Regulatory Response of Rice Seedlings to Exogenously Applied Kinetin During Oxidative Stress

Hydroxyurea (HU) is a known suppressor of ribonucleotide reductase enzyme through enhanced hydrogen peroxide (H₂O₂) production, causing oxidative damage to DNA in plants. Kinetin (KI) has emerged as an important phytohormone in regulating development processes and antioxidant protection effects against environmental stresses. Therefore, this study aimed to investigate the potential and regulating mechanism of KI application on tolerance of Oryza sativa to HU-induced oxidative stress. Three-day-old rice seedlings were grown in 1/2 MS medium for seven days following different treatments: control, HU (1 mM), KI (40 nM), and HU + KI. The results showed that, compared to control, HU treatment significantly reduced the growth (e.g., dry weight and root length: 36% and 48%, respectively) and photosynthetic rate (e.g., Fv/Fm: 31%) and pigments (e.g., chlorophyll and carotenoid: 52% and 67%, respectively), by stimulating oxidative stress (e.g., H₂O₂) markers and malondialdehyde levels, causing DNA damage and G1/S (growth/synthesis) and G2/M (growth/mitotic) phase arrest on seven-day-old rice seedlings. Meanwhile, the follow-up treatment of KI to the HU stress plants enhanced the growth (14–31%) and photosynthetic (13–29%) parameters by regulating antioxidant enzyme (e.g., catalase, ascorbate peroxidase, peroxidase, and superoxide dismutase) activities as well as abscisic acid, salicylic acid, gibberellic acid, and indole-3-acetic acid hormone contents, coupled with a significant reduction in reactive oxygen species accumulation. Additionally, KI reduced the DNA damage in the plants exposed to HU stress by reducing the relative density of apurinic/apyrimidinic sites, as evidenced by both decrease and increase in transcriptional regulation of genes (e.g., ATM, ATR, PARP, RAD51A2, and RAD51C) involved in DNA damage response and cell cycle progression. Our findings indicate that exogenous application of KI to plants affected by oxidative stress improves the antioxidant defense system and phytohormone homeostasis as well as DNA damage response alleviating G1/S and G2/M arrest, contributing to enhancement of the rice seedling performance.