Arbuscular mycorrhizal colonization alleviates Fusarium wilt in watermelon and modulates the composition of root exudates

Citrullus lanatus (watermelon) is susceptible to Fusarium oxysporum f. sp. niveum (FON) infection, which causes wilt disease. Arbuscular mycorrhizal fungus (AMF) colonization can alleviate biotic and abiotic stresses encountered by plants, as plant roots secrete allelopathic substances when they come into contact with soilborne pathogens. We investigated whether AMF colonization would alleviate watermelon wilt disease and alter the plant's responses to the pathogen. The results showed that arbuscular mycorrhizal (AM) colonization reduced the disease index by 89.3 % by suppressing pathogen development in the roots and rhizosphere of watermelon. Inoculation with both AMF and FON decreased the number of colony forming units of FON by 58.1 and 47.7 % in watermelon roots and the rhizosphere, respectively, and decreased by 52.1-84.5 % free amino acid secretion from the roots. By contrast, p-coumaric acid and malic acid secretion increased by 62.1 and 142.4 % upon double inoculation with AMF and FON, respectively. FON infection triggered phenolic acids and organic acids of succinic acid and oxalic acid production, but this response was reduced following double inoculation. Thus, AM colonization alleviates watermelon wilt disease and modulates the composition of root exudates formed in response to FON infection, which increased allelopathic substance of p-coumaric acid and malic acid exudation and decreased common root exudates secretion.