Unlocking the potential of brown algae as a biostimulant and its role in disease resistance and stress tolerance in crop plants.

Brown algal extracts increase crop yield by stimulating growth and enhancing resistance to environmental stress, making them a sustainable and effective biostimulant for modern agriculture. Population growth, climate change, and intensive agrochemical use pose significant challenges to environmental sustainability and food security. Seaweeds, particularly brown algae, have attracted considerable attention as promising biostimulants for sustainable agricultural applications. Brown algae, the second most prevalent group of marine macroalgae, are rich in polysaccharides (alginates, fucoidans, and laminarins), vitamins, minerals, and polyphenols, which contribute to their biostimulant properties. Previous studies have provided important insights into the mechanisms of action of seaweed extracts and the physiological and biochemical changes they induce in crop plants. Although the molecular mechanisms underlying the effects of seaweed biostimulants remain incompletely understood, recent research efforts have substantially advanced our understanding of their functional roles. This review discusses conventional and advanced extraction techniques used to obtain bioactive compounds from seaweeds. In addition, it examines the composition of brown algae and their roles in promoting plant growth, development, and stress tolerance in various crop species. Furthermore, this review highlights the molecular mechanisms underlying growth promotion, biotic stress resistance, and abiotic stress tolerance in brown algae-treated plants, along with key findings from recent metabolomics studies. The use of brown algal extracts or their components influences crop plants by enhancing nutrient uptake, regulating phytohormone signalling, boosting antioxidant defence, facilitating osmotic adjustment, and stimulating stress-responsive genes and pathways. Collectively, these properties highlight the potential of brown algae-derived biostimulants to support sustainable agriculture by reducing the need for synthetic agrochemicals while increasing food security amid growing environmental challenges.