Spermidine mitigates wheat copper toxicity by modulating ascorbate and glutathione metabolism, copper accumulation and photosynthetic performance

The influence of spermidine (Spd) on wheat ascorbate and glutathione metabolism, copper (Cu) accumulation and photosynthetic performance under Cu stress was studied. The findings displayed that Cu stress boosted reduced ascorbate (AsA) and reduced glutathione (GSH) contents by improving ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), L-galactono-1,4-lactone dehydrogenase (GalLDH) and gamma-glutamylcysteine synthetase (γ-ECS) activities. Nevertheless, Cu stress promoted malondialdehyde (MDA) accumulation and electrolyte leakage (EL) level, and lowered AsA/dehydroascorbic acid (DHA) and GSH/oxidised glutathione (GSSG). Meanwhile, Cu stress promoted Cu accumulation in plant tissues. It declined net photosynthetic rate (Pn), chlorophyll fluorescence parameter maximum photochemical efficiency of PSII (Fv/Fm), chlorophyll (Chl) and carotenoids (Car) contents, and wheat height and biomass. In this way, Cu stresses limited wheat growth. Compared with Cu stress, Spd plus Cu stress enhanced APX, GR, DHAR, MDHAR, GalLDH and γ-ECS activities to 4.75, 5.14, 3.77, 2.96, 3.24 and 2.83 U/g FW (fresh weight), respectively. This way, Spd further increased AsA and GSH contents to 4.62 and 0.78 µmol/g FW under Cu stress. Meanwhile, Spd increased AsA/DHA to 14.60 and GSH/GSSG to 15.97 and declined MDA content to 11.68 nmol/g FW and EL to 17.00% under Cu stress. Besides, Spd declined Cu content in leaves to 68.8 µg/g DW and roots to 152.9 µg/g DW and respectively increased Pn, Fv/Fm and Chl and Car contents to 15.22 µmol/m2/s, 0.74, 1.55 mg/g FW and 0.38 mg/g FW. In this way, Spd promoted wheat growth under Cu stress. Meanwhile, we found that Spd alone also improved the ascorbate and glutathione metabolism, photosynthetic performance, and wheat growth compared to the control. These results illustrated that Spd mitigated wheat Cu toxicity by reducing Cu accumulation and improving ascorbate and glutathione metabolism and photosynthetic performance. Hence, using Spd will be a good strategy to improve the Cu tolerance of wheat crops in the future.