The formation of nitrogen-fixing nodules on legumes requires an integration of infection by rhizobia at the root epidermis and the initiation of cell division in the cortex, several cell layers away from the sites of infection. Several recent developments have added to our understanding of the signaling events in the epidermis associated with the perception of rhizobial nodulation factors and the role of plant hormones in the activation of cell division leading to nodule morphogenesis. This review focuses on the tissue-specific nature of the developmental processes associated with nodulation and the mechanisms by which these processes are coordinated during the formation of a nodule.

Coordinating nodule morphogenesis with rhizobial infection in legumes.

(1988). Inhibition of legume nodule formation and N2 fixation by nitrate. Critical Reviews in Plant Sciences: Vol. 7, No. 1, pp. 1-23.

Inhibition of legume nodule formation and N2 fixation by nitrate

Rhizobium nodulation (Nod) factors are lipo-chitooligosaccharides that act as symbiotic signals, eliciting several key developmental responses in the roots of legume hosts. Using nodulation-defective mutants of Medicago truncatula, we have started to dissect the genetic control of Nod factor transduction. Mutants in four genes (DMI1, DMI2, DMI3, and NSP) were pleiotropically affected in Nod factor responses, indicating that these genes are required for a Nod factor–activated signal transduction pathway that leads to symbiotic responses such as root hair deformations, expressions of nodulin genes, and cortical cell divisions. Mutant analysis also provides evidence that Nod factors have a dual effect on the growth of root hair: inhibition of endogenous (plant) tip growth, and elicitation of a novel tip growth dependent on (bacterial) Nod factors. dmi1, dmi2, and dmi3 mutants are also unable to establish a symbiotic association with endomycorrhizal fungi, indicating that there are at least three common steps to nodulation and endomycorrhization in M. truncatula and providing further evidence for a common signaling pathway between nodulation and mycorrhization.

http://www.plantcell.org/content/plantcell/12/9/1647.full.pdf

Four Genes of Medicago truncatula Controlling Components of a Nod Factor Transduction Pathway

Legumes are highly important food, feed and biofuel crops. With few exceptions, they can enter into an intricate symbiotic relationship with specific soil bacteria called rhizobia. This interaction results in the formation of a new root organ called the nodule in which the rhizobia convert atmospheric nitrogen gas into forms of nitrogen that are useable by the plant. The plant tightly controls the number of nodules it forms, via a complex root-to-shoot-to-root signaling loop called autoregulation of nodulation (AON). This regulatory process involves peptide hormones, receptor kinases and small metabolites. Using modern genetic and genomic techniques, many of the components required for nodule formation and AON have now been isolated. This review addresses these recent findings, presents detailed models of the nodulation and AON processes, and identifies gaps in our understanding of these process that have yet to be fully explained.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1744-7909.2010.00899.x

Molecular Analysis of Legume Nodule Development and Autoregulation

In legumes, root nodule organogenesis is activated in response to morphogenic lipochitin oligosaccharides that are synthesized by bacteria, commonly known as rhizobia. Successful symbiotic interaction results in the formation of highly specialized organs called root nodules, which provide a unique environment for symbiotic nitrogen fixation. In wild-type plants the number of nodules is regulated by a signalling mechanism integrating environmental and developmental cues to arrest most rhizobial infections within the susceptible zone of the root. Furthermore, a feedback mechanism controls the temporal and spatial susceptibility to infection of the root system. This mechanism is referred to as autoregulation of nodulation, as earlier nodulation events inhibit nodulation of younger root tissues. Lotus japonicus plants homozygous for a mutation in the hypernodulation aberrant root (har1) locus escape this regulation and form an excessive number of nodules. Here we report the molecular cloning and expression analysis of the HAR1 gene and the pea orthologue, Pisum sativum, SYM29. HAR1 encodes a putative serine/threonine receptor kinase, which is required for shoot-controlled regulation of root growth, nodule number, and for nitrate sensitivity of symbiotic development.

