Abiotic stresses such as drought, cold, salt and heat cause reduction of plant growth and loss of crop yield worldwide. Reactive oxygen species (ROS) including hydrogen peroxide (H2O2), superoxide anions (O2•‾), hydroxyl radical (OH•) and singlet oxygen (1O2) are by-products of physiological metabolisms, and are precisely controlled by enzymatic and non-enzymatic antioxidant defense systems. ROS are significantly accumulated under abiotic stress conditions, which cause oxidative damage and eventually resulting in cell death. Recently, ROS have been also recognized as key players in the complex signaling network of plants stress responses. The involvement of ROS in signal transduction implies that there must be coordinated function of regulation networks to maintain ROS at non-toxic levels in a delicate balancing act between ROS production, involving ROS generating enzymes and the unavoidable production of ROS during basic cellular metabolism, and ROS-scavenging pathways. Increasing evidence showed that ROS play crucial roles in abiotic stress responses of crop plants for the activation of stress-response and defense pathways. More importantly, manipulating ROS levels provides an opportunity to enhance stress tolerances of crop plants under a variety of unfavorable environmental conditions. This review presents an overview of current knowledge about homeostasis regulation of ROS in crop plants. In particular, we summarize the essential proteins that are involved in abiotic stress tolerance of crop plants through ROS regulation. Finally, the challenges toward the improvement of abiotic stress tolerance through ROS regulation in crops are discussed.

Drought stress is an important environmental factor limiting plant productivity. In this study, we screened drought-resistant transgenic plants from 65 promoter-pyrabactin resistance 1-like (PYL) abscisic acid (ABA) receptor gene combinations and discovered that pRD29A::PYL9 transgenic lines showed dramatically increased drought resistance and drought-induced leaf senescence in both Arabidopsis and rice. Previous studies suggested that ABA promotes senescence by causing ethylene production. However, we found that ABA promotes leaf senescence in an ethylene-independent manner by activating sucrose nonfermenting 1-related protein kinase 2s (SnRK2s), which subsequently phosphorylate ABA-responsive element-binding factors (ABFs) and Related to ABA-Insensitive 3/VP1 (RAV1) transcription factors. The phosphorylated ABFs and RAV1 up-regulate the expression of senescence-associated genes, partly by up-regulating the expression of Oresara 1. The pyl9 and ABA-insensitive 1-1 single mutants, pyl8-1pyl9 double mutant, and snrk2.2/3/6 triple mutant showed reduced ABA-induced leaf senescence relative to the WT, whereas pRD29A::PYL9 transgenic plants showed enhanced ABA-induced leaf senescence. We found that leaf senescence may benefit drought resistance by helping to generate an osmotic potential gradient, which is increased in pRD29A::PYL9 transgenic plants and causes water to preferentially flow to developing tissues. Our results uncover the molecular mechanism of ABA-induced leaf senescence and suggest an important role of PYL9 and leaf senescence in promoting resistance to extreme drought stress. Significance We identified transgenic plants that are extremely resistant to drought from a large-scale screening of transgenic plants overexpressing the pyrabactin resistance 1-like (PYL) family of abscisic acid (ABA) receptors. We explored how these plants resist drought by examining both short-term responses, such as stomatal closure, and long-term responses, such as senescence. The physiological roles of ABA-induced senescence under stress conditions and the underlying molecular mechanism are unclear. Here, we found that ABA induces senescence by activating ABA-responsive element-binding factors and Related to ABA-Insensitive 3/VP1 transcription factors through core ABA signaling. Our results suggest that PYL9 promotes drought resistance by not only limiting transpirational water loss but also, causing summer dormancy-like responses, such as senescence, in old leaves and growth inhibition in young tissues under severe drought conditions.

Sesame is an important oil crop due to its high oil, antioxidant, and protein content. Drought stress is a major abiotic stress that affects sesame production as well as the quality of sesame seed. To reveal the adaptive mechanism of sesame in response to water deficient conditions, transcriptomic and metabolomics were applied in drought-tolerant (DT) and drought-susceptible (DS) sesame genotypes. Transcriptomic analysis reveals a set of core drought-responsive genes (684 up-regulated and 1346 down-regulated) in sesame that was robustly differently expressed in both genotypes. Most enriched drought-responsive genes are mainly involved in protein processing in endoplasmic reticulum, plant hormone signal transduction photosynthesis, lipid metabolism, and amino acid metabolism. Drought-susceptible genotype was more disturbed by drought stress at both transcriptional and metabolic levels, since more drought-responsive genes/metabolites were identified in DS. Drought-responsive genes associated with stress response, amino acid metabolism, and reactive oxygen species scavenging were more enriched or activated in DT. According to the partial least-squares discriminate analysis, the most important metabolites which were accumulated under drought stress in both genotypes includes ABA, amino acids, and organic acids. Especially, higher levels of ABA, proline, arginine, lysine, aromatic and branched chain amino acids, GABA, saccharopine, 2-aminoadipate, and allantoin were found in DT under stress condition. Combination of transcriptomic and metabolomic analysis highlights the important role of amino acid metabolism (especially saccharopine pathway) and ABA metabolism and signaling pathway for drought tolerance in sesame. The results of the present study provide valuable information for better understanding the molecular mechanism underlying drought tolerance of sesame, and also provide useful clues for the genetic improvement of drought tolerance in sesame.

Abiotic stresses such as drought cause a reduction of plant growth and loss of crop yield. Stomatal aperture controls CO₂ uptake and water loss to the atmosphere, thus playing important roles in both the yield gain and drought tolerance of crops. Here, a rice homologue of SRO (similar to RCD one), termed OsSRO1c, was identified as a direct target gene of SNAC1 (stress-responsive NAC 1) involved in the regulation of stomatal aperture and oxidative response. SNAC1 could bind to the promoter of OsSRO1c and activate the expression of OsSRO1c. OsSRO1c was induced in guard cells by drought stress. The loss-of-function mutant of OsSRO1c showed increased stomatal aperture and sensitivity to drought, and faster water loss compared with the wild-type plant, whereas OsSRO1c overexpression led to decreased stomatal aperture and reduced water loss. Interestingly, OsSRO1c-overexpressing rice showed increased sensitivity to oxidative stress. Expression of DST, a reported zinc finger gene negatively regulating H₂O₂-induced stomatal closure, and the activity of H₂O₂-scavenging related enzymes were significantly suppressed, and H₂O₂ in guard cells was accumulated in the overexpression lines. OsSRO1c interacted with various stress-related regulatory and functional proteins, and some of the OsSRO1c-interacting proteins are predicted to be involved in the control of stomatal aperture and oxidative stress tolerance. The results suggest that OsSRO1c has dual roles in drought and oxidative stress tolerance of rice by promoting stomatal closure and H₂O₂ accumulation through a novel pathway involving regulators SNAC1 and DST.