Legume nodule senescence: roles for redox and hormone signalling in the orchestration of the natural aging process

Abstract

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.

Aging in Legume Symbiosis. A Molecular View on Nodule Senescence in Medicago truncatula

“…This induced nodule senescence develops much more rapidly than developmental senescence and present features of oxidative stress and plant cell death (Puppo et al, 2005).…”

“…Developmental nodule senescence has been proposed to result from a decrease in the ascorbategluthatione antioxidant pool coupled to decreasing carbon-to-nitrogen ratios inside the nodule (Puppo et al, 2005). These changes would be sensed and transduced via an abscisic acid-mediated signaling pathway mobilizing proteolytic activities.…”

Sinorhizobium meliloti lsrB is involved in alfalfa root nodule development and nitrogen-fixing bacteroid differentiation

“…Since reactive oxygen species (ROS) are involved in the regulation of leguminous nodule senescence [5], the accumulation of ROS was examined in three-week old alfalfa root nodules. DAB staining of H 2 O 2 indicated that much increased levels of H 2 O 2 were generated in the majority of alfalfa root nodules induced by the lsrB1-2 mutant ( Figure 4(b)-(c)), but only slight H 2 O 2 levels were detectable in alfalfa root nodules induced by the wild type strain, S. meliloti 1021 (Figure 4(a)).…”

“…In the host plant, ascorbate and glutathione play regulatory roles in delaying nodule senescence, whereas ethylene and abscisic acid promote premature nodule senescence [5]. A large number of host genes are upregulated in aging nodules, as indicated by transcriptomic data.…”

“…Infected cells decay and collapse finally. Nitrogenase activity is then significantly decreased or completely abolished [5].…”