Split-Root Assays Using Trifolium subterraneum Show that Rhizobium Infection Induces a Systemic Response That Can Inhibit Nodulation of Another Invasive Rhizobium Strain

Split-root assays for studying legume-rhizobia symbioses, rhizodeposition, and belowground nitrogen transfer in legumes

Split-root assays have been used widely in studies focused on understanding the complex regulatory mechanisms in legume-rhizobia symbioses, root nitrogen rhizodeposition, and belowground nitrogen transfer, and the effects of different biotic/abiotic factors on this symbiotic interaction. This assay allows a plant to have a root system that is physically divided into two distinct sections that are both still attached to a common shoot. Thus, each root section can be treated separately to monitor local and systemic plant responses. Different techniques are used to establish split-root assemblies, including double-pot systems, divided growth pouches, elbow root assembly, twin-tube systems, a single pot or chamber with a partition in the center, and divided agar plates. This review is focused on discussing the various types of split-root assays currently used in legume-based studies, and their associated advantages and limitations. Furthermore, this review also focuses on how split-root assays have been used for studies on nitrogen rhizodeposition, belowground nitrogen transfer, systemic regulation of nodulation, and biotic and abiotic factors affecting legume-rhizobia symbioses.

Spatial heterogeneity in root litter and soil legacies differentially affect legume root traits

BACKGROUND AND AIMS

Plants affect the soil environment via litter inputs and changes in biotic communities, which feed back to subsequent plant growth. Here we investigated the individual contributions of litter and biotic communities to soil feedback effects, and plant ability to respond to spatial heterogeneity in soil legacy.

METHODS

We tested for localised and systemic responses of Trifolium repens to soil biotic and root litter legacy of seven grassland species by exposing half of a root system to control soil and the other half to specific inoculum or root litter.

RESULTS

Soil inoculation triggered a localised reduction in root length while litter locally increased root biomass independent of inoculum or litter species identity. Nodule formation was locally suppressed in response to soil conditioned by another legume (Vicia cracca) and showed a trend towards systemic reduction in response to conspecific soil. V. cracca litter also caused a systemic response with thinner roots produced in the part of the root system not directly exposed to the litter.

CONCLUSIONS

Spatial heterogeneity in root litter distribution and soil communities generate distinct local and systemic responses in root morphology and nodulation. These responses can influence plant-mutualist interactions and nutrient cycling, and should be included in plant co-existence models.

Different Pathways Act Downstream of the CEP Peptide Receptor CRA2 to Regulate Lateral Root and Nodule Development

C-TERMINALLY ENCODED PEPTIDEs (CEPs) control root system architecture in a non-cell-autonomous manner. In Medicago truncatula, MtCEP1 affects root development by increasing nodule formation and inhibiting lateral root emergence by unknown pathways. Here, we show that the MtCEP1 peptide-dependent increase in nodulation requires the symbiotic signaling pathway and ETHYLENE INSENSITIVE2 (EIN2)/SICKLE (SKL), but acts independently of SUPER NUMERIC NODULES. MtCEP1-dependent inhibition of lateral root development acts through an EIN2-independent mechanism. MtCEP1 increases nodulation by promoting rhizobial infections, the developmental competency of roots for nodulation, the formation of fused nodules, and an increase in frequency of nodule development that initiates at proto-phloem poles. These phenotypes are similar to those of the ein2/skl mutant and support that MtCEP1 modulates EIN2-dependent symbiotic responses. Accordingly, MtCEP1 counteracts the reduction in nodulation induced by increasing ethylene precursor concentrations, and an ethylene synthesis inhibitor treatment antagonizes MtCEP1 root phenotypes. MtCEP1 also inhibits the development of EIN2-dependent pseudonodule formation. Finally, mutants affecting the COMPACT ROOT ARCHITECTURE2 (CRA2) receptor, which is closely related to the Arabidopsis CEP Receptor1, are unresponsive to MtCEP1 effects on lateral root and nodule formation, suggesting that CRA2 is a CEP peptide receptor mediating both organogenesis programs. In addition, an ethylene inhibitor treatment counteracts the cra2 nodulation phenotype. These results indicate that MtCEP1 and its likely receptor, CRA2, mediate nodulation and lateral root development through different pathways.

