Microalgae are photosynthetic microorganisms that produce several bioactive molecules that have received considerable attention in scientific and industrial communities. Today, many plant biostimulants including seaweed extracts and polysaccharides are used in agriculture. However, microalgae have not been largely exploited in this field as a potential source of plant bio stimulants. This study investigated the biostimulatory effects of microalgae polysaccharides on different metabolomic and biochemical pathways related to plant defense. 0.2mgmL(-1) of crude polysaccharides extracted from four green microalgae strains was injected into tomato plants (Solanum lycopersicum). -1,3-glucanase activity, lipid remodeling, phenylalanine ammonia lyase (PAL), Lipoxygenase (LOX), and antioxidant enzyme (APX, POD and CAT) activities were evaluated 48h after treatment. Plants treated with crude polysaccharides extracted from. C. vulgaris and C. sorokiniana exhibited a significant increase in -1,3-glucanase activity. Accordingly, C. sorokiniana crude polysaccharides had a significant stimulatory effect on PAL activity with a percentage increase of 188.73% compared to the control. GC/MS quantitative lipidomics analysis revealed that treatment with D. salina, C. sorokiniana, and C. reinhardtii crude polysaccharides increased PUFA content by 50.37%, 34.46%, and 33.37% respectively. Microalgae polysaccharides also enhanced stearic acid, palmitic acid, and VLCFA content, the optimal value of which increased by 45.50%, 32.83%, and 60.60% respectively under treatment with C. reinhardtii crude polysaccharides compared with the control. C. vulgaris and C. reinhardtii crude polysaccharides also exhibited higher APX and POD activity respectively. The present results therefore indicate the potentiality of microalgae crude polysaccharides as a promising renewable bio resource in the development plant bio stimulants.