Alfalfa Controls Nodulation during the Onset of Rhizobium-induced Cortical Cell Division

Lotus japonicus HAR1 regulates root morphology locally and systemically under a moderate nitrate condition in the absence of rhizobia

The local and long-distance signaling pathways mediated by the leucine-rich repeat receptor kinase HAR1 suppress root branching and promote primary root length in response to nitrate supply. The root morphology of higher plants changes plastically to effectively absorb nutrients and water from the soil. In particular, legumes develop root organ nodules, in which symbiotic rhizobia fix atmospheric nitrogen in nitrogen-poor environments. The number of nodules formed in roots is negatively regulated by a long-distance signaling pathway that travels through shoots called autoregulation of nodulation (AON). In the model plant Lotus japonicus, defects in AON genes, such as a leucine-rich repeat receptor kinase HYPERNODULATION ABERRANT ROOT FORMATION 1 (HAR1), an orthologue of CLAVATA1, and the F-box protein TOO MUCH LOVE (TML), induce the formation of an excess number of nodules. The loss-of-function mutant of HAR1 exhibits a short and bushy root phenotype in the absence of rhizobia. We show that the har1 mutant exhibits high nitrate sensitivity during root development. The uninfected har1 mutant significantly increased lateral root number and reduced primary root length in the presence of 3 mM nitrate, compared with the wild-type and tml mutant. Grafting experiments indicated that local and long-distance signaling pathways via root- and shoot-acting HAR1 additively regulated root morphology under the moderate nitrate concentrations. These findings allow us to propose that HAR1-mediated signaling pathways control the root system architecture by suppressing lateral root branching and promoting primary root elongation in response to nitrate availability.

Characterisation of Medicago truncatula CLE34 and CLE35 in nitrate and rhizobia regulation of nodulation

Legumes form a symbiosis with atmospheric nitrogen (N₂ )-fixing soil rhizobia, resulting in new root organs called nodules that enable N₂ -fixation. Nodulation is a costly process that is tightly regulated by the host through autoregulation of nodulation (AON) and nitrate-dependent regulation of nodulation. Both pathways require legume-specific CLAVATA/ESR-related (CLE) peptides. Nitrogen-induced nodulation-suppressing CLE peptides have not previously been investigated in Medicago truncatula, for which only rhizobia-induced MtCLE12 and MtCLE13 have been characterised. Here, we report on novel peptides MtCLE34 and MtCLE35 in nodulation control. The nodulation-suppressing CLE peptides of five legume species were classified into three clades based on sequence homology and phylogeny. This approached identified MtCLE34 and MtCLE35 and four new CLE peptide orthologues of Pisum sativum. Whereas MtCLE12 and MtCLE13 are induced by rhizobia, MtCLE34 and MtCLE35 respond to both rhizobia and nitrate. MtCLE34 was identified as a pseudogene lacking a functional CLE-domain. MtCLE35 was found to inhibit nodulation in a SUNN- and RDN1-dependent manner via overexpression analysis. Together, our findings indicate that MtCLE12 and MtCLE13 have a specific role in AON, while MtCLE35 regulates nodule numbers in response to both rhizobia and nitrate. MtCLE34 likely had a similar role to MtCLE35, but its function was lost due to a premature nonsense mutation.

Unraveling new molecular players involved in the autoregulation of nodulation in Medicago truncatula

The number of legume root nodules resulting from a symbiosis with rhizobia is tightly controlled by the plant. Certain members of the CLAVATA3/Embryo Surrounding Region (CLE) peptide family, specifically MtCLE12 and MtCLE13 in Medicago truncatula, act in the systemic autoregulation of nodulation (AON) pathway that negatively regulates the number of nodules. Little is known about the molecular pathways that operate downstream of the AON-related CLE peptides. Here, by means of a transcriptome analysis, we show that roots ectopically expressing MtCLE13 deregulate only a limited number of genes, including three down-regulated genes encoding lysin motif receptor-like kinases (LysM-RLKs), among which are the nodulation factor (NF) receptor NF Perception gene (NFP) and two up-regulated genes, MtTML1 and MtTML2, encoding Too Much Love (TML)-related Kelch-repeat containing F-box proteins. The observed deregulation was specific for the ectopic expression of nodulation-related MtCLE genes and depended on the Super Numeric Nodules (SUNN) AON RLK. Moreover, overexpression and silencing of these two MtTML genes demonstrated that they play a role in the negative regulation of nodule numbers. Hence, the identified MtTML genes are the functional counterpart of the Lotus japonicus TML gene shown to be central in the AON pathway. Additionally, we propose that the down-regulation of a subset of LysM-RLK-encoding genes, among which is NFP, might contribute to the restriction of further nodulation once the first nodules have been formed.