Small-peptide signals that control root nodule number, development, and symbiosis

Many legumes have the capacity to enter into a symbiotic association with soil bacteria generically called 'rhizobia' that results in the formation of new lateral organs on roots called nodules within which the rhizobia fix atmospheric nitrogen (N). Up to 200 million tonnes of N per annum is fixed by this association. Therefore, this symbiosis plays an integral role in the N cycle and is exploited in agriculture to support the sustainable fixation of N for cropping and animal production in developing and developed nations. Root nodulation is an expendable developmental process and competency for nodulation is coupled to low-N conditions. Both nodule initiation and development is suppressed under high-N conditions. Although root nodule formation enables sufficient N to be fixed for legumes to grow under N-deficient conditions, the carbon cost is high and nodule number is tightly regulated by local and systemic mechanisms. How legumes co-ordinate nodule formation with the other main organs of nutrient acquisition, lateral roots, is not fully understood. Independent mechanisms appear to regulate lateral roots and nodules under low- and high-N regimes. Recently, several signalling peptides have been implicated in the local and systemic regulation of nodule and lateral root formation. Other peptide classes control the symbiotic interaction of rhizobia with the host. This review focuses on the roles played by signalling peptides during the early stages of root nodule formation, in the control of nodule number, and in the establishment of symbiosis. Here, we highlight the latest findings and the gaps in our understanding of these processes.

Multiple Autoregulation of Nodulation (AON) Signals Identified through Split Root Analysis of Medicago truncatula sunn and rdn1 Mutants

Nodulation is energetically costly to the host: legumes balance the nitrogen demand with the energy expense by limiting the number of nodules through long-distance signaling. A split root system was used to investigate systemic autoregulation of nodulation (AON) in Medicago truncatula and the role of the AON genes RDN1 and SUNN in the regulatory circuit. Developing nodule primordia did not trigger AON in plants carrying mutations in RDN1 and SUNN genes, while wild type plants had fully induced AON within three days. However, despite lacking an early suppression response, AON mutants suppressed nodulation when roots were inoculated 10 days or more apart, correlated with the maturation of nitrogen fixing nodules. In addition to correlation between nitrogen fixation and suppression of nodulation, suppression by extreme nutrient stress was also observed in all genotypes and may be a component of the observed response due to the conditions of the assay. These results suggest there is more than one systemic regulatory circuit controlling nodulation in M. truncatula. While both signals are present in wild type plants, the second signal can only be observed in plants lacking the early repression (AON mutants). RDN1 and SUNN are not essential for response to the later signal.

Split-root systems applied to the study of the legume-rhizobial symbiosis: what have we learned?

Split-root system (SRS) approaches allow the differential treatment of separate and independent root systems, while sharing a common aerial part. As such, SRS is a useful tool for the discrimination of systemic (shoot origin) versus local (root/nodule origin) regulation mechanisms. This type of approach is particularly useful when studying the complex regulatory mechanisms governing the symbiosis established between legumes and Rhizobium bacteria. The current work provides an overview of the main insights gained from the application of SRS approaches to understand how nodule number (nodulation autoregulation) and nitrogen fixation are controlled both under non-stressful conditions and in response to a variety of stresses. Nodule number appears to be mainly controlled at the systemic level through a signal which is produced by nodule/root tissue, translocated to the shoot, and transmitted back to the root system, involving shoot Leu-rich repeat receptor-like kinases. In contrast, both local and systemic mechanisms have been shown to operate for the regulation of nitrogenase activity in nodules. Under drought and heavy metal stress, the regulation is mostly local, whereas the application of exogenous nitrogen seems to exert a regulation of nitrogen fixation both at the local and systemic levels.

The peptide-encoding CEP1 gene modulates lateral root and nodule numbers in Medicago truncatula

The role of MtCEP1, a member of the CEP (C-terminally encoded peptide) signaling peptide family, was examined in Medicago truncatula root development. MtCEP1 was expressed in root tips, vascular tissue, and young lateral organs, and was up-regulated by low nitrogen levels and, independently, by elevated CO2. Overexpressing MtCEP1 or applying MtCEP1 peptide to roots elicited developmental phenotypes: inhibition of lateral root formation, enhancement of nodulation, and the induction of periodic circumferential root swellings, which arose from cortical, epidermal, and pericycle cell divisions and featured an additional cortical cell layer. MtCEP peptide addition to other legume species induced similar phenotypes. The enhancement of nodulation by MtCEP1 is partially tolerant to high nitrate, which normally strongly suppresses nodulation. These nodules develop faster, are larger, and fix more nitrogen in the absence and presence of inhibiting nitrate levels. At 25mM nitrate, nodules formed on pre-existing swelling sites induced by MtCEP1 overexpression. RNA interference-mediated silencing of several MtCEP genes revealed a negative correlation between transcript levels of MtCEP1 and MtCEP2 with the number of lateral roots. MtCEP1 peptide-dependent phenotypes were abolished or attenuated by altering or deleting key residues in its 15 amino acid domain. RNA-Seq analysis revealed that 89 and 116 genes were significantly up- and down-regulated, respectively, by MtCEP1 overexpression, including transcription factors WRKY, bZIP, ERF, and MYB, homologues of LOB29, SUPERROOT2, and BABY BOOM. Taken together, the data suggest that the MtCEP1 peptide modulates lateral root and nodule development in M. truncatula.

