Glutathione is involved in thiol redox signaling and acts as a major redox buffer against reactive oxygen species, helping to maintain a reducing environment in vivo. Glutathione reductase (GR) catalyzes the reduction of glutathione disulfide (GSSG) into reduced glutathione (GSH). The Arabidopsis thaliana genome encodes two GRs: GR1 and GR2. Whereas the cytosolic/ peroxisomal GR1 is not crucial for plant development, we show here that the plastid-localized GR2 is essential for root growth and root apical meristem (RAM) maintenance. We identify a GR2 mutant, miao, that displays strong inhibition of root growth and severe defects in the RAM, with GR activity being reduced to ;50%. miao accumulates high levels of GSSG and exhibits increased glutathione oxidation. The exogenous application of GSH or the thiol-reducing agent DTT can rescue the root phenotype of miao, demonstrating that the RAM defects in miao are triggered by glutathione oxidation. Our in silico analysis of public microarray data shows that auxin and glutathione redox signaling generally act independently at the transcriptional level. We propose that glutathione redox status is essential for RAM maintenance through both auxin/PLETHORA (PLT)-dependent and auxin/PLT-independent redox signaling pathways.

In the present study, the significance of sulfite oxidase (SO) for sulfite detoxification and sulfur assimilation was investigated. In response to sulfur dioxide (SO2) exposure, a remarkable expansion of sulfate and a significant increase of GSH pool were observed in wild-type and SO-overexpressing Arabidopsis. These metabolic changes were connected with a negative feedback inhibition of adenosine 5Ј-phosphosulfate reductase (APR), but no alterations in gas exchange parameters or visible symptoms of injury. However, Arabidopsis SO-KO mutants were consistently negatively affected upon 600 nL L -1 SO2 exposure for 60 h and showed phenotypical symptoms of injury with small necrotic spots on the leaves. The mean g H O 2 ( ) was reduced by about 60% over the fumigation period, accompanied by a reduction of net CO2 assimilation and SO2 uptake of about 50 and 35%. Moreover, sulfur metabolism was completely distorted. Whereas sulfate pool was kept constant, thiol-levels strongly increased. This demonstrates that SO should be the only protagonist for back-oxidizing and detoxification of sulfite. Based on these results, it is suggested that co-regulation of SO and APR controls sulfate assimilation pathway and stabilizes sulfite distribution into organic sulfur compounds. In conclusion, a sulfate-sulfite cycle driven by APR and SO can be postulated for fine-tuning of sulfur distribution that is additionally used for sulfite detoxification, when plants are exposed to atmospheric SO2.

SUMMARYElevated [CO^] has been shown to protect photosynthesis and growth of wheat against moderately eIe-\-ated [O^]. To investigate the role of ozone exclusion and detoxification in this protection, spring wheat {Triticum aestivitm L. cv. Wembley) was grown from seed, in controlled-environment chambers, under reciprocal combinations of [CO^] at 350 or 700 //mol mol"' and [Og] peaking at < 5 or 60 nmol mol"', respectively. Cumulative ozone dose to the mesophyll and antioxidant status were determined throughout flag leaf development. Catalase activity correlated with rates of photorespiration and declined in response to elevated [COJ and/or [O,J. Superoxide dismutase activity was not significantly affected by either condition. Neither ascorbate nor glutathione content was enhanced hy elevated [COJ. In wheat, at moderately elevated [Oj], our results show that stomatal exclusion plays a major role in the protective effect of elevated [CO^] against O3 damage.

To study whether responses of antioxidative enzymes to enhanced atmospheric CO2 concentrations are affected by plant nutrition, the activities of superoxide dismutase, catalase and peroxidase were investigated in leaves of 3-year-old beech trees grown with low (O-l x optimum), intermediate (0-5 x optimum) and high (2 x optimum) nutrient supply rates in open-top chambers at either ambient (=355;t;niol mol"') or elevated (700;UnioI moP') CO2 concentrations. These treatments resulted in foliar C/N ratios of about 20 in the presenee of bigli and > 30 in the presence of low nutrient supply rates. Pigment and malondialdehyde contents were determined to assess plant stress levels. Low nutrient supply rates caused pigment loss, whereas elevated CO2 bad no effect on pigmentation. Guaiacol peroxidase activities did not respond to eitber CO2 or nutrient treatment. Catalase activity decreased witb decreasing nutrient supply rate and also in response to elevated CO2. Superoxidase dismutase activity was affected by botb nutrient supply and CO2 concentration. In leaves lrom trees grown with the high-nutrient treatment, superoxide dismutase activity was low irrespective of CO2 concentration. In chlorotic leaves, superoxide dismutase activity was increased, suggesting an enhanced need for detoxification of reactive oxygen species. Leaves from plants grown under elevated CO2 witb medium nutrient supply rates showed decreased malondialdehyde contents and superoxide dismutase activities. Tbis suggests tbat tbe intrinsic oxidative stress of leaves was decreased under tbese conditions. Tbese results imply tbat intrinsic oxidative stress is modulated by tbe balance between N and C assimilation.

The Venus flytrap Dionaea muscipula captures insects and consumes their flesh. Prey contacting touch-sensitive hairs trigger traveling electrical waves. These action potentials (APs) cause rapid closure of the trap and activate secretory functions of glands, which cover its inner surface. Such prey-induced haptoelectric stimulation activates the touch hormone jasmonate (JA) signaling pathway, which initiates secretion of an acidic hydrolase mixture to decompose the victim and acquire the animal nutrients. Although postulated since Darwin's pioneering studies, these secretory events have not been recorded so far. Using advanced analytical and imaging techniques, such as vibrating ion-selective electrodes, carbon fiber amperometry, and magnetic resonance imaging, we monitored stimuluscoupled glandular secretion into the flytrap. Trigger-hair bending or direct application of JA caused a quantal release of oxidizable material from gland cells monitored as distinct amperometric spikes. Spikes reminiscent of exocytotic events in secretory animal cells progressively increased in frequency, reaching steady state 1 d after stimulation. Our data indicate that trigger-hair mechanical stimulation evokes APs. Gland cells translate APs into touch-inducible JA signaling that promotes the formation of secretory vesicles. Early vesicles loaded with H + and Cl − fuse with the plasma membrane, hyperacidifying the "green stomach"-like digestive organ, whereas subsequent ones carry hydrolases and nutrient transporters, together with a glutathione redox moiety, which is likely to act as the major detected compound in amperometry. Hence, when glands perceive the haptoelectrical stimulation, secretory vesicles are tailored to be released in a sequence that optimizes digestion of the captured animal.amperometry | exocytosis | Dionaea muscipula | secretion | plant digestion