Specific response mechanism to autotoxicity in melon (Cucumis melo L.) root revealed by physiological analyses combined with transcriptome profiling

Melon is of great value in food, medicine and industry. In recent years, the continuous cropping obstacles of melon is increasingly prominent, which seriously affects the cultivation. Autotoxicity is the key factor for the obstacles. Root is the first line against autotoxicity and main organs for autotoxins secretion. Some physiological responses and differentially expressed genes (DEGs) related to autotoxicity are only limited to root system. Considering the lack of relevant research, physiological researches combined with transcriptome sequencing of melon seedling after autotoxicity stress mediated by root exudates (RE) was performed to help characterize the response mechanism to autotoxicity in melon roots. The results showed that autotoxicity inhibited root morphogenesis of melon seedlings, induced the excessive accumulation of reactive oxygen species (ROS) and lipid peroxidation in roots, and activated most antioxidant enzymes. Compared with the control group, the osmoregulation substance content was always at a high level. DEGs response to autotoxicity in roots were distinguished from that in leaves. Functional annotation of these DEGs suggested that autotoxicity affected biological regulation in a negative manner. DEGs were mainly involved in the synthesis of antioxidants, DNA damage and metabolism, and stress response. These setbacks were associated with the deterioration of root morphogenesis, generation of dwarf and slender roots, and ultimately leading to plant death. The results may provide important information for revealing the response mechanism of root to autotoxicity, and provide theoretical basis for solving the continuous cropping obstacles in melon.