Nitrogen modulation of legume root architecture signaling pathways involves phytohormones and small regulatory molecules

Nitrogen, particularly nitrate is an important yield determinant for crops. However, current agricultural practice with excessive fertilizer usage has detrimental effects on the environment. Therefore, legumes have been suggested as a sustainable alternative for replenishing soil nitrogen. Legumes can uniquely form nitrogen-fixing nodules through symbiotic interaction with specialized soil bacteria. Legumes possess a highly plastic root system which modulates its architecture according to the nitrogen availability in the soil. Understanding how legumes regulate root development in response to nitrogen availability is an important step to improving root architecture. The nitrogen-mediated root development pathway starts with sensing soil nitrogen level followed by subsequent signal transduction pathways involving phytohormones, microRNAs and regulatory peptides that collectively modulate the growth and shape of the root system. This review focuses on the current understanding of nitrogen-mediated legume root architecture including local and systemic regulations by different N-sources and the modulations by phytohormones and small regulatory molecules.

Too much love, a novel Kelch repeat-containing F-box protein, functions in the long-distance regulation of the legume-Rhizobium symbiosis

The interaction of legumes with N2-fixing bacteria collectively called rhizobia results in root nodule development. The number of nodules formed is tightly restricted through the systemic negative feedback control by the host called autoregulation of nodulation (AON). Here, we report the characterization and gene identification of TOO MUCH LOVE (TML), a root factor that acts during AON in a model legume Lotus japonicus. In our genetic analyses using another root-regulated hypernodulation mutant, plenty, the tml-1 plenty double mutant showed additive effects on the nodule number, whereas the tml-1 har1-7 double mutant did not, suggesting that TML and PLENTY act in different genetic pathways and that TML and HAR1 act in the same genetic pathway. The systemic suppression of nodule formation by CLE-RS1/RS2 overexpression was not observed in the tml mutant background, indicating that TML acts downstream of CLE-RS1/RS2. The tml-1 Snf2 double mutant developed an excessive number of spontaneous nodules, indicating that TML inhibits nodule organogenesis. Together with the determination of the deleted regions in tml-1/-2/-3, the fine mapping of tml-4 and the next-generation sequencing analysis, we identified a nonsense mutation in the Kelch repeat-containing F-box protein. As the gene knockdown of the candidate drastically increased the number of nodules, we concluded that it should be the causative gene. An expression analysis revealed that TML is a root-specific gene. In addition, the activity of ProTML-GUS was constitutively detected in the root tip and in the nodules/nodule primordia upon rhizobial infection. In conclusion, TML is a root factor acting at the final stage of AON.

Simple and efficient methods to generate split roots and grafted plants useful for long-distance signaling studies in Medicago truncatula and other small plants

BACKGROUND

Long distance signaling is a common phenomenon in animal and plant development. In plants, lateral organs such as nodules and lateral roots are developmentally regulated by root-to-shoot and shoot-to-root long distance signaling. Grafting and split root experiments have been used in the past to study the systemic long distance effect of endogenous and environmental factors, however the potential of these techniques has not been fully realized because data replicates are often limited due to cumbersome and difficult approaches and many plant species with soft tissue are difficult to work with. Hence, developing simple and efficient methods for grafting and split root inoculation in these plants is of great importance.

RESULTS

We report a split root inoculation system for the small legume M. truncatula as well as robust and reliable techniques of inverted-Y grafting and reciprocal grafting. Although the split root technique has been historically used for a variety of experimental purposes, we made it simple, efficient and reproducible for M. truncatula. Using our split root experiments, we showed the systemic long distance suppression of nodulation on a second wild type root inoculated after a delay, as well as the lack of this suppression in mutants defective in autoregulation. We demonstrated inverted-Y grafting as a method to generate plants having two different root genotypes. We confirmed that our grafting method does not affect the normal growth and development of the inserted root; the composite plants maintained normal root morphology and anatomy. Shoot-to-root reciprocal grafts were efficiently made with a modification of this technique and, like standard grafts, demonstrate that the regulatory signal defective in rdn1 mutants acts in the root.