Legume nodulation: The host controls the party

Global demand to increase food production and simultaneously reduce synthetic nitrogen fertilizer inputs in agriculture are underpinning the need to intensify the use of legume crops. The symbiotic relationship that legume plants establish with nitrogen-fixing rhizobia bacteria is central to their advantage. This plant-microbe interaction results in newly developed root organs, called nodules, where the rhizobia convert atmospheric nitrogen gas into forms of nitrogen the plant can use. However, the process of developing and maintaining nodules is resource intensive; hence, the plant tightly controls the number of nodules forming. A variety of molecular mechanisms are used to regulate nodule numbers under both favourable and stressful growing conditions, enabling the plant to conserve resources and optimize development in response to a range of circumstances. Using genetic and genomic approaches, many components acting in the regulation of nodulation have now been identified. Discovering and functionally characterizing these components can provide genetic targets and polymorphic markers that aid in the selection of superior legume cultivars and rhizobia strains that benefit agricultural sustainability and food security. This review addresses recent findings in nodulation control, presents detailed models of the molecular mechanisms driving these processes, and identifies gaps in these processes that are not yet fully explained.

Biogenic amines in rhizobia and legume root nodules

Root-nodule bacteria (rhizobia) are of great importance for nitrogen acquisition through symbiotic nitrogen fixation in a wide variety of leguminous plants. These bacteria differ from most other soil microorganisms by taking dual forms, i.e. a free-living form in soils and a symbiotic form inside of host legumes. Therefore, they should have a versatile strategy for survival, whether inhabiting soils or root nodules formed through rhizobia-legume interactions. Rhizobia generally contain large amounts of the biogenic amine homospermidine, an analog of spermidine which is an essential cellular component in most living systems. The external pH, salinity and a rapid change in osmolarity are thought to be significant environmental factors affecting the persistence of rhizobia. The present review describes the regulation of homospermidine biosynthesis in response to environmental stress and its possible functional role in rhizobia. Legume root nodules, an alternative habitat of rhizobia, usually contain a variety of biogenic amines besides homospermidine and the occurrence of some of these amines is closely associated with rhizobial infections. In the second half of this review, novel biogenic amines found in certain legume root nodules and the mechanism of their synthesis involving cooperation between the rhizobia and host legume cells are also described.

Inoculation- and nitrate-induced CLE peptides of soybean control NARK-dependent nodule formation

Systemic autoregulation of nodulation in legumes involves a root-derived signal (Q) that is perceived by a CLAVATA1-like leucine-rich repeat receptor kinase (e.g. GmNARK). Perception of Q triggers the production of a shoot-derived inhibitor that prevents further nodule development. We have identified three candidate CLE peptide-encoding genes (GmRIC1, GmRIC2, and GmNIC1) in soybean (Glycine max) that respond to Bradyrhizobium japonicum inoculation or nitrate treatment. Ectopic overexpression of all three CLE peptide genes in transgenic roots inhibited nodulation in a GmNARK-dependent manner. The peptides share a high degree of amino acid similarity in a 12-amino-acid C-terminal domain, deemed to represent the functional ligand of GmNARK. GmRIC1 was expressed early (12 h) in response to Bradyrhizobium-sp.-produced nodulation factor while GmRIC2 was induced later (48 to 72 h) but was more persistent during later nodule development. Neither GmRIC1 nor GmRIC2 were induced by nitrate. In contrast, GmNIC1 was strongly induced by nitrate (2 mM) treatment but not by Bradyrhizobium sp. inoculation and, unlike the other two GmCLE peptides, functioned locally to inhibit nodulation. Grafting demonstrated a requirement for root GmNARK activity for nitrate regulation of nodulation whereas Bradyrhizobium sp.-induced regulation was contingent on GmNARK function in the shoot.

