title: Early signaling, synthesis, transport and metabolism of ureides.
abstract: The symbiosis between α nitrogen (N2)-fixing Proteobacteria (family Rhizobiaceae) and legumes belonging to the Fabaceae (a single phylogenetic group comprising three subfamilies: Caesalpinioideae, Mimosoideae and Papilionoideae) results in the formation of a novel root structure called a nodule, where atmospheric N2 is fixed into NH3(+). In the determinate type of nodules harbored by Rhizobium-nodulated Fabaceae species, newly synthesized NH3(+) is finally converted into allantoin (C4H6N4O3) and allantoic acid (C4H8N4O4) (ureides) through complex pathways involving at least 20 different enzymes that act synchronously in two types of nodule cells with contrasting ultrastructure, including the tree nodule cell organelles. Newly synthesized ureides are loaded into the network of nodule-root xylem vessels and transported to aerial organs by the transpirational water current. Once inside the leaves, ureides undergo an enzymatically driven reverse process to yield NH4(+) that is used for growth. This supports the role of ureides as key nitrogen (N)-compounds for the growth and yield of legumes nodulated by Rhizobium that grow in soils with a low N content. Thus, a concrete understanding of the mechanisms underlying ureide biogenesis and catabolism in legumes may help agrobiologists to achieve greater agricultural discoveries. In this review we focus on the transmembranal and transorganellar symplastic and apoplastic movement of N-precursors within the nodules, as well as on the occurrence, localization and properties of enzymes and genes involved in the biogenesis and catabolism of ureides. The synthesis and transport of ureides are not unique events in Rhizobium-nodulated N2-fixing legumes. Thus, a brief description of the synthesis and catabolism of ureides in non-legumes was included for comparison. The establishment of the symbiosis, nodule organogenesis and the plant's control of nodule number, synthesis and translocation of ureides via feed-back inhibition mechanisms are also reviewed.
authors.affiliation: Faculty of Agriculture and Forestry, University of Helsinki, P.O. Box 27, Latokartanonkaari 7, FIN-00014 Helsinki, Finland. Electronic address: bikash.baral@helsinki.fi.
authors.forename: Bikash
authors.initials: B
authors.lastname: Baral
authors.affiliation: P.O. Box 7, Miki-cho Post Office, Ikenobe 3011-2, Kagawa-ken 761-0799, Japan. Electronic address: jaimetex@yahoo.com.
authors.forename: Jaime A
authors.initials: JA
authors.lastname: Teixeira da Silva
authors.affiliation: Biological Nitrogen Fixation Laboratory, Chemistry Department, Faculty of Science, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa. Electronic address: mlizaguirre@gmail.com.
authors.forename: Maria Luisa
authors.initials: ML
authors.lastname: Izaguirre-Mayoral
mesh_terms: D000215:Acylation; D000481:Allantoin / Q000378:metabolism; D001692:Biological Transport; D007887:Fabaceae / Q000382:microbiology / Q000502:physiology*; D018506:Gene Expression Regulation, Plant; D009584:Nitrogen / Q000378:metabolism*; D009586:Nitrogen Fixation; D018515:Plant Leaves / Q000382:microbiology / Q000502:physiology; D055170:Plant Root Nodulation; D018517:Plant Roots / Q000382:microbiology / Q000502:physiology; D012231:Rhizobium / Q000502:physiology*; D013559:Symbiosis; D014508:Urea / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism*; D052584:Xylem / Q000382:microbiology / Q000502:physiology
chemical_list: C024081:allantoic acid; D000481:Allantoin; D014508:Urea; D009584:Nitrogen
keywords: Biological nitrogen fixation; Nod and nif genes; Rhizobium; Transporters; Xylem ureides