CONCLUSIONS

Our split root inoculation protocol shows marked improvement over existing methods in the number and quality of the roots produced. The dual functions of the inverted-Y grafting approach are demonstrated: it is a useful system to produce a plant having roots of two different genotypes and is also more efficient than published shoot-to-root reciprocal grafting techniques. Both techniques together allow dissection of long distance plant developmental regulation with very simple, efficient and reproducible approaches.

Never too many? How legumes control nodule numbers

Restricted availability of nitrogen compounds in soils is often a major limiting factor for plant growth and productivity. Legumes circumvent this problem by establishing a symbiosis with soil-borne bacteria, called rhizobia that fix nitrogen for the plant. Nitrogen fixation and nutrient exchange take place in specialized root organs, the nodules, which are formed by a coordinated and controlled process that combines bacterial infection and organ formation. Because nodule formation and nitrogen fixation are energy-consuming processes, legumes develop the minimal number of nodules required to ensure optimal growth. To this end, several mechanisms have evolved that adapt nodule formation and nitrogen fixation to the plant's needs and environmental conditions, such as nitrate availability in the soil. In this review, we give an updated view on the mechanisms that control nodulation.

Lipopolysaccharides in Rhizobium-legume symbioses

The establishment of nitrogen-fixing symbiosis between a legume plant and its rhizobial symbiont requires that the bacterium adapt to changing conditions that occur with the host plant that both promotes and allows infection of the host root nodule cell, regulates and resists the host defense response, permits the exchange of metabolites, and contributes to the overall health of the host. This adaptive process involves changes to the bacterial cell surface and, therefore, structural modifications to the lipopolysaccharide (LPS). In this chapter, we describe the structures of the LPSs from symbiont members of the Rhizobiales, the genetics and mechanism of their biosynthesis, the modifications that occur during symbiosis, and their possible functions.

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.

Split-root study of autoregulation of nodulation in the model legume Lotus japonicus

We used a split-root system to determine the timing for induction of the autoregulation of nodulation (AUT) in Lotus japonicus (Regel) Larsen after inoculation with Mesorhizobium loti. The signal took at least five days for full induction of AUT and inhibition of infection thread formation. Strain ML108 (able to nodulate but unable to fix nitrogen) induced full AUT, but ML101 (unable to nodulate or to fix nitrogen) did not induce autoregulation. These results indicate that Nod factor-producing strains induce AUT, but that the nitrogen fixed by rhizobia and supplied to the plant as ammonia does not elicit the AUT in L. japonicus.

Legume nodulation: successful symbiosis through short- and long-distance signalling

Nodulation in legumes provides a major conduit of available nitrogen into the biosphere. The development of nitrogen-fixing nodules results from a symbiotic interaction between soil bacteria, commonly called rhizobia, and legume plants. Molecular genetic analysis in both model and agriculturally important legume species has resulted in the identification of a variety of genes that are essential for the establishment, maintenance and regulation of this symbiosis. Autoregulation of nodulation (AON) is a major internal process by which nodule numbers are controlled through prior nodulation events. Characterisation of AON-deficient mutants has revealed a novel systemic signal transduction pathway controlled by a receptor-like kinase. This review reports our present level of understanding on the short- and long-distance signalling networks controlling early nodulation events and AON.

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.

Methods to evaluate nodulation competitiveness between Sinorhizobium meliloti strains using melanin production as a marker

Three methods to evaluate the relative ability of different strains of Sinorhizobium meliloti to occupy nodules formed on alfalfa after co-inoculation were compare in this study. Results obtained using the three methods of evaluation together, provided insight into the relative nodulation competitiveness between two given sinorhizobial strains. A simple visual phenotypic marker, i.e., melanin production was used to distinguish individual strains in a given assay. As such, melanin producing strains were compared with melanin non-producing strains throughout this study. Method 1 required the use of an ELISA plate, took 35 min for the analysis of 40 nodules, and allowed strain identification by melanin production 2 days after nodule harvest. Method 2 required 3 h for the analysis of 40 nodules, used an ELISA plate, growth of bacteria on Petri dishes, and melanin production was analysed after 48 h of cell culture. Finally, method 3 involved the whole nodulated plant root, required less material than the above methods, and results were obtained after 24 h. Only method 2 was useful in determining if both a melanin producing strain and a melanin non-producing strain had occupied an individual nodule. Each of the three methods represented a rapid way of studying strain competition for field studies, using a natural trait as a marker.

