Identification and characterization of defensin genes conferring Phytophthora infestans resistance in tomato

Defensins are widely distributed in all plants, and these proteins play important roles in the resistance to pathogens. In this study, two tomato defensin genes (SlyDF1 and SlyDF2) were cloned and characterized from tomato Solanum lycopersicum Zaofen No.2. Phylogenetic analysis classified SlyDF1 and SlyDF2 into Group II of the defensin family, and the two proteins were shown to have a closer genetic relationship with MtDef4 and PDF2.5. Both SlyDF1 and SlyDF2 transcripts were observed in the root, stem, leaf, flower and mature fruit of the tomato plant, and were strongly upregulated 1 and 2 days after inoculation of the whole plant with Phytophthora infestans. The tomato plants that overexpressed SlyDF1 displayed greater resistance to P. infestans infection, as evidenced by decreased P. infestans abundance, disease index, number of necrotic cells, lesion sizes, and number of sporangia per leaf compared to the control plants. SlyDF1 enhanced the activities of peroxidase (POD) and superoxide dismutase (SOD) to decrease the accumulation of H2O2, thereby preventing damage to the tomato cell membrane during resistance to P. infestans infection. The results suggest that tomato defensin might play a role of positive regulation in the response to P. infestans infection and could therefore be considered as a candidate gene for enhancing biotic stress-resistance in tomato.