Search for nodulation-related CLE genes in the genome of Glycine max

CLE peptides are potentially involved in nodule organ development and in the autoregulation of nodulation (AON), a systemic process that restricts nodule number. A genome-wide survey of CLE peptide genes in the soybean glycine max genome resulted in the identification of 39 GmCLE genes, the majority of which have not yet been annotated. qRT-PCR analysis indicated two different nodulation-related CLE expression patterns, one linked with nodule primordium development and a new one linked with nodule maturation. Moreover, two GmCLE gene pairs, encoding group-III CLE peptides that were previously shown to be involved in AON, had a transient expression pattern during nodule development, were induced by the essential nodulation hormone cytokinin, and one pair was also slightly induced by the addition of nitrate. Hence, our data support the hypothesis that group-III CLE peptides produced in the nodules are involved in primordium homeostasis and intertwined in activating AON, but not in sustaining it.

A holistic view of nitrogen acquisition in plants

Nitrogen (N) is the mineral nutrient required in the greatest amount and its availability is a major factor limiting growth and development of plants. As sessile organisms, plants have evolved different strategies to adapt to changes in the availability and distribution of N in soils. These strategies include mechanisms that act at different levels of biological organization from the molecular to the ecosystem level. At the molecular level, plants can adjust their capacity to acquire different forms of N in a range of concentrations by modulating the expression and function of genes in different N uptake systems. Modulation of plant growth and development, most notably changes in the root system architecture, can also greatly impact plant N acquisition in the soil. At the organism and ecosystem levels, plants establish associations with diverse microorganisms to ensure adequate nutrition and N supply. These different adaptive mechanisms have been traditionally discussed separately in the literature. To understand plant N nutrition in the environment, an integrated view of all pathways contributing to plant N acquisition is required. Towards this goal, in this review the different mechanisms that plants utilize to maintain an adequate N supply are summarized and integrated.

Early action of pea symbiotic gene NOD3 is confirmed by adventitious root phenotype

A supernodulating and Nts (nitrate-tolerant symbiosis) symbiotic mutation of pea (Pisum sativum L.) line RisfixC was found to retain its expression in the distant genetic background of pea lines Afghanistan L1268, Zhodino E900, and cv. Arvika. This finding allowed for reliable scoring for the trait in mapping crosses. The RisfixC mutation was localized 8.2cM apart from SYM2 and cosegregated with molecular markers for SYM2-NOD3 region Psc923 and OA-1. Grafting experiments showed that supernodulation is root-determined, consistently with mutants in the NOD3 locus. Therefore, the mutation of RisfixC can be localized in gene NOD3. Like in other published nod3 alleles, the RisfixC mutation determines supernodulation when it is expressed in the root but not in the shoot. Supernodulated adventitious roots that are spontaneously formed in the wild-type scions on mutant rootstocks indicate that the descending systemic signal, which is inhibitory to nodule formation, is absent in this type of chimeric plants. Since the descending signal production in the wild-type shoot reflects the presence of the ascending root signal, the nod3-associated lesion must be located in the beginning of the systemic circuit regulating nodule number.

Adaptation of Medicago truncatula to nitrogen limitation is modulated via local and systemic nodule developmental responses

Adaptation of Medicago truncatula to local nitrogen (N) limitation was investigated to provide new insights into local and systemic N signaling. The split-root technique allowed a characterization of the local and systemic responses of NO(3)(-) or N(2)-fed plants to localized N limitation. (15)N and (13)C labeling were used to monitor plant nutrition. Plants expressing pMtENOD11-GUS and the sunn-2 hypernodulating mutant were used to unravel mechanisms involved in these responses. Unlike NO(3)(-)-fed plants, N(2)-fixing plants lacked the ability to compensate rapidly for a localized N limitation by up-regulating the N(2)-fixation activity of roots supplied elsewhere with N. However they displayed a long-term response via a growth stimulation of pre-existing nodules, and the generation of new nodules, likely through a decreased abortion rate of early nodulation events. Both these responses involve systemic signaling. The latter response is abolished in the sunn mutant, but the mutation does not prevent the first response. Local but also systemic regulatory mechanisms related to plant N status regulate de novo nodule development in Mt, and SUNN is required for this systemic regulation. By contrast, the stimulation of nodule growth triggered by systemic N signaling does not involve SUNN, indicating SUNN-independent signaling.