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.

Preference in the nodulation ofPhaseolus vulgariscv. RAB39. II. Effect of delayed inoculation or low cell representation in the inoculant on nodule occupancy byRhizobium tropiciUMR1899

Common bean (Phaseolus vulgaris L.) is a traditional crop in much of Latin America, where it is often planted into soils containing numerous, sometimes ineffective, indigenous rhizobia. The presence of these indigenous organisms can limit response to inoculation. Because of this, we have sought bean cultivars that will nodulate preferentially with the inoculant strain, and have previously reported on the preference between the bean cultivar RAB39 and strains of Rhizobium tropici. We have detailed this interaction using the inoculant-quality strain UMR1899. In the present study the root tip marking (RTM) technique was used to demonstrate that this preference in nodulation was evident, even when inoculation with UMR1899 was delayed up to 8 relative to that with Rhizobium etli UMR1632. In contrast to studies with other legumes, roots of RAB39 were not predisposed to nodulate with UMR1632, even though preexposed to this strain for considerable periods of time. The presence of UMR1899 actually reduced nodulation by UMR1632 substantially, even when inoculation with UMR1899 was significantly delayed. When UMR1899 and UMR1632 were applied to separate halves of a split-root system, the number of nodules on the side receiving UMR1632 was less than for the half root inoculated with UMR1899, but the differences were not significant. This suggests that the preference response is not systemic but requires proximity between the strains involved. UMR1899 produced more than 50% of the nodules even when the ratio of UMR1632:UMR1899 in the inoculant was 10:1. The results are further evidence of a stable and marked preference of RAB39 for UMR1899, which warrants a more detailed study at the field level.Key words: Phaseolus vulgaris L., common bean, delayed inoculation, strain preference, cell proportions.

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.

Growth and Movement of Spot Inoculated Rhizobium meliloti on the Root Surface of Alfalfa

Inoculum droplets of approximately 10 nanoliter volume and containing about 10 Rhizobium meliloti cells were placed onto the root surface of alfalfa seedlings in plastic growth pouches at either the root tip, the position of the smallest emergent root hairs, or at a site midway between these points. The droplets were initially confined to an area of about 0.2 square millimeter at the point of application. By 48 and 96 hours after inoculation, the inoculum bacteria and their progeny were distributed over several centimeters of the root between the initial site of deposition and the growing root tip, reaching densities of 10(3) to 10(4) bacteria per centimeter near the site of initial deposition and decreasing exponentially from that point toward the root tip. Graphite particles deposited on the root surface close to the growing tip were similarly distributed along the root length by 48 and 96 hours, suggesting that passive displacement by root cell elongation was primarily responsible for the spread of bacteria. A nonmotile mutant of R. meliloti colonized alfalfa roots to the same extent as the wild type and was usually distributed in the same manner, indicating that bacterial motility contributed little under these conditions to long distance spread of the bacteria. However, when applied in low numbers, R. meliloti mutants defective in motility or chemotaxis were considerably less efficient in initiating nodules near the point of inoculation than the wild type. This implies that motility and/or chemotaxis contribute significantly to local exploration for suitable infection sites. Almost all nodules on the primary root formed within a few millimeters of the spot-inoculation site, indicating that, under our experimental conditions, movement and multiplication of R. meliloti on the root surface were not sufficient to maintain an adequate population in the infectible region of the root during root growth.

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.

nodT, a positively-acting cultivar specificity determinant controlling nodulation of Trifolium subterraneum by Rhizobium leguminosarum biovar trifolii

