Glutathione has numerous roles in cellular defence and in sulphur metabolism. These functions depend or impact on the concentration and/or redox state of leaf glutathione pools. Effective function requires homeostatic control of concentration and redox state, with departures from homeostasis acting as signals that trigger adaptive responses. Intercellular and intracellular glutathione pools are linked by transport across membranes. It is shown that glutathione can cross the chloroplast envelope at rates similar to the speed of biosynthesis. Control of glutathione concentration and redox state is therefore due to a complex interplay between biosynthesis, utilization, degradation, oxidation/reduction, and transport. All these factors must be considered in order to evaluate the significance of glutathione as a signalling component during development, abiotic stress, or pathogen attack.

SummaryPhotorespiration is a light-dependent source of H 2 O 2 in the peroxisomes, where concentrations of this signalling molecule are regulated by catalase. Growth of Arabidopsis knock-out mutants for CATALASE2 (cat2) in ambient air caused severely decreased rosette biomass, intracellular redox perturbation and activation of oxidative signalling pathways. These effects were absent when cat2 was grown at high CO 2 levels to inhibit photorespiration, but were re-established following a subsequent transfer to air. Growth of cat2 in air at different daylengths revealed that photoperiod is a critical determinant of the oxidative stress response. Decreased growth was observed in 8-h, 12-h and 16-h photoperiods, but lesion development was dependent on long days. Experiments at different light fluence rates showed that cell death in cat2 was linked to long days and not to total light exposure or the severity of oxidative stress. Perturbed intracellular redox state and oxidative signalling pathway induction were more prominent in short days than in long days, as evidenced by glutathione status and induction of defence genes and oxidative stress-responsive transcripts. Similar daylength-dependent effects were observed in the response of mature plants transferred from short days in high CO 2 conditions to ambient air conditions. Prior growth of plants with short days in air alleviated the cat2 cell-death phenotype in long days. Together, the data reveal the influence of photoperiodic events on redox signalling, and define distinct photoperiod-dependent strategies in the acclimation versus cell-death decision in stress conditions.

Contents  Summary 1 Introduction 2 The characteristics of legume nodule senescence 3 Redox homeostasis and programmed cell death 6 Antioxidants and programmed cell death during the senescence process  10 Perspectives for manipulating nodule senescence 13 Conclusions and perspectives 14  Acknowledgements 15  References 15 Summary Research on legume nodule development has contributed greatly to our current understanding of plant–microbe interactions. However, the factors that orchestrate root nodule senescence have received relatively little attention. Accumulating evidence suggests that redox signals contribute to the establishment of symbiosis and senescence. Although degenerative in nature, nodule senescence is an active process programmed in development in which reactive oxygen species (ROS), antioxidants, hormones and proteinases have key roles. Nodules have high levels of the redox buffers, ascorbate and glutathione, which are important in the nodulation process and in senescence. These metabolites decline with N‐fixation as the nodule ages but the resultant decrease in redox buffering capacity does not necessarily lead to enhanced ROS or oxidative stress. We propose models by which ROS and antioxidants interact with hormones such as abscisic acid in the orchestration of nodule senescence.

H 2 O 2 production and changes in glutathione, catalase, and peroxidase were followed in whole-leaf extracts from the susceptible (AlgS [Algerian/4* (F14) Man.(S)]; ml-a1 allele) and resistant (AlgR [Algerian/4* (F14) Man.(R)]; Ml-a1 allele) barley (Hordeum vulgare) isolines between 12 and 24 h after inoculation with powdery mildew (Blumeria graminis [DC]. Speer [syn. Erysiphe graminis DC] f.sp hordei Marchal). Localized papilla responses and cell death hypersensitive responses were not observed within the same cell. In hypersensitive response sites, H 2 O 2 accumulation first occurred in the mesophyll underlying the attacked epidermal cell. Subsequently, H 2 O 2 disappeared from the mesophyll and accumulated around attacked epidermal cells. In AlgR, transient glutathione oxidation coincided with H 2 O 2 accumulation in the mesophyll. Subsequently, total foliar glutathione and catalase activities transiently increased in AlgR. These changes, absent from AlgS, preceded inoculation-dependent increases in peroxidase activity that were observed in both AlgR and AlgS at 18 h. An early intercellular signal precedes H 2 O 2 , and this elicits anti-oxidant responses in leaves prior to events leading to death of attacked cells.