Abstract
Harpins play a key role in inducing disease resistance in crops, and identifying their core functional regions and establishing a system for their efficient expression would be very valuable. In this study, large amounts of soluble fusion proteins of harpin HrpZ and its subpeptides were obtained via the optimized induction conditions (28 °C with 0.5 mmol·L−1 IPTG for 6 h) in Escherichia coli BL21 (DE3). Hypersensitive response (HR) assays demonstrated that the C-terminal 66 aa of HrpZ (HrpZ_C_2_2) elicited a strong HR in tobacco (Nicotiana benthamiana) and grape (Flame Seedless) leaves. Additionally, treatment with HrpZ, and particularly HrpZ_C_2_2, significantly reduced the disease incidence and severity index of field vine leaves and those inoculated with downy mildew. The determination of the physiological parameters indicated that HrpZ, and especially HrpZ_C_2_2, improved the photosynthesis- and chlorophyll fluorescence-related parameters, enhanced the activity of defense-related enzymes, including SOD, POD, CAT and PAL, and increased the H2O2 level. Collectively, we efficiently expressed a core peptide of HrpZ and elucidated its strong ability to elicit a HR and resistance to downy mildew. This research provides insight into understanding the structure and function of HrpZ and will advance the application of HrpZ_C_2_2 to increase the resistance of grapevine to downy mildew.
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