HO Functions as a Downstream Signal of IAA to Mediate HS-Induced Chilling Tolerance in Cucumber.

Hydrogen sulfide (HS) plays a crucial role in regulating chilling tolerance. However, the role of hydrogen peroxide (HO) and auxin in HS-induced signal transduction in the chilling stress response of plants was unclear. In this study, 1.0 mM exogenous HO and 75 μM indole-3-acetic acid (IAA) significantly improved the chilling tolerance of cucumber seedlings, as demonstrated by the mild plant chilling injury symptoms, lower chilling injury index (CI), electrolyte leakage (EL), and malondialdehyde content (MDA) as well as higher levels of photosynthesis and cold-responsive genes under chilling stress. IAA-induced chilling tolerance was weakened by N, N'-dimethylthiourea (DMTU, a scavenger of HO), but the polar transport inhibitor of IAA (1-naphthylphthalamic acid, NPA) did not affect HO-induced mitigation of chilling stress. IAA significantly enhanced endogenous HO synthesis, but HO had minimal effects on endogenous IAA content in cucumber seedlings. In addition, the HO scavenger DMTU, inhibitor of HO synthesis (diphenyleneiodonium chloride, DPI), and IAA polar transport inhibitor NPA reduced HS-induced chilling tolerance. Sodium hydrosulfide (NaHS) increased HO and IAA levels, flavin monooxygenase (FMO) activity, and respiratory burst oxidase homolog () and FMO-like protein () mRNA levels in cucumber seedlings. DMTU, DPI, and NPA diminished NaHS-induced HO production, but DMTU and DPI did not affect IAA levels induced by NaHS during chilling stress. Taken together, the present data indicate that HO as a downstream signal of IAA mediates HS-induced chilling tolerance in cucumber seedlings.