Abstract:  【Aim】To explore the effects of single and combined seed soaking treatments with calcium chloride (CaCl₂) and salicylic acid (SA) on rice resistance to the brown planthopper (BPH), Nilaparvata lugens.【Methods】Rice seeds were soaked in 20 mmol/L CaCl₂ solution (CaCl₂), 1 mmol/L SA solution (SA) or a solution combined with them (SA+CaCl₂) for 48 h, meanwhile the control seeds were soaked in distilled water. During the tillering stage, the expression levels of defense genes including transcription factor genes OsWRKY70 and OsWRKY53, OsNPR1 (one of key regulators of SA signaling pathway) and OsPAL1 (a key gene involved in SA synthesis), and the activities of defense enzymes induding phenylalanine ammonia lyase (PAL), peroxidase (POD), polyphenol oxidase (PPO) and β-1, 3-glucanase (β-1,3-GA) in leaf sheaths of rice plants subjected to different seed soaking treatments, under the infestation of the 3rd instar nymphs of N. lugens were tested, and the survival rates and developmental duration of N. lugens fed on rice plants subjected to various seed soaking treatments were detected as well. 【Results】Without infestation of N. lugens, the expression levels of OsWRKY53, OsNPR1 and OsPAL1 in leaf sheaths of rice plants subjected to single and combined seed soaking treatments with CaCl₂ and SA showed no significant difference. The expression levels of OsWRKY53, OsNPR1 and OsPAL1 in leaf sheaths of the control rice plants were significantly increased at 180 min after infestation by the 3rd instar nymphs of N. lugens as compared to those in leaf sheaths of the control rice plants uninfested by N. lugens. OsNPR1 and OsPAL1 in leaf sheaths of rice plants subjected to seed soaking treatment with CaCl₂ reached the highest expression levels at 90 and 120 min after infestation of N. lugens, respectively, which were significantly higher than those in leaf sheaths of the control rice plants at the corresponding infestation time points. OsWRKY70 exhibited higher expression level in leaf sheaths of rice plants subjected to seed soaking treatment with CaCl₂ than in leaf sheaths of rice plants subjected to seed soaking treatments with SA and SA+CaCl₂ at each infestation time. The expression levels of OsWRKY53 and OsPAL1 in leaf sheaths of rice plants subjected to seed soaking treatment with SA were significantly higher than those subjected to the other seed soaking treatments at 180 and 90 min after infestation of N. lugens, respectively. The expression levels of OsWRKY53 in leaf sheaths of rice plants subjected to seed soaking treatment with SA+CaCl₂ at 90 and 120 min after infestation of N. lugens, and those of OsNPR1 in leaf sheaths of rice plants subjected to seed soaking treatment with SA+CaCl₂ at 60 and 180 min after infestation of N. lugens were significantly higher than those subjected to the other seed soaking treatments at the corresponding infestation time, while the expression levels of OsPAL1 in leaf sheaths of rice plants subjected to seed soaking treatment with SA+CaCl₂ were significantly lower than those subjected to the other seed soaking treatments at all the tested infestation time points. The activities of PAL and β-1,3-GA in leaf sheaths of rice plants subjected to seed soaking treatment with CaCl₂, those of PAL, POD, PPO and β-1,3-GA in leaf sheaths of rice plants subjected to seed soaking treatments with SA and SA+CaCl₂ were significantly higher than those in leaf sheaths of the control rice plants. The activities of all the four defense enzymes in leaf sheaths of the control rice plants infested by N. lugens increased as compared to those in leaf sheaths of the control rice plants uninfested by N. lugens, and the ascending degrees of PAL, POD and PPO activities in leaf sheaths of rice plants subjected to single and combined seed soaking treatments with CaCl₂ and SA were higher than those in leaf sheaths of the control rice plants. Seed soaking treatment with SA+CaCl₂ had obvious induction effect on the POD activity under infestation of N. lugens. The survival rates of N. lugens nymphs fed on rice plants subjected to seed soaking treatments with CaCl₂, SA and SA+CaCl₂ were 9.03%, 13.82% and 19.61%, respectively, lower than that fed on the control rice plants at 8 d after infestation, and the nymphal duration of N. lugens fed on the rice plants subjected to seed soaking treatments with CaCl₂, SA and SA+CaCl₂ was significantly prolonged as compared to that fed on the control rice plants. The N. lugens nymphs fed on the rice plants subjected to seed soaking treatment with SA+CaCl₂ displayed the lowest survival rate. 【Conclusion】Single and combined seed soaking treatments with CaCl₂ and SA have different induction effects on the expression levels of OsWRKY70, OsWRKY53, OsNPR1 and OsPAL1 and the activities of defense enzymes PAL, POD, PPO and β-1,3-GA, while the combined seed soaking treatment with CaCl₂ and SA has a synergistic effect on rice resistance to N. lugens.

Key words: Nilaparvata lugens, rice, calcium chloride, salicylic acid, seed soaking treatment, synergistic effect