Abstract
In nitrogen poor soils legumes establish a symbiotic interaction with rhizobia that results in the formation of root nodules. These are unique plant organs where bacteria differentiate into bacteroids, which express the nitrogenase enzyme complex that reduces atmospheric N 2 to ammonia. Nodule metabolism requires a tight control of the concentrations of reactive oxygen and nitrogen species (RONS) so that they can perform useful signaling roles while avoiding nitro-oxidative damage. In nodules a thiol-dependent regulatory network that senses, transmits and responds to redox changes is starting to be elucidated. A combination of enzymatic, immunological, pharmacological and molecular analyses has allowed us to conclude that glutathione and its legume-specific homolog, homoglutathione, are abundant in meristematic and infected cells, that their spatio-temporally distribution is correlated with the corresponding (homo)glutathione synthetase activities, and that they are crucial for nodule development and function. Glutathione is at high concentrations in the bacteroids and at moderate amounts in the mitochondria, cytosol and nuclei. Less information is available on other components of the network. The expression of multiple isoforms of glutathione peroxidases, peroxiredoxins, thioredoxins, glutaredoxins and NADPH-thioredoxin reductases has been detected in nodule cells using antibodies and proteomics. Peroxiredoxins and thioredoxins are essential to regulate and in some cases to detoxify RONS in nodules. Further research is necessary to clarify the regulation of the expression and activity of thiol redox-active proteins in response to abiotic, biotic and developmental cues, their interactions with downstream targets by disulfide-exchange reactions, and their participation in signaling cascades. The availability of mutants and transgenic lines will be crucial to facilitate systematic investigations into the function of the various proteins in the legume-rhizobial symbiosis.
Highlights
Reviewed by: Sona Pandey, Donald Danforth Plant Science Center, USA Vasileios Fotopoulos, Cyprus University of Technology, Cyprus
The mutation in the gshA gene of Bradyrhizobium sp. 6144-S7Z appears to affect the ability of the bacterium to compete during peanut (Arachis hypogaea) nodulation, but not its capacity to form effective nodules (Sobrevals et al, 2006). These results show that the bacterial GSH pool plays a critical role in the rhizobia-plant interaction and that different cellular processes are regulated by, or are dependent on, GSH in free-living rhizobia and in N2-fixing bacteroids
Notable accomplishments include the findings that the pathway is compartimentalized in plant cells, with export of γ-glutamylcysteine from the plastids to the cytosol; the crystallization and subsequent structure elucidation of γECS, GSH synthetase (GSHS) and hGSH synthetase (hGSHS); the role of thiol compounds and associated enzymes in redox signaling and in controlling the cell cycle; the transcriptional regulation of γECS, GSHS and hGSHS by reactive oxygen and nitrogen species (RONS) and hormones; and the post-translational regulation of γECS by a redox switch involving conserved cysteine residues
Summary
Reviewed by: Sona Pandey, Donald Danforth Plant Science Center, USA Vasileios Fotopoulos, Cyprus University of Technology, Cyprus. ROLES OF THIOLS IN NODULE FORMATION AND FUNCTIONING A recent electron microscopy study of pea (Pisum sativum) nodules with a GSH-specific antibody revealed that this thiol is present in the bacteroids, mitochondria, cytosol and nuclei of infected cells (Matamoros et al, 2013).
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