Modulation of zinc-induced oxidative damage in Solanum melongena by 6-benzylaminopurine involves ascorbate–glutathione cycle metabolism

• Effect of 6-benzylaminopurine (BAP) on AsA–GSH cycle was studied in eggplants under Zn stress.
• BAP alleviates Zn-induced oxidative damage by regulating the redox state of AsA and GSH.
• BAP regulates related enzyme activities and gene expressions involved in AsA–GSH cycle.
• BAP shifted the hormonal balance (CK, IAA, and ABA) in the leaves with or without Zn treatment.
• A hypothesis is that BAP-regulated AsA–GSH cycle is mediated by CK signal pathway via IAA and ABA.

Cytokinins (CKs) play an important role in plant adaptation to environmental stresses. Ascorbate–glutathione (AsA–GSH) cycle is regarded as an essential mechanism for resistance of plants under stress conditions. In this study, the effects of 6-benzylaminopurine (BAP), a synthetic CK, on the growth, oxidative damage, AsA–GSH cycle metabolism, and hormone contents in the leaves of eggplant (Solanum melongena L.) seedlings were investigated under 10 mM zinc (Zn) stress. Zn stress resulted in oxidative damage as evidenced by a decrease in eggplant growth and an increase in reactive oxygen species (ROS, H2O2, and O2−) formation and membrane damage (lipid peroxidation and electrolyte leakage). However, application of exogenous BAP significantly alleviated the growth inhibition and oxidative damage caused by Zn stress. Importantly, BAP further enhanced the redox states of AsA and GSH, and the related enzyme activities involved in the AsA–GSH cycle such as ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), and glutathione synthetase (GS) in eggplants grown under Zn stress. Quantitative analysis revealed that BAP could induce the transcript levels of MDHAR and GS, but not DHAR in AsA–GSH cycle under Zn stress or non-stress conditions. GR expression was further increased by BAP application in the leaves of eggplants under Zn stress. Moreover, BAP shifted the hormonal balance toward a substantial increase in the levels of CK and auxin (IAA) but a decrease in abscisic acid (ABA) content in the leaves with or without Zn treatment. The results of the present study strongly suggest that BAP-enhanced resistance of eggplants against Zn toxicity is associated with the regulation of AsA–GSH cycle metabolism as an effective antioxidant system and the balance of hormones.