Overexpressing <Emphasis Type="Italic">SgNCED1</Emphasis> in Tobacco Increases ABA Level,Antioxidant Enzyme Activities,and Stress Tolerance

Abscisic acid (ABA) regulates plant adaptive responses to various environmental stresses. 9-cis-epoxycarotenoid dioxygenase (NCED) is the key enzyme of ABA biosynthesis in higher plants. A NCED gene, SgNCED1, was overexpressed in transgenic tobacco plants which resulted in 51–77% more accumulation of ABA in leaves. Transgenic tobacco plants decreased stomatal conductance, transpiration rate, and photosynthetic rate but induced activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate-peroxidase (APX). Hydrogen peroxide (H₂O₂) and nitric oxide (NO) in leaves were also induced in the transgenic plants. Compared to the wild-type control, the transgenic plants improved growth under 0.1 M mannitol-induced drought stress and 0.1 M NaCl-induced salinity stress. It is suggested that the ABA-induced H₂O₂ and NO generation upregulates the stomatal closure and antioxidant enzymes, and therefore increases drought and salinity tolerance in the transgenic plants.     

PeCHYR1, a ubiquitin E3 ligase from Populus euphratica,enhances drought tolerance via ABA‐induced stomatal closure by ROS production in Populus

Drought, a primary abiotic stress, seriously affects plant growth and productivity. Stomata play a vital role in regulating gas exchange and drought adaptation. However, limited knowledge exists of the molecular mechanisms underlying stomatal movement in trees. Here, PeCHYR1, a ubiquitin E3 ligase, was isolated from Populus euphratica, a model of stress adaptation in forest trees. PeCHYR1 was preferentially expressed in young leaves and was significantly induced by ABA (abscisic acid) and dehydration treatments. To study the potential biological functions of PeCHYR1, transgenic poplar 84K (Populus alba × Populus glandulosa) plants overexpressing PeCHYR1 were generated. PeCHYR1 overexpression significantly enhanced H₂O₂ production and reduced stomatal aperture. Transgenic lines exhibited increased sensitivity to exogenous ABA and greater drought tolerance than that of WT (wild‐type) controls. Moreover, up‐regulation of PeCHYR1 promoted stomatal closure and decreased transpiration, resulting in strongly elevated WUE (water use efficiency). When exposed to drought stress, transgenic poplar maintained higher photosynthetic activity and biomass accumulation. Taken together, these results suggest that PeCHYR1 plays a crucial role in enhancing drought tolerance via ABA‐induced stomatal closure caused by hydrogen peroxide (H₂O₂) production in transgenic poplar plants.     

Hydrogen peroxide pretreatment induces osmotic stress tolerance by influencing osmolyte and abscisic acid levels in maize leaves

Hydrogen peroxide (H₂O₂) functions as a signal molecule in plants under abiotic and biotic stresses. Leaves of detached maize (Zea mays L.) seedlings were used to study the function of H₂O₂ pretreatment in osmotic stress resistance. Low H₂O₂ concentration (10 mM) which did not cause a visual symptom of water deficit (leaf rolling) was applied to the seedlings. Exogenous H₂O₂ alone increased leaf water potential, endogenous H₂O₂ content, abscisic acid (ABA) concentration, and metabolite levels including soluble sugars, proline, and polyamines while it decreased lipid peroxidation and stomatal conductance. Osmotic stress induced by polyethylene glycol (PEG 6000) decreased leaf water potential and stomatal conductance but enhanced lipid peroxidation, endogenous H₂O₂ content, the metabolite levels, and ABA content. H₂O₂ pretreatment also induced the metabolite accumulation and improved water status, stomatal conductance, lipid peroxidation, ABA, and H₂O₂ levels under osmotic stress. These results indicated that H₂O₂ pretreatment may alleviate water loss and induce osmotic stress resistance by increasing the levels of soluble sugars, proline, and polyamines thus ABA and H₂O₂ production slightly decrease in maize seedlings under osmotic stress.     

Orchestration of hydrogen peroxide and nitric oxide in brassinosteroid‐mediated systemic virus resistance in Nicotiana benthamiana

Brassinosteroids (BRs) play essential roles in modulating plant growth, development and stress responses. Here, involvement of BRs in plant systemic resistance to virus was studied. Treatment of local leaves in Nicotiana benthamiana with BRs induced virus resistance in upper untreated leaves, accompanied by accumulations of H₂O₂ and NO. Scavenging of H₂O₂ or NO in upper leaves blocked BR‐induced systemic virus resistance. BR‐induced systemic H₂O₂ accumulation was blocked by local pharmacological inhibition of NADPH oxidase or silencing of respiratory burst oxidase homolog gene NbRBOHB, but not by systemic NADPH oxidase inhibition or NbRBOHA silencing. Silencing of the nitrite‐dependent nitrate reductase gene NbNR or systemic pharmacological inhibition of NR compromised BR‐triggered systemic NO accumulation, while local inhibition of NR, silencing of NbNOA1 and inhibition of NOS had little effect. Moreover, we provide evidence that BR‐activated H₂O₂ is required for NO synthesis. Pharmacological scavenging or genetic inhibiting of H₂O₂ generation blocked BR‐induced systemic NO production, but BR‐induced H₂O₂ production was not sensitive to NO scavengers or silencing of NbNR. Systemically applied sodium nitroprusside rescued BR‐induced systemic virus defense in NbRBOHB‐silenced plants, but H₂O₂ did not reverse the effect of NbNR silencing on BR‐induced systemic virus resistance. Finally, we demonstrate that the receptor kinase BRI1(BR insensitive 1) is an upstream component in BR‐mediated systemic defense signaling, as silencing of NbBRI1 compromised the BR‐induced H₂O₂ and NO production associated with systemic virus resistance. Together, our pharmacological and genetic data suggest the existence of a signaling pathway leading to BR‐mediated systemic virus resistance that involves local Respiratory Burst Oxidase Homolog B (RBOHB)‐dependent H₂O₂ production and subsequent systemic NR‐dependent NO generation.     

