Mechanisms of reactive oxygen species in plants under drought stress

Stress is one of the main issues in agricultural production. Drought stress has a significant impact on plant growth and development, reduces crop yield, and can result in environmental degradation. Plants have evolved a series of adaptations to natural environmental stress at morphological, physiological, biochemical, cellular, and molecular levels, such as stress escape, stress tolerance, and stress resistance. Therefore, it is very important to understand the effects of drought stress on plants, and plant sensing, transduction, and response to drought signals, for improving crop yields. ROS, as an essential metabolite of aerobic metabolism in all aerobic organisms on earth, mainly exists in the form of free radicals and non-free radicals, including hydrogen peroxide (H2O2), superoxide anion radicals (O2−), hydroxyl radicals (·OH), singlet oxygen (1O2), organic oxygen radicals (RO· and ROO·). In plants, ROS is widely involved in plant growth, development, differentiation, metabolic transport of substances, defense response, programmed cell death and biological and abiotic stress responses as a byproduct of aerobic energy metabolism and plant exposure to various biological and abiotic stresses. Under normal environmental conditions, ROS produced can be maintained at a relatively stable level through the antioxidant enzyme system in vivo in plant cells. When plants are stimulated by drought stress, this balance is destroyed. Plant cells may be oxidative stress due to the consumption of antioxidant enzymes or excessive accumulation of ROS. Reactive oxygen species (ROS) play an important dual regulatory role in plant growth and development processes as intracellular by-products of aerobic metabolism. Under normal conditions, the production and elimination of ROS maintain dynamic homeostasis in plants. However, this situation can be broken when plants are subjected to drought stresses, leading to ROS production and metabolism disorders. ROS-mediated oxidative stress can cause many adverse cytological effects such as plasma membrane peroxidation, nucleus impairment, photosynthesis retardation, and abnormal respiratory. On the other hand, ROS, as an important signal molecule, interacts with other signal molecules, such as CaM, G protein, MAKP, miRNA, and NO to form a large and complex signal network, which plays a very important regulatory role in plant growth and development, physiological and biochemical reactions, programmed cell death, hormone metabolism, and biotic and abiotic stress responses. In addition, plant cells have evolved a variety of antioxidant mechanisms (enzymatic and non-enzymatic systems) to eliminate toxicity in order to prevent oxidative damage to cells arising from over accumulation of ROS. Enzymatic scavenging system mainly includes superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione peroxidase (GPX), guaiacol peroxidase (GOPX), etc. Non-enzymatic antioxidants are mainly some organic compounds with relatively small molecular weight, such as proline, ascorbic acid (ASA), glutathione (GSH), tocopherols, flavonoids, phenolic compounds and non-protein amino acids and so on. In this paper, the effects of drought stress on plant growth and development, morphological structure, and physiological and biochemical characteristics, and the relationships between plant responses to drought stress are summarized. The possible signaling network between ROS as a second messenger and other signaling molecules is also discussed. A theoretical basis is provided to further understand the mechanisms of ROS and improve plant stress resistance.