Rhizobium leguminosarum biovar trifolii strain TA1 nodulates a range of Trifolium plants including red, white and subterranean clovers. Nitrogen-fixing nodules are promptly initiated on the tap roots of these plants at the site of inoculation. In contrast to these associations, strain TA1 has a 'Nod-' phenotype on a particular cultivar of subterranean clover called Woogenellup (A.H. Gibson, Aust J Agric Sci 19: (1968) 907-918) where it induces rare, poorly developed, slow-to-appear and ineffective lateral root nodules. By comparing the nodulation gene region of strain TA1 with that of another R. leguminosarum bv. trifolii strain ANU843, which is capable of efficiently nodulating cv. Woogenellup, we have shown that the nodT gene (B.P. Surin et al., Mol Microbiol 4: (1990) 245-252) is essential for nodulation on cv. Woogenellup. The nodT gene is naturally absent in strain TA1. A cosmid clone spanning the entire nodulation gene region of strain TA1 was capable of conferring nodulation ability to R.l. bv. trifolii strains deleted for nodulation genes, but only on cultivars of subterranean clovers nodulated by strain TA1. This shows that cultivar recognition events are, in part, determined by genes in the nodulation region of strain TA1. Complementation studies also indicated that strain TA1 contains negatively-acting genes located on the Sym plasmid and elsewhere, which specifically block nodulation of cv. Woogenellup.

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

The formation of first nodules inhibits subsequent nodulation in younger regions of alfalfa (Medicago sativa L.) roots by a feedback regulatory mechanism that controls nodule number systemically (G Caetano-Anollés, WD Bauer [1988] Planta 175: 546-557). Following inoculation with wild-type Rhizobium meliloti, almost all infections associated with cortical cell division developed into mature nodules. While the distribution of Rhizobium- induced cell divisions closely paralleled the distribution of first emergent nodules, only 9 to 15% of total cell division foci failed to become functional nodules. Nodule formation was restricted to the primary root when plants were inoculated before lateral root emergence. Excision of these primary root nodules allowed nodules to reappear in lateral roots clustered around the location of the root tip at the time of nodule removal. Apparently, this region regained susceptibility to infection within the first hours after excision of primary nodules and suppression of nodulation was restored a day later probably due to the development of new infection foci. Our results suggest that alfalfa controls nodulation during the onset of cell division in the root cortex and not during infection development as in soybean.

Early autoregulation of symbiotic root nodulation in soybeans

Autoregulation of symbiotic root nodulation in soybean seedlings (Glycine max L. Merrill cv Pride 216) was studied following double inoculation of primary roots with Bradyrhizobium japonicum 110. When the second inoculation was given 10 or 17 hours after the first, the nodulation in the first-inoculated region of the root was suppressed. The effect was eliminated if B. japonicum 110 containing Tn5 insertions in the ;common' nod ABC genes was used for the second inoculation, indicating the requirement for changes in the root mediated by these bacterial genes. When the root cortex in the suppressed basal region was examined 3 days after inoculation, cell division centers were present in numbers not significantly different from the numbers in control roots given a sham second inoculation; their size distribution, however, showed a failure of enlargement compared with controls.

Rhizobium meliloti exopolysaccharide Mutants Elicit Feedback Regulation of Nodule Formation in Alfalfa

Nodule formation by wild-type Rhizobium meliloti is strongly suppressed in younger parts of alfalfa (Medicago sativum L.) root systems as a feedback response to development of the first nodules (G Caetano-Anollés, WD Bauer [1988] Planta 175: 546-557). Mutants of R. meliloti deficient in exopolysaccharide synthesis can induce the formation of organized nodular structures (pseudonodules) on alfalfa roots but are defective in their ability to invade and multiply within host tissues. The formation of empty pseudonodules by exo mutants was found to elicit a feedback suppression of nodule formation similar to that elicited by the wild-type bacteria. Inoculation of an exo mutant onto one side of a split-root system 24 hours before inoculation of the second side with wild-type cells suppressed wild-type nodule formation on the second side in proportion to the extent of pseudonodule formation by the exo mutants. The formation of pseudonodules is thus sufficient to elicit systemic feedback control of nodulation in the host root system: infection thread development and internal proliferation of the bacteria are not required for elicitation of feedback. Pseudonodule formation by the exo mutants was found to be strongly suppressed in split-root systems by prior inoculation on the opposite side with the wild type. Thus, feedback control elicited by the wild-type inhibits Rhizobium-induced redifferentiation of host root cells.

Growth and Nodulation Responses of Rhizobium meliloti to Water Stress Induced by Permeating and Nonpermeating Solutes

Isolates of Rhizobium meliloti , representing antigenically distinct indigenous serogroups 31 and 17, were grown in yeast extract-mannitol broth (YEM) containing NaCl or polyethylene glycol (PEG) to provide external water potentials ranging from −0.15 to −1.5 MPa. Several differences were found between representatives of the two groups in their abilities to adapt to water stress induced by the nonpermeating solute PEