Abstract:

【Objective】 Apoplast in plants acts as the structural and physiological barrier against pathogen attacks. In present paper, the role of apoplastic responses in the systemic resistance of rice seedlings to bacterial blight induced by nickel were investigated so as to explore the physiological mechanisms involved in the cross-resistance to pathogens and heavy metals. 【Method】 The second leaf and its lower parts of three-leaf stage rice seedlings were sprayed with nitrate nickel at 0.5, 1.0 and 2.0 mmol L-1. The untreated third leaf was inoculated with Xanthomonas oryzae pv. oryzae (Xoo) 3 d after spraying, and the disease symptoms were evaluated 12 d after inoculation. The activities of apoplastic guaiacol peroxidase (G-POD), NADH-POD, diamine oxidase (DAO) and the contents of H2O2 and lignin were determined in the third leaves of seedlings treated with 2.0 mmol L-1 nickel or/and inoculated with Xoo. 【Result】 The systemic resistance of seedlings to bacterial blight was induced by nickel. The best induction effect occurred at 2.0 mmol L-1 nickel. Nickel not only led to the enhancement in the activities of G-POD and NADH-POD, which are present in apoplastic water fluid (AWF) and are associated with cell wall, but also the enhancement in DAO activity in AWF. Concurrently, a rapid increase in apoplastic H2O2 level and a significant increase in lignin content were also observed in treatment with nickel. On the other hand, the parameters mentioned above in the nickel-induced and Xoo-inoculated seedlings were always higher than those in the non-induced but Xoo-inoculated ones. Additionally, NaN3, an inhibitor of PODs mediating in apoplastic H2O2 formation, compromised the H2O2 accumulation and systemic resistance against bacterial blight induced by nickel. However, diphenyleneiodonium chloride, an inhibitor of NADPH oxidase, exerted no effects on them. 【Conclusion】 The results suggest that the increase in the activities of NADH-POD and DAO contributes to the production of apoplastic H2O2 induced by nickel, and the rapid enhancement in apoplastic H2O2 level and lignin accumulation in cell wall may be involved in the systemic resistance to bacterial blight induced by nickel.

Key words: apoplast, peroxidase, H2O2, systemic resistance, bacterial blight, nickel, rice (Oryza sativa)