Melissa officinalis (lemon balm) is a susceptible plant to various pathogens such as Fusarium species, which significantly affect the plant’s yield. The mycorrhizal symbiosis can increase the plant’s resistance to many fungal pathogens. Therefore, this study was conducted to elucidate the effect of pre-inoculation of lemon balm with the arbuscular mycorrhizal (AM) fungus Glomus mosseae on the disease severity caused by Fusarium culmorum. The obtained results indicated that mycorrhizal colonization significantly reduced disease index of F. culmorum in lemon balm leaves and roots by 46 % compared to non-mycorrhizal plants. Also, it has been observed that AM symbiosis results in compensation of pathogen-induced losses in the length of shoots (58 %) and roots (53 %), dry weight of shoot (49 %) and root (51 %) and, total chlorophyll content (46 %). Furthermore, stronger pathogeninduced defense responses were found in lemon balm pre-inoculated with AM compared to non-mycorrhizal plants. The activity of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POX), and polyphenol oxidase (PPO), as well as the contents of non-enzymatic antioxidants (carotenoid, ascorbate, and phenolic compounds) in mycorrhizal plants infected with pathogen was higher than in non-mycorrhizal ones. Stronger antioxidant reactions led to a reduction in the pathogeninduced hydrogen peroxide (H2O2) and malondialdehyde (MDA) accumulation in mycorrhizal plants. Symbiosis increased phenylalanine ammonia lyase (PAL) and lipoxygenase (LOX) activities which resulted in the accumulation of salicylic acid (SA) and jasmonic acid (JA). In conclusion, obtained results suggest G. mosseae colonization of lemon balm increases resistance against F. culmorum infection through recovering photosynthesis rate, strengthening antioxidants machinery, and triggering SA and JA signals.

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