Hydrogen peroxide mediates spermidine-induced autophagy to alleviate salt stress in cucumber.

Autophagy, an evolutionally conserved cellular degradation process, plays critical roles in plant development and stress response. Despite the wealth of information on the vital role of autophagy in responses to environmental stresses, little is known about the regulation of autophagy. In this study, we demonstrated that spermidine (Spd), a kind of polyamine, was involved in the regulation of salt tolerance through activating the expression of (autophagy-related) genes and the formation of autophagosomes in cucumber under salt stress. Furthermore, NADPH oxidase-derived apoplastic HO-mediated Spd-induced salt tolerance and autophagy. Exogenous Spd significantly increased the tolerance to salt stress and inhibited the accumulation and ubiquitination of insoluble proteins. Foliar application of Spd promoted the transcript levels of genes and autophagosomes formation. Besides, Spd induced the expression of (respiratory burst oxidase homolog), and the accumulation of HO both in leaves and roots. However, either pretreatment with dimethylthiourea (DMTU, an HO scavenger) or diphenyleneiodonium chloride (DPI, an inhibitor of NADPH oxidase) reduced Spd-induced accumulation of apoplastic HO. Importantly, Spd-induced salt tolerance and autophagy were compromised when plants were pretreated with DMTU or DPI. Furthermore, the silencing of and reduced Spd-induced salt tolerance and autophagosomes formation. Taken together, these results revealed that -dependent HO mediated the Spd-induced autophagy and salt tolerance in cucumber.: Asat: light-saturated rate of CO assimilation; ATG: autophagy-related; DCF-DA: 2, 7-dichlorofluorescein diacetate; DMTU: dimethylthiourea; DPI: diphenyleneiodonium chloride; DW: dry weight; EL: electrolyte leakage; FW: fresh weight; Fv/Fm: the maximum quantum yield of photosystem II; GFP: green fluorescent protein; MDC: monodansylcadaverine; PDS: phytoene desaturase; PE: phosphatidylethanolamine; PLD: phospholipase D; RBOH: respiratory burst oxidase homolog; ROS: reactive oxygen species; SDS-PAGE: sodium dodecyl sulfate-polyacrylamide gel electrophoresis; SIN1: salt induced NAC1; Spd: spermidine; TOR: target of rapamycin; VIGS: virus-induced gene silencing.