Shoot control of root development and nodulation is mediated by a receptor-like kinase

The availability of soybean mutants with altered symbiotic properties allowed an investigation of the shoot or root control of the relevant phenotype. By means of grafts between these mutants and wild-type plants (cultivar Bragg and Williams), we demonstrated that supernodulation as well as hypernodulation (nitrate tolerance in nodulation and lack of autoregulation) is shoot controlled in two mutants (nts382 and nts1116) belonging most likely to two separate complementation groups. The supernodulation phenotype was expressed on roots of the parent cultivar Bragg as well as the roots of cultivar Williams. Likewise it was shown that non-nodulation (resistance to Bradyrhizobium) is root controlled in mutant nod49. The shoot control of nodule initiation is epistatically suppressed by the non-nodulation, root-expressed mutation. These findings suggest that different plant organs can influence the expression of the nodulation phenotype.

Regulation of the Soybean-Rhizobium Nodule Symbiosis by Shoot and Root Factors

Nitrate inhibition of nodulation in legumes.

The early events in the development of nodules induced byBradyrhizobium japonicum were studied in serial sections of a wild type (cv. Bragg), a supernodulating mutant (nts 382) and four non-nodulating mutants (nod49, nod139, nod772, andrj
1) of soybean (Glycine max [L.] Merrill). Cultivar Bragg responded to inoculation in a similar manner to that described previously for cv. Williams; centres of sub-epidermal cell divisions were observed both with and without associated infection threads and most infection events were blocked before the formation of a nodule meristem. The non-nodulating mutants (nod49, nod772, andrj
1) had, at most, a few centres of sub-epidermal cell divisions. In general, these were devoid of infection threads and did not develop beyond the very early stages of nodule ontogeny. Sub-epidermal cell divisions or infection threads were never observed on mutant nodl39. This mutant is not allelic to the other non-nodulating mutants and represents a defect in a separate complementation group or gene that is required for nodulation. The supernodulating mutant nts382, which is defective in autoregulation of nodulation, had a similar number of sub-epidermal cell divisions as the wild-type Bragg, but a much greater proportion of these developed to an advanced stage of nodule ontogeny. Mutant nts382, like Bragg, possessed other infection events that were arrested at an early stage of development. The results are discussed in the context of the progression of events in nodule formation and autoregulation of nodulation in soybean.

Development of Bradyrhizobium infections in supernodulating and non-nodulating mutants of soybean ( Glycine max [L.] Merrill)

Characterization of non-nodulation mutants of soybean [glycine-max (L) merr] - bradyrhizobium effects and absence of root hair curling

The root exudates and extracts of soybean nonnodulation (nod¯) mutants nod49, nod139, and nod772 were compared with those of the wild-type parent cultivar Bragg and supernodulation (nitrate tolerant symbiotic) mutant nts382 to assess their effects on the early stages of nodulation by Bradyrhizobium japonicum strain USDA110. When nod¯ mutants were cocultured in proximity with wild-type or mutant nts382, nodulation was as expected for the individual genotypes. Coculturing nts382 with wild-type or nonnodulating plants decreased nodule number on the supernodulating mutant when compared to similar plant density of nts382 alone. Pretreatment of B. japonicumwith collected root exudates of the soybean mutants did not affect nodulation efficiency or total nodule number per plant. Analysis of in vivo labeled proteins in root exudates of wild type, nts382, and nod49 showed similar patterns both on one- and two-dimensional gels. Radioactively (35S) labeled root exudates were subjected to immunoprecipitation with the antibody to soybean lectin (SBL) and similar amounts were found in the nod49, nts382, and wild-type soybean. Seedling root extracts of nts382, nod49, and Bragg were tested for their ability to induce a nodC::lacZ fusion in B. japonicum strain USDA110. Similar levels of induction were found in all cases, indicating that uninoculated seedlings of cultivar Bragg, nod49, and nts382 contained similar stimulating compounds. Based on the observation that mutants nod49, nod139, and nod772 lack root hair curling, we suggest that the nonnodulation phenotype may be caused by an inability to respond efficiently to bacterial factors designed to stimulate plant cell divisions and concomitant root hair curling.

Anne Mathews

Papers

Regulation of the Soybean-Rhizobium Nodule Symbiosis by Shoot and Root Factors

Nitrate inhibition of nodulation in legumes.

Development of Bradyrhizobium infections in supernodulating and non-nodulating mutants of soybean ( Glycine max [L.] Merrill)

Characterization of non-nodulation mutants of soybean [glycine-max (L) merr] - bradyrhizobium effects and absence of root hair curling

Biological characterization of root exudates and extracts from nonnodulating and supernodulating soybean mutants