**1. Introduction**

Sulfated polysaccharides are increasingly recognized for their broad spectrum of biological activities. They usually found in large quantities in brown seaweeds. In addition, the polysaccharides structures depend on the algae species. Thus, various biological activities could be discovered with each new sulfated polysaccharide extraction [1].

Amongst these polysaccharides, the most studied was carrageenan [2], ulvan [3] and fucoidans. Fucoidans at the molecular level constitute a polymer of L-fucose linked by (1,3) and (1,4) with residues mainly sulfated on C-4 [4,5]. The characteristic structure of fucoidans rich in L-fucose and sulfated ester groups has generated widespread interest due to their therapeutic e ffects. Several works reported the biological proprieties of fucoidans [6], namely antioxidant [7,8], antitumor [9–12] and anticoagulant [13]. In addition to these biological applications, fucoidans and their oligosaccharides have also been the

subject of other studies on biostimulants of defence mechanisms in plants [14,15]. They were also suggested as biological approaches to control plants disease [16], by benefiting from their stimulating effect of early and late defensive responses. It is reported that biological and biostimulant properties of fucoidans depend in particular on the degree of sulfation [8] and on their various physico–chemical properties [5].

The aim of this work is to study the potential activity of fucoidans to elicit the natural defence mechanisms in date palm (*Phoenix dactylifera* L.) roots as a monocotyledon plant. Through an innovative elicitation model allowing the treatment of the roots which are the site of infection of the date palm by *Fusarium oxysporum* f. sp. *albedinis* (Foa), a telluric pathogen causing the fatal disease (Bayoud) of date palm [17]. In response to Bayoud disease, date palm develops numerous defence mechanisms in roots such as the induction of phytoalexins [18], the accumulation of ca ffeoyl shikimic acids [19,20] and the reinforcement of the cell walls by lignin and phenolic compounds [21]. These defence mechanisms all depend on, phenylalanine ammonia-lyase (PAL) activity, triggering the phenlypropanoid pathway [17]. The activity of this enzyme governs the defence mechanisms induced in sensitive and resistant varieties during a date palm and Foa interactions [22]. In this context crude fucoidans, extracted from two brown algae *Bifurcaria bifurcata* and *Fucus spiralis* from the Atlantic coast of Morocco were structurally characterized and tested for their possible eliciting e ffect on the defence mechanisms of the date palm roots.