Autoregulation of nodulation (AON) in Pisum sativum (pea) involves signalling events associated with both nodule primordia development and nitrogen fixation

To define the signalling events required for the activation of AON, we utilised approach grafts between wild-type pea plants and their mutants defective at successive stages of nodule formation. AON signalling strength was monitored by prior inoculation of mutant root portions (as so-called 'sensor') and quantifying nodule formation on connected roots of delayed inoculated wild type (the 'reporter'). Detectable AON sensing and associated signal exchange between root and shoot started after root hair curling but before the initiation of visible cortical and pericycle cell divisions. The strength of AON signalling was correlated with the stage of nodule development and size of nodule, with mature nitrogen-fixing nodules possessing the strongest AON-inducing signal. We demonstrated that the pea supernodulating mutant nod3 may function pre-NARK in the root. A model for the activation of AON signalling and its potential relationship with cell division, nitrogen fixation and/or cytokinin signal transduction are presented.

Infection and invasion of roots by symbiotic, nitrogen-fixing rhizobia during nodulation of temperate legumes

Bacteria belonging to the genera Rhizobium, Mesorhizobium, Sinorhizobium, Bradyrhizobium, and Azorhizobium (collectively referred to as rhizobia) grow in the soil as free-living organisms but can also live as nitrogen-fixing symbionts inside root nodule cells of legume plants. The interactions between several rhizobial species and their host plants have become models for this type of nitrogen-fixing symbiosis. Temperate legumes such as alfalfa, pea, and vetch form indeterminate nodules that arise from root inner and middle cortical cells and grow out from the root via a persistent meristem. During the formation of functional indeterminate nodules, symbiotic bacteria must gain access to the interior of the host root. To get from the outside to the inside, rhizobia grow and divide in tubules called infection threads, which are composite structures derived from the two symbiotic partners. This review focuses on symbiotic infection and invasion during the formation of indeterminate nodules. It summarizes root hair growth, how root hair growth is influenced by rhizobial signaling molecules, infection of root hairs, infection thread extension down root hairs, infection thread growth into root tissue, and the plant and bacterial contributions necessary for infection thread formation and growth. The review also summarizes recent advances concerning the growth dynamics of rhizobial populations in infection threads.

Organogenesis of legume root nodules

The N(2)-fixing nodules elicited by rhizobia on legume roots represent a useful model for studying plant development. Nodule formation implies a complex progression of temporally and spatially regulated events of cell differentiation/dedifferentiation involving several root tissues. In this review we describe the morphogenetic events leading to the development of these histologically well-structured organs. These events include (1) root hair deformation, (2) development and growth of infection threads, (3) induction of the nodule primordium, and (4) induction, activity, and persistence of the nodular meristem and/or of foci of meristematic activities. Particular attention is given to specific aspects of the symbiosis, such as the early stages of intracellular invasion and to differentiation of the intracellular form of rhizobia, called symbiosomes. These developmental aspects were correlated with (1) the regulatory signals exchanged, (2) the plant genes expressed in specific cell types, and (3) the staining procedures that allow the recognition of some cell types. When strictly linked with morphogenesis, the nodulation phenotypes of plant and bacterial mutants such as the developmental consequence of the treatment with metabolic inhibitors, metabolic intermediates, or the variation of physical parameters are described. Finally, some aspects of nodule senescence and of regulation of nodulation are discussed.

Nodule initiation involves the creation of a new symplasmic field in specific root cells of medicago species

The organogenesis of nitrogen-fixing nodules in legume plants is initiated in specific root cortical cells and regulated by long-distance signaling and carbon allocation. Here, we explore cell-to-cell communication processes that occur during nodule initiation in Medicago species and their functional relevance using a combination of fluorescent tracers, electron microscopy, and transgenic plants. Nodule initiation induced symplasmic continuity between the phloem and nodule initials. Macromolecules such as green fluorescent protein could traffic across short or long distances from the phloem into these primordial cells. The created symplasmic field was regulated throughout nodule development. Furthermore, Medicago truncatula transgenic plants expressing a viral movement protein showed increased nodulation. Hence, the establishment of this symplasmic field may be a critical element for the control of nodule organogenesis.

