LIN, aMedicago truncatulaGene Required for Nodule Differentiation and Persistence of Rhizobial Infections

Abstract

Ethyl methanesulfonate mutagenesis of the model legume Medicago truncatula has previously identified several genes required for early steps in nodulation. Here, we describe a new mutant that is defective in intermediate steps of nodule differentiation. The lin (lumpy infections) mutant is characterized by a 4-fold reduction in the number of infections, all of which arrest in the root epidermis, and by nodule primordia that initiate normally but fail to mature. Genetic analyses indicate that the symbiotic phenotype is conferred by a single gene that maps to the lower arm of linkage group 1. Transcriptional markers for early Nod factor responses (RIP1 and ENOD40) are induced in lin, as is another early nodulin, ENOD20, a gene expressed during the differentiation of nodule primordia. By contrast, other markers correlated with primordium differentiation (CCS52A), infection progression (MtN6), or nodule morphogenesis (ENOD2 and ENOD8) show reduced or no induction in homozygous lin individuals. Taken together, these results suggest that LIN functions in maintenance of rhizobial infections and differentiation of nodules from nodule primordia.In response to specific soil bacteria called rhizobia, legume roots initiate a unique developmental program that culminates in the formation of nitrogen-fixing root nodules. The highly specialized microenvironment of the nodule provides conditions necessary for the conversion of atmospheric nitrogen to ammonium by the rhizobial nitrogenase enzyme. Nod factors, lipooligosaccharide signal molecules produced by the bacterial partner, are required for the parallel processes of bacterial infection and nodule morphogenesis (Geurts and Bisseling, 2002). In recent years, major advances have been made in defining the host's responses to Nod factors, culminating in the cloning of several plant genes required for Nod factor signal transduction (Schauser et al., 1999;Endre et al., 2002;Stracke et al., 2002;Limpens and Bisseling, 2003;Madsen et al., 2003;Radutoiu et al., 2003;Ané et al., 2004;Lévy et al., 2004). The characterization of these genes and their mutant phenotypes indicates that receptor-like kinases play multiple roles in the perception of Nod factor and that transduction of the signal proceeds via calcium-mediated responses and transcriptional regulation (Cullimore and Dénarié, 2003;Parniske and Downie, 2003;Riely et al., 2004).Current understanding of the plant's contribution to symbiotic development subsequent to Nod factor perception, including rhizobial infection, primordium initiation, and nodule differentiation, comes largely from microscopic examination and characterization of plant genes that are differentially expressed during these processes. Early in the interaction, the bacteria gain access to the root through selective penetration of root hair cells. Invagination of the plant plasma membrane and deposition of cell wall material enable bacterial invasion via infection threads. Some, but not all, infection threads traverse the epidermal cells to invade root cortical cel...

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

“…Late-initiating infection threads that abort are associated with necrotic cells, and with the accumulation of defence-related proteins and phenolic compounds in plant cells and cell walls 134 . This type of cellular damage is also found near aborted infection threads and failed symbiosomes formed by M. truncatula lin, nip and sym6 mutants, and aborted infection threads formed on wild-type M. sativa plants by S. meliloti exoY mutants 29,43,72,122,[134][135][136] . M. truncatula plants inoculated with the exoY mutant also express several plant defence genes more strongly than plants inoculated with wild-type S. meliloti (K. M. J. and G. C. W., unpublished data).…”

“…The M. truncatula lin mutant, when inoculated with wild-type S. meliloti, has an abortive infection thread phenotype that is similar to that of a wild-type plant inoculated with an S. meliloti exoY mutant 39,43 (TABLE 1). In both of these cases, infection arrests at the CCRH stage (FIG.…”

A Medicago truncatula Cystathionine-β-Synthase-like Domain-Containing Protein Is Required for Rhizobial Infection and Symbiotic Nitrogen Fixation

“…Like rpg, the nf-ya1 mutant also has thickened, slowly progressing ITs that appear bulbous and swollen (Laporte et al, 2014). The lin/cerberus, Ljcyclops, and vpy mutants form abnormal-looking ITs that terminate very early in the root hairs (Kuppusamy et al, 2004;Yano et al, 2009;Murray 2011). The recently reported arf16a mutant has a decreased number of microcolonies (Breakspear et al, 2014), while microcolony numbers in cbs1 were increased relative to wild type.…”

Genetics and functional genomics of legume nodulation

“…The M. truncatula lumpy infections (lin) mutant is defective in infection-thread formation in root cortical cells despite the fact that the early nodulation responses still occur in this mutant [32]. Therefore, LIN probably acts downstream of the first events in the Nod signaling pathway [32]. The crinkle mutant (Ljsym79) from L. japonicus is defective in plant development (i.e.…”