichum_ spp., is also one of the most severe diseases of this woody crop [10]. Furthermore, the olive fruit fly poses a serious economic problem. It reduces about 15% of production, equivalent to an average of 800 million dollars per year [11]. Today, pesticides are an important input in the protection of the olive tree [7]. Despite their effectiveness, the residues of these agrochemicals in fruits and olive oil present a major risk to public health [7; 12]. Recently, researchers have focused on using biological control agents (BCAs) such as arbuscular mycorrhizal fungi (AMF), plant growth-promoting rhizobacteria (PGPR), plant growth-promoting fungi (PGPF), and endophytes as an eco-friendly alternative to control olive diseases [12]. However, the use of BCAs is qualified by some restrictions, namely negative impact on non-target microorganisms, issues of adaptation to the target ecological niche, and especially direct effects leading to resistance increases in pathogen communities [9]. Therefore, sustainable approaches such as the stimulation of natural defenses of plants using natural elicitors could be a promising alternative to preventing disease development in the olive tree and reducing dependence on pesticides. This strategy aims to induce plant resistance using molecules, such as lipids, glycoproteins, peptides, proteins, oligosaccharides, or polysaccharides [13]. Systemic acquired resistance (SAR) and induced systemic resistance (ISR), are two types of plant resistance involved depending on the nature of the elicitors and the regulatory pathway, leading to protection against a wide range of phytopathogens [14]. Seaweed or marine macoralgae constitute an important source of bioactive products [15]. Due to their abundance, research has been focused on exploiting seaweed extracts and seaweed-derived polymers in agriculture as inducers of plant defense against a wide range of phytopathogens [16; 17]. Currently, a great deal of attention has been directed at using algal polysaccharides [16]. Their applications have shown to activate natural defenses in a variety of plants, including tomato, palm date, apple fruits, and tobacco [18; 19; 20; 21; 22; 23; 24]. However, only a few studies have been conducted on the olive tree. This study aims to characterize the cell wall polysaccharides of the Moroccan brown seaweed _S. muticum_ and _C. myriophylloides_, and to evaluate for the first time, the ability of alginates, and their derivatives, fucoidans, and glycoproteins from the green seaweed _C. decorticatum_ to induce natural defenses in the leaves of the olive tree (Picholine Marocaine).