Glucuronan and oligoglucuronans isolated from green algae activate natural defense responses in apple fruit and reduce postharvest blue and gray mold decay El Faïza Abouraïcha 1 & Zainab El Alaoui-Talibi 1 & Ahmed Tadlaoui-Ouafi 1 & Redouan El Boutachfaiti 2 & Emmanuel Petit 2 & Allal Douira 3 & Bernard Courtois 2 & Josiane Courtois 2 & Cherkaoui El Modafar 1 Received: 3 March 2016 /Revised and accepted: 3 August 2016 # Springer Science+Business Media Dordrecht 2016 Abstract The efficacy of two algal saccharides, glucuronan and oligoglucuronans (average degree of polymerization = 3), against postharvest blue mold (Penicillium expansum) and gray mold (Botrytis cinerea) on apple fruit (Malus domestica Borkh. cv Golden Delicious) and the related-defense re- sponses involved were evaluated. The severity of both blue and gray mold in apple fruits was reduced with the greatest efficacy being achieved using oligoglucuronans. Moreover, glucuronan and its oligomers trigger a rapid and transient ac- cumulation of hydrogen peroxide (H 2 O 2 ) as well as the acti- vation of antioxidant-related enzymes, namely catalase (CAT) and superoxide dismutase (SOD). These algal saccharides in- creased also the activities of phenylalanine ammonialyase (PAL), peroxydase (POD), and polyphenoloxydase (PPO) as well as the levels of lignin and phenolic compounds. These results suggest that the protective effects of glucuronan and oligoglucuronans on apple might be due to its ability on acti- vating an onset-related defensive enzymes and metabolites instead of its direct antifungal activity on the pathogens. These findings showed that glucuronan and even more oligoglucuronans treatments could be a promising method to reduce dependency on synthetic fungicides. Keywords Bioelicitors . Glucuronan . Oligoglucuronans . Ulva lactuca . Apple fruit . Natural defenses Introduction Most investigations related to crop protection have attracted recently a special interest in order to develop new and safer control methods instead of those based on chemical pesticides. One strategy might be the induction of the plant’ s own defenses using natural products (Terry and Joyce 2004; Walters et al. 2005; Benhamou and Rey 2012). Seaweeds are a rich but still underexploited source of bio- active compounds (Craigie 2011; Sharma et al. 2014). Among them, carbohydrates are probably the most abundant organic compounds in the oceans with a great molecular diversity (Alves et al. 2013; Sharma et al. 2014; Stadnik and de Freitas 2014). Some of these carbohydrates have been identi- fied as polysaccharides such as linear β-1,3 glucan (laminar- in), carrageenan, and ulvan (Elyakova and Zvyagintseva 1974; Usov 1998; Lahaye and Robic 2007). The structure and the composition of these algal polysaccharides have high- ly contributed to their potential on activating signaling path- ways and enhancing defense mechanisms in various plants (Abouraïcha et al. 2015; Arman and Qader 2012; de Freitas et al. 2015; El Modafar et al. 2012; Klarzynski et al. 2000; Mercier et al. 2001). For example, laminarin, a linear β-1,3- glucan polymer, from brown algae induces the formation of antifungal compounds in alfalfa cotyledons (Kobayashi et al. 1993) and several defense responses in tobacco cell suspen- sion cultures (Klarzynski et al. 2000), rice (Inui et al. 1997), and grapevine (Aziz et al. 2003). Carrageenans, which form a family of sulfated linear galactans found in the cell walls of many red algae, are potent elicitors of defense in tobacco * Cherkaoui El Modafar elmodafar@uca.ma 1 Laboratoire de Biotechnologies et Bio-ingénierie Moléculaire, Faculté des Sciences et Techniques Guéliz, Université Cadi Ayyad, B.P. 549, 40000 Marrakech, Morocco 2 Laboratoire des Polysaccharides Microbiens et Végétaux, Université de Picardie Jules Verne, IUT-GB Amiens, Avenue des facultés, Le Bailly, F-80025 Amiens Cedex 1, France 3 Laboratoire de Botanique et de Protection des Plantes, Faculté des Sciences, Université Ibn Tofail, B.P. 133, 14000 Kénitra, Morocco J Appl Phycol DOI 10.1007/s10811-016-0926-0