Root-knot nematodes (RKNs) are one of the most serious pests of carrot plants. Resistance cultivars are a cost-effective and ecofriendly management strategy for nematodes. To develop these, it is essential to understand the genetic, molecular and biochemical aspects underpinning RKN infection. Here, Meloidogyne javanica penetration and development, defence reactions and antioxidant enzyme activity were investigated in a resistant carrot cultivar (TN-99-129)compared to the susceptible cultivar TN-99-62.No significant difference was found in the number of J2s penetrating the roots of both cultivars. At 2 days post-inoculation(DPI), necrosis symptoms were observed around the head of penetrated nematodes. Cell death also increased in the roots of the nematode-infectedTN-99-129cultivar. Callose deposition happened at 6 DPI, and lignin content increased with delay (21 DPI) in the inoculated resistant cultivar. In addition, the reactive oxygen species metabolism process in resistant and susceptible cultivars was significantly different. In TN-99-129,catalase activity inhibition was observed at the early (2 DPI) phases of infection, and increasing superoxide dismutase activity and a relative decrease in catalase activity during nematode development. The activity of guaiacol peroxidase also increased significantly in the resistant cultivar, but no significant difference was observed in the activity of ascorbate peroxidase. This study explains the process of defence reactions in the TN-99-129 carrot cultivar during RKN penetration and development. The exhaustive characterization of the defence mechanism of a novel source of resistance to RKN in carrots constitutes a significant step toward their use in crop breeding.

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