Medicago truncatula ENOD11: a novel RPRP-encoding early nodulin gene expressed during mycorrhization in arbuscule-containing cells

Leguminous plants establish endosymbiotic associations with both rhizobia (nitrogen fixation) and arbuscular mycorrhizal fungi (phosphate uptake). These associations involve controlled entry of the soil microsymbiont into the root and the coordinated differentiation of the respective partners to generate the appropriate exchange interfaces. As part of a study to evaluate analogies at the molecular level between these two plant-microbe interactions, we focused on genes from Medicago truncatula encoding putative cell wall repetitive proline-rich proteins (RPRPs) expressed during the early stages of root nodulation. Here we report that a novel RPRP-encoding gene, MtENOD11, is transcribed during preinfection and infection stages of nodulation in root and nodule tissues. By means of reverse transcription-polymerase chain reaction and a promoter-reporter gene strategy, we demonstrate that this gene is also expressed during root colonization by endomycorrhizal fungi in inner cortical cells containing recently formed arbuscules. In contrast, no activation of MtENOD11 is observed during root colonization by a nonsymbiotic, biotrophic Rhizoctonia fungal species. Analysis of transgenic Medicago spp. plants expressing pMtENOD11-gusA also revealed that this gene is transcribed in a variety of nonsymbiotic specialized cell types in the root, shoot, and developing seed, either sharing high secretion/metabolite exchange activity or subject to regulated modifications in cell shape. The potential role of early nodulins with atypical RPRP structures such as ENOD11 and ENOD12 in symbiotic and nonsymbiotic cellular contexts is discussed.

Partitioning of 14 C-labelled photosynthate to developing nodules and roots of soybean (Glycine max)

A split-root growth system was used to study photosynthate partitioning to developing nodules and roots of soybean (Glycine max L., Merr). Opposite sides of the root systems were inoculated with Bradyrhizobium japonicum at 8 and 12 d after planting (early/delayed inoculation treatment) or, alternatively, only one side was inoculated 8 d after planting (early/uninoculated treatment). Plants were incubated with ¹⁴ CO₂ at 24-h intervals from early inoculation until the onset of N₂ fixation (acetylene reduction). After staining with Eriochrome black, root and nodule meristematic structures were excised under a dissecting microscope and their radioactivity determined by scintillation counting. The specific radioactivity of nodule structures increased with nodule development, and was as much as 4 times higher in early nodules than in roots and nodules on half-roots receiving delayed inoculation By the time that N₂ fixation could be measured in the first mature nodules, the early inoculated half-root contained over 70% of the radioactivity recovered from the entire root systems of both early/delayed and early/uninotulated treatments. These results suggest that developing nodules create a strong sink for photosynthate, and that nodules and roots compete for current photosynthate. Early initiated nodules might develop at the expense of late initiated nodules, as well as at the expense of the roots themselves.

Shoot versus Root Signal Involvement in Nodulation and Vegetative Growth in Wild-Type and Hypernodulating Soybean Genotypes

Grafting studies involving Williams 82 (normally nodulating) and NOD1-3 (hypernodulating) soybean (Glycine max [L.] Merr.) lines and Lablab purpureus were used to evaluate the effect of shoot and root on nodulation control and plant growth. A single- or double-wedge graft technique, with superimposed partial defoliation, was used to separate signal control from a photosynthate supply effect. Grafting of hypernodulated soybean shoots to roots of Williams 82 or L. purpureus resulted in increased nodule numbers. Grafting of two shoots to one root enhanced root growth in both soybean genotypes, whereas the nodule number was a function of shoot genotype but not of the photosynthetic area. In double-shoot, single-root-grafted plants, removing trifoliolate leaves from either Williams 82 or NOD1-3 shoots decreased root and shoot dry matter, attributable to decreased photosynthetic source. Concurrently, Williams 82 shoot defoliation increased the nodule number, whereas NOD1-3 shoot defoliation decreased the nodule number on both soybean and L. purpureus roots. It was concluded that (a) soybean leaves are the dominant site of autoregulatory signal production, which controls the nodule number; (b) soybean and L. purpureus have a common, translocatable, autoregulatory control signal; (c) seedling vegetative growth and nodule number are independently controlled; and (d) two signals, inhibitor and promoter, may be involved in controlling legume nodule numbers.

