Global climate change and associated adverse abiotic stress conditions, such as drought, salinity, heavy metals, waterlogging, extreme temperatures, oxygen deprivation, etc., greatly influence plant growth and development, ultimately affecting crop yield and quality, as well as agricultural sustainability in general. Plant cells produce oxygen radicals and their derivatives, so-called reactive oxygen species (ROS), during various processes associated with abiotic stress. Moreover, the generation of ROS is a fundamental process in higher plants and employs to transmit cellular signaling information in response to the changing environmental conditions. One of the most crucial consequences of abiotic stress is the disturbance of the equilibrium between the generation of ROS and antioxidant defense systems triggering the excessive accumulation of ROS and inducing oxidative stress in plants. Notably, the equilibrium between the detoxification and generation of ROS is maintained by both enzymatic and nonenzymatic antioxidant defense systems under harsh environmental stresses. Although this field of research has attracted massive interest, it largely remains unexplored, and our understanding of ROS signaling remains poorly understood. In this review, we have documented the recent advancement illustrating the harmful effects of ROS, antioxidant defense system involved in ROS detoxification under different abiotic stresses, and molecular cross-talk with other important signal molecules such as reactive nitrogen, sulfur, and carbonyl species. In addition, state-of-the-art molecular approaches of ROS-mediated improvement in plant antioxidant defense during the acclimation process against abiotic stresses have also been discussed.

Quercetin (Qu) is a strong antioxidant among the phenolic compounds having physiological and biochemical roles in plants. Hence, we have studied the Qu evolved protection against salinity in tomato (Solanum lycopersicum L.). Salinity caused ionic toxicity by increasing Na⁺ content in seedlings along with nutritional starvation of K⁺, Ca²⁺, and Mg²⁺. While osmotic stress was detected by higher free proline (Pro) content and lower leaf relative water content (LRWC) in salt-stressed seedlings. Salt toxicity also induced higher H₂O₂ generation, malondialdehyde (MDA) content and lipoxygenase (LOX) activity as a sign of oxidative stress. Tomato seedlings suffered from methylglyoxal (MG) toxicity, degradation of chlorophyll, along with lower biomass accumulation and growth due to salt exposure. However, Qu application under salinity resulted in lower Na⁺/K⁺ due to reduced Na⁺ content, higher LRWC, increased Pro, and reduction of H₂O₂ and MDA content, and LOX activity, which indicated alleviation of ionic, osmotic, and oxidative stress respectively. Quercetin caused oxidative stress, lessening through the strengthening of both enzymatic and non-enzymatic antioxidants. In addition, Qu increased glutathione S-transferase activity in salt-invaded seedlings, which might be stimulated reactive oxygen species (ROS) scavenging along with higher GSH content. As a result, toxic MG was detoxified in Qu supplemented salt-stressed seedlings by increasing both Gly I and Gly II activities. Moreover, Qu insisted on better plant growth and photosynthetic pigments synthesis in saline or without saline media. Therefore, exogenous applied Qu may become an important actor to minimize salt-induced toxicity in crops.

Sulfur (S) is a macronutrient for the plant, which has an immense role in basic plant processes and regulation of several metabolic pathways. It has also a major role in providing protection against adverse conditions. Sulfur-containing amino acids and metabolites maintain plant cell mechanisms to improve stress tolerance. It interacts with several biomolecules such as phytohormones, polyamines, nitric oxide (NO), and even with other plant nutrients, which can produce some derivatives those are essential for abiotic stress tolerance. Different S derivatives stimulate signaling cascades, for the upregulation of different cellular messengers such as abscisic acid, Ca²⁺, and NO. Sulfur is also known to interact with some essential plant nutrients by influencing their uptake and transport, hence, confers nutrient homeostasis efficiencies. This review focuses on how S is interacted with several signaling molecules like NO, glutathiones, phytohormones, hydrogen sulfide, polyamines, etc. This is a concise summary aimed at guiding the researchers to study S-related plant processes in the light of abiotic stress tolerance.