Cross-talk between salicylic acid and NaCl-generated reactive oxygen species and nitric oxide in tomato during acclimation to high salinity

Hydrogen peroxide (H₂O₂) and nitric oxide (NO) generated by salicylic acid (SA) are considered to be functional links of cross‐tolerance to various stressors. SA‐stimulated pre‐adaptation state was beneficial in the acclimation to subsequent salt stress in tomato (Solanum lycopersicum cv. Rio Fuego). At the whole‐plant level, SA‐induced massive H₂O₂ accumulation only at high concentrations (10^?3–10^?2M), which later caused the death of plants. The excess accumulation of H₂O₂ as compared with plants exposed to 100 mM NaCl was not associated with salt stress response after SA pre‐treatments. In the root tips, 10^?3–10^?2M SA triggered the production of reactive oxygen species (ROS) and NO with a concomitant decline in the cell viability. Sublethal concentrations of SA, however, decreased the effect of salt stress on ROS and NO production in the root apex. The attenuation of oxidative stress because of high salinity occurred not only in pre‐adapted plants but also at cell level. When protoplasts prepared from control leaves were exposed to SA in the presence of 100 mM NaCl, the production of NO and ROS was much lower and the viability of the cells was higher than in salt‐treated samples. This suggests that, the cross‐talk of signalling pathways induced by SA and high salinity may occur at the level of ROS and NO production. Abscisic acid (ABA), polyamines and 1‐aminocyclopropane‐1‐carboxylic acid, the compounds accumulating in pre‐treated plants, enhanced the diphenylene iodonium‐sensitive ROS and NO levels, but, in contrast to others, ABA and putrescine preserved the viability of protoplasts.     

Abscisic acid improves drought tolerance of triploid bermudagrass and involves H2O2- and NO-induced antioxidant enzyme activities

Drought is a major limiting factor for turfgrass growth. Protection of triploid bermudagrass against drought stress by abscisic acid (ABA) and its association with hydrogen peroxide (H₂O₂) and nitric oxide (NO) were investigated. ABA treatment increased relative water content, decreased ion leakage and the percentage of dead plants significantly under drought stress. Superoxide dismutase (SOD) and catalase (CAT) activities increased in both ABA-treated and control plants, but more in ABA-treated plants, under drought stress. Malondialdehyde, an indicator of plant lipid peroxidation, was lower in ABA-treated plants than in control plants, indicating that ABA alleviated drought-induced oxidative injury. ABA treatment increased H₂O₂ and NO contents. ABA-induced SOD and CAT activities could be blocked by scavengers of H₂O₂ and NO, and inhibitors of H₂O₂ and NO generation. The results indicated that H₂O₂ and NO were essential for ABA-induced SOD and CAT activities. Both H₂O₂ and NO could induce SOD and CAT activities individually. SOD and CAT induced by H₂O₂ could be blocked by scavenger of NO and inhibitors of NO generation, while SOD and CAT induced by NO could not be blocked by scavenger of H₂O₂ and inhibitor of H₂O₂. The results revealed that ABA-induced SOD and CAT activities were mediated sequentially by H₂O₂ and NO, and NO acted downstream of H₂O₂.     

WD40‐REPEAT 5a functions in drought stress tolerance by regulating nitric oxide accumulation in Arabidopsis

Nitric oxide (NO) generation by NO synthase (NOS) in guard cells plays a vital role in stomatal closure for adaptive plant response to drought stress. However, the mechanism underlying the regulation of NOS activity in plants is unclear. Here, by screening yeast deletion mutants with decreased NO accumulation and NOS‐like activity when subjected to H₂O₂ stress, we identified TUP1 as a novel regulator of NOS‐like activity in yeast. Arabidopsis WD40‐REPEAT 5a (WDR5a), a homolog of yeast TUP1, complemented H₂O₂‐induced NO accumulation of a yeast mutant Δtup1, suggesting the conserved role of WDR5a in regulating NO accumulation and NOS‐like activity. This note was further confirmed by using an Arabidopsis RNAi line wdr5a‐1 and two T‐DNA insertion mutants of WDR5a with reduced WDR5a expression, in which both H₂O₂‐induced NO accumulation and stomatal closure were repressed. This was because H₂O₂‐induced NOS‐like activity was inhibited in the mutants compared with that of the wild type. Furthermore, these wdr5a mutants were more sensitive to drought stress as they had reduced stomatal closure and decreased expression of drought‐related genes. Together, our results revealed that WDR5a functions as a novel factor to modulate NOS‐like activity for changes of NO accumulation and stomatal closure in drought stress tolerance.     

Hydrogen peroxide is involved in nitric oxide‐induced cell death in maize leaves

Recent evidence indicates that nitric oxide (NO) plays an important role in plant hypersensitive cell death. Here, we report that NO treatment led to rapid cell death and induced hydrogen peroxide (H₂O₂) accumulation in maize leaves. We also show that NO induced the expression of Zmrboh genes. Pharmacological study suggests that NO‐induced cell death is in part mediated via H₂O₂. In addition, semi‐quantitative RT‐PCR revealed that NO induced expression of the systemic acquired resistance (SAR) genes, ZmPR1 and ZmPR5.