Gene structure and differential regulation of the Rhizobium-induced peroxidase gene rip1

Primary expression of the Rhizobium meliloti-induced peroxidase gene rip1 occurs prior to nodule morphogenesis, specifically at the site of impending rhizobial infection (D. Cook, D. Dreyer, D. Bonnet, M. Howell, E. Nony, K. VandenBosch [1995] Plant Cell 7: 43-55). We examined the distribution and structure of rip1 transcript throughout nodule development. We determined that expression of rip1 in root tips is correlated with the competence of this zone for symbiotic association, whereas after rhizobial infection rip1 transcript is specifically associated with the zone of nodule development, including nascent nodule primordia. rip1 transcripts are characterized by multiple polyadenylation sites distributed within 200 to 400 bp of the translation stop site, and a single major transcription initiation site in close proximity to the rip1 open reading frame. Thus, rip1 expression is likely to be mediated through effects on a single transcription unit. Immediately 5' of the rip1 transcription unit DNA sequence analysis identified a 377-bp DNA element containing extensive repeat structure that is widely distributed in the Medicago truncatula genome.

Root nodulation of Sesbania rostrata suppresses stem nodulation by Sinorhizobium teranga but not Azorhizobium caulinodans

Azorhizobium caulinodans ORS571 and Sinorhizobium teranga ORS51 and ORS52 are symbionts of the same host plant Sesbania rostrata. In nature, A. caulinodans nodulates more competitively the stem-located infection sites of Sesbania rostrata. Sinorhizobium strains, although frequently present in root nodules, are seldom found in stem nodules. One probable explanation for this phenomenon is the more abundant presence of Azorhizobium on the leaf and stem surfaces of the host plant. Work presented here hints at other plausible factors that determine the greater "stem specificity" of Azorhizobium. We found that under experimental conditions in which roots are not inoculated, all strains nodulated stems very well. However, ORS51 and ORS52 were much more sensitive than ORS571 to suppression of stem nodulation by previous root inoculation. The introduction of the regulatory nodD gene from A. caulinodans diminished the sensitivity to this suppression, probably by enhanced nod gene expression and subsequent Nod factor production. Our hypothesis is that the greater infectivity of ORS571 is due to a more efficient production of mitogenic Nod factors at stem-located infection sites, thereby more readily overcoming systemic suppression caused by previous root inoculations.Key words: autoregulation, nitrogen fixation, rhizobial ecology, systemic suppression of nodulation.

Transient induction of a peroxidase gene in Medicago truncatula precedes infection by Rhizobium meliloti

Although key determinative events of the Rhizobium-legume symbiosis are likely to precede bacterial infection, no plant genes have been identified that are expressed strongly prior to infection and nodule morphogenesis. A subtractive hybridization-polymerase chain reaction technique was used to enrich for genes induced during the early phases of the R. meliloti-Medicago truncatula symbiosis. One gene so identified encodes a putative plant peroxidase protein, which we have named Rip1 for Rhizobium-induced peroxidase. The accumulation of rip1 transcript was rapidly and transiently induced by R. meliloti and by the corresponding lipooligosaccharide signal molecule Nod factor RmIV, which was both necessary and sufficient for rip1 induction. The duration of maximal rip1 expression coincided with the preinfection period: transcript levels for rip1 were near maximal by 3 hr postinoculation and declined by 48 hr, coincident with early infection events and the onset of nodule morphogenesis. Furthermore, although rip1 induction preceded bacterial infection by at least 24 hr, the transcript was localized to epidermal cells in the differentiating root zone that was subsequently infected by Rhizobium. Thus, a defining feature of the Rhizobium infection court is the prior induction of rip1 expression.

The root epidermis-specific pea gene RH2 is homologous to a pathogenesis-related gene

Two-dimensional gel electrophoresis of pea root and root hair proteins revealed the existence of at least 10 proteins present at elevated levels in root hairs. One of these, named RH2, was isolated and a partial amino acid sequence was determined from two tryptic peptides. Using this sequence information oligonucleotides were designed to isolate by PCR an RH2 cDNA clone. In situ hybridization studies with this cDNA clone showed that rh2 is not only expressed in root hairs, but also in root epidermal cells lacking these tubular outgrowths. During post-embryonic development the gene is switched on after the transition of protoderm into epidermis and since rh2 is already expressed in a globular pea embryo in the protoderm at the side attached to the suspensor, we conclude that the expression of rh2 is developmentally regulated. At the amino acid level RH2 is 95% homologous to the pea PR protein I49a. These gene encoding I49a is induced in pea pods upon inoculation with the pathogen Fusarium solani [12]. We postulate that rh2 contributes to a constitutive defence barrier in the root epidermis. A similar role has been proposed for chalcone synthase (CHS) and chitinase, pathogenesis-related protein that are also constitutively present in certain epidermal tissues.

