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.

Plants co-ordinate information derived from many diverse external and internal signals to ensure appropriate control of gene expression under optimal and stress conditions. In this work, the relationships between catalase (CAT) and H2O2 during drought in wheat (Triticum aestivum L.) are studied. Drought-induced H2O2 accumulation correlated with decreases in soil water content and CO2 assimilation. Leaf H2O2 content increased even though total CAT activity doubled under severe drought conditions. Diurnal regulation of CAT1 and CAT2 mRNA abundance was apparent in all conditions and day/night CAT1 and CAT2 expression patterns were modified by mild and severe drought. The abundance of CAT1 transcripts was regulated by circadian controls that persisted in continuous darkness, while CAT2 was modulated by light. Drought decreased abundance, and modified the pattern, of CAT1 and CAT2 mRNAs. It was concluded that the complex regulation of CAT mRNA, particularly at the level of translation, allows precise control of leaf H2O2 accumulation.

A Sebacinales species was recovered from a clone library made from a pooled rhizosphere sample of Nicotiana attenuata plants from 14 native populations. Axenic cultures of the related species, Piriformospora indica and Sebacina vermifera, were used to examine their effects on plant performance. Inoculation of N. attenuata seeds with either fungus species stimulated seed germination and increased growth and stalk elongation. S. vermifera inoculated plants flowered earlier, produced more flowers and matured more seed capsules than did non-inoculated plants. Jasmonate treatment during rosette-stage growth, which slows growth and elicits herbivore resistance traits, erased differences in vegetative, but not reproductive performance resulting from S. vermifera inoculation. Total nitrogen and phosphorous contents did not differ between inoculated and control plants, suggesting that the performance benefits of fungal inoculation did not result from improvements in nutritional status. Since the expression of trypsin proteinase inhibitors (TPI), defensive proteins which confer resistance to attack from Manduca sexta larvae, incur significant growth and fitness costs for the plant, we examined the effect of S. vermifera inoculation on herbivore resistance and TPI activity. After 10 days of feeding on S. vermifera-inoculated plants, larval mass was 46% higher and TPI activity was 48% lower than that on non-inoculated plants. These results suggest that Sebacina spp. may interfere with defense signaling and allow plants to increase growth rates at the expense of herbivore resistance mediated by TPIs.