The ROOT DETERMINED NODULATION1 Gene Regulates Nodule Number in Roots of Medicago truncatula and Defines a Highly Conserved, Uncharacterized Plant Gene Family      

Abstract

The formation of nitrogen-fixing nodules in legumes is tightly controlled by a long-distance signaling system in which nodulating roots signal to shoot tissues to suppress further nodulation. A screen for supernodulating Medicago truncatula mutants defective in this regulatory behavior yielded loss-of-function alleles of a gene designated ROOT DETERMINED NODULATION1 (RDN1). Grafting experiments demonstrated that RDN1 regulatory function occurs in the roots, not the shoots, and is essential for normal nodule number regulation. The RDN1 gene, Medtr5g089520, was identified by genetic mapping, transcript profiling, and phenotypic rescue by expression of the wild-type gene in rdn1 mutants. A mutation in a putative RDN1 ortholog was also identified in the supernodulating nod3 mutant of pea (Pisum sativum). RDN1 is predicted to encode a 357-amino acid protein of unknown function. The RDN1 promoter drives expression in the vascular cylinder, suggesting RDN1 may be involved in initiating, responding to, or transporting vascular signals. RDN1 is a member of a small, uncharacterized, highly conserved gene family unique to green plants, including algae, that we have named the RDN family.

Pea nodule gradients explain N nutrition and limited symbiotic fixation in hypernodulating mutants

“…In the case of pea (Pisum sativum L.), hypernodulating mutants have been selected after mutagenesis and three genes have been shown to be involved in the regulation of nodulation, namely NOD3 (Postma et al 1988), SYM29, and SYM28 (Sagan and Duc 1996). On the basis of homologies with Medicago truncatula or Lotus japonicus, these three genes have been identified (Krusell et al 2002(Krusell et al , 2011Schnabel et al 2011), NOD3 being of unknown function when the two other genes encode a leucine-rich receptor like kinase.…”

Lignin Modification Leads to Increased Nodule Numbers in Alfalfa

“…Several legume mutants affected in different steps of nodule initiation, development, and autoregulation exhibit root growth defects (Wopereis et al, 2000;Veereshlingam et al, 2004;Bright et al, 2005;Kuppusamy et al, 2009;Popp and Ott, 2011), although nodulation was not altered in the short roots of the cra1 lignin mutant (Laffont et al, 2010). Some legume mutants can also exhibit hypernodulation phenotypes and normal root growth (Penmetsa and Cook, 1997;Yokota et al, 2009;Schnabel et al, 2011). These apparently contradictory results mean that it is not clear how root growth and nodulation may be impacted by the modification of lignin biosynthesis.…”

TheM. truncatula SUNNgene is expressed in vascular tissue, similarly toRDN1, consistent with the role of these nodulation regulation genes in long distance signaling

“…Outside of legumes, the SUNN kinase is most closely related by sequence to the Arabidopsis CLV1 kinase and related proteins in monocots involved in meristem maintenance, 5 and AtCLV1, in addition to its well characterized expression in apical meristems, is expressed in the phloem companion cells. 10 Combined with the observation that the RDN1 protein is completely unknown in function but highly conserved across all green plants including moss, 4 the functions of RDN1 and SUNN cannot be limited to signal transduction in the autoregulation of nodulation. The fact that they are expressed in the vasculature with little change in expression in response to rhizobia could suggest the two proteins are involved in the same signal transduction event and we are investigating this further.…”

“…2,3 We recently identified a gene, RDN1 (ROOT DETERMINED NODULATION 1), that controls nodule number from the root but is expressed in both shoots and roots. 4 A fusion of the RDN1 promoter to the uidA (GUS) gene gave staining in the vasculature of transgenic roots. 4 The SUNN gene in M. truncatula encodes a leucine-rich repeat receptor kinase that controls nodule number from the shoot but like RDN1 is expressed in both shoots and roots.…”

“…4 A fusion of the RDN1 promoter to the uidA (GUS) gene gave staining in the vasculature of transgenic roots. 4 The SUNN gene in M. truncatula encodes a leucine-rich repeat receptor kinase that controls nodule number from the shoot but like RDN1 is expressed in both shoots and roots. 5 To assess tissue-level expression of SUNN, we constructed a transgenic M. truncatula plant containing 1360 bp of sequence upstream of SUNN driving expression of an mGFP-GUS gene fusion.…”