Five Nodulation Mutants of White Sweetclover (Melilotus alba Desr.) Exhibit Distinct Phenotypes Blocked at Root Hair Curling, Infection Thread Development, and Nodule Organogenesis

In an effort to obtain a developmental sequence of mutations in the Rhizobium-legume interaction within a single legume species, we have characterized the early events of nodule development in 10 nodulation mutants of sweetclover, Melilotus alba Desr. cv U389, representing five genetic loci. Both seed and root exudates from all of the sweetclover mutants induced expression of the nod genes of Rhizobium meliloti. Mutants in three loci were blocked in the early stages of root hair curling. Of these, a mutant in the sym-3 locus exhibited root hair deformations in response to inoculation with R. meliloti but produced no nodules or emerging nodule primordia, suggesting a blockage in the signal transduction events leading to nodule organogenesis. In contrast, mutants in both the sym-1 and sym-5 loci formed ineffective nodules in response to inoculation but differed slightly in the type of root hair response observed. None of these three early mutants formed infection threads. Infection threads were observed in mutant sym-2 as well as in ineffective nodules. Mutant sym-4 also formed infection threads but lacked nodules. The phenotypes observed for mutants from these five loci suggest that a secondary receptor or signal produced by the plant is required for nodule development.

Plant genetic suppression of the non-nodulation phenotype of Rhizobium meliloti host-range nodH mutants: gene-for-gene interaction in the alfalfa-Rhizobium symbiosis?

Rhizobium nodulation genes can produce active extracellular signals for legume nodulation. The R. meliloti host-range nodH gene has been postulated to mediate the transfer of a sulfate to a modified lipo-oligosaccharide, which in its sulfated form is a specific nodulation factor for alfalfa (Medicago sativa L.). We found that alfalfa was capable of effective nodulation with signal-defective and non-nodulating nodH mutants (Nnr) defining a novel gene-for-gene interaction that conditions nodulation. Bacteria-free nodules that formed spontaneously at about a 3-5% rate in unselected seed populations of alfalfa cv 'Vernal' in the total absence of Rhizobium (Nar) exhibited all the histological, regulatory and ontogenetic characteristics of alfalfa nodules. Inoculation of such populations with nodH mutants, but not with nodA or nodC mutants, produced a four- to five-fold increase in the percentage of nodulated plants. Some 10-25% of these nodulated plants formed normal pink nitrogen-fixing nodules instead of white empty nodules. About 70% of the S1 progeny of such Nnr(+) plants retained the parental phenotype; these plants were also able to form nodules in the absence of Rhizobium. If selected Nar(+) plants were self-pollinated almost the entire progeny exhibited the parental Nar(+) phenotype. Segregation analysis of S1 and S2 progeny from selected Nar(+) plants suggests that the Nar character is monogenic dominant and that the nodulation phenotype is controlled by a gene dose effect. The inoculation of different S1 Nar(+) progeny with nodH mutant bacteria gave only empty non-fixing nodules. Our results indicate that certain alfalfa genotypes can be selected for suppression of the non-nodulation phenotype of nodH mutants. The fact that the Nnr plant phenotype behaved as a dominant genetic trait and that it directly correlated with the ability of the selected plants to form nodules in the absence of Rhizobium suggests that the interaction of plant and bacterial alleles occurs early during signal transduction through the alteration of a signal reception component of the plant so that it responds to putative signal precursors.

Genetic Analysis of Rhizobium leguminosarum bv. Phaseoli Mutants Defective in Nodulation and Nodulation Suppression

Nodulation-defective rhizobia and their nodule-forming derivatives containing cloned DNA from the wild type were used to study nodulation suppression in Phaseolus vulgaris L. Non-nitrogen-fixing derivatives which formed rhizobia-containing white nodules induced partial suppression. Comparison of this with the complete suppression by Fix + derivatives and a Fix - mutant which formed rhizobia-containing pink nodules suggests that the extent of suppression may be related to successive stages of nodule development.

Nitrate inhibition of nodulation can be overcome by the ethylene inhibitor aminoethoxyvinylglycine

Previously, we reported (a) a positive correlation between the nitrate concentrations in growth medium and ethylene evolved from uninoculated and inoculated alfalfa (Medicago sativa) roots and (b) a negative correlation between ethylene evolution and nodulation. Here, we report that the inhibitory effect of NO(3) (-) on nodulation of alfalfa can be eliminated by the ethylene inhibitor aminoethoxyvinylglycine (AVG). This effect was probably related to the strong inhibition (90%) of ethylene biosynthesis caused by AVG in these inoculated and NO(3) (-)-treated roots. These results support our hypothesis that the inhibitory effect of NO(3) (-) is mediated through the phytohormone ethylene. A possible role of endogenous ethylene in the autoregulation of nodulation also is discussed. AVG at 10 micromolar significantly (P < 0.05) increased total nitrogenase activity (acetylene reduction) in 2.5 and 5 millimolar NO(3) (-)-fed plants probably as a result of the very high stimulation of nodulation.

Autoregulatory response of Phaseolus vulgaris L. to symbiotic mutants of Rhizobium leguminosarum bv. phaseoli

In Rhizobium-legume symbiosis, the plant host controls and optimizes the nodulation process by autoregulation. Tn5 mutants of Rhizobium leguminosarum bv. phaseoli TAL 182 which are impaired at various stages of symbiotic development, were used to examine autoregulation in the common bean (Phaseolus vulgaris L.). Class I mutants were nonnodulating, class II mutants induced small, distinct swellings on the roots, and a class III mutant formed pink, bacterium-containing, but ineffective nodules. A purine mutant (Ade-) was nonnodulating, while a pyrimidine mutant (Ura-) formed small swellings on the roots. Amino acid mutants (Leu-, Phe-, and Cys-) formed mostly empty white nodules. Each of the mutants was used as a primary inoculant on one side of a split-root system to assess its ability to suppress secondary nodulation by the wild type on the other side. All mutants with defects in nodulation ability, regardless of the particular stage of blockage, failed to induce a suppression response from the host. Only the nodulation-competent, bacterium-containing, but ineffective class III mutant induced a suppression response similar to that induced by the wild type. Suppression was correlated with the ability of the microsymbiont to proliferate inside the nodules but not with the ability to initiate nodule formation or the ability to fix nitrogen. Thus, the presence of bacteria inside the nodules may be required for the induction of nodulation suppression in the common bean.

Efficiency of Nodule Initiation and Autoregulatory Responses in a Supernodulating Soybean Mutant

We compared the formation of nodules on the primary roots of a soybean cultivar ( Glycine max (L.) Merr. cv. Bragg) and a supernodulating mutant derivative, nts382. Inoculation with Bradyrhizobium japonicum USDA 110 at different times after seed imbibition showed that the roots acquired full susceptibility to infection only between 3 and 4 days postgermination. When the plants were inoculated with serial dilutions of a bacterial suspension, the number of nodules formed in the initially susceptible region of the roots was linearly dependent on the logarithm of the inoculum dose until an optimum dose was reached. At least 10-fold-lower doses were required to induce half-maximal nodulation responses on nts382 than on the wild type. However, at optimal doses, about six times as many nodules formed in the initially susceptible region of the roots in nts382. Since there was no appreciable difference in the apparent rates of nodule emergence, the increased efficiency of nodule initiation in the supernodulating mutant could have resulted from a lower threshold of response to bacterial symbiotic signals. Two inoculations (24 h apart) of G. max cv. Bragg revealed that there was a host-mediated regulatory response that suppressed nodulation in younger portions of the primary roots, as reported previously for other soybean cultivar- Bradyrhizobium combinations. Similar experiments with nts382 revealed a comparable suppressive response, but this response was not as pronounced as it was in the wild type. This and other results suggest that there are additional control mechanisms for nodulation that are different from the systemic autoregulatory control of nodulation altered in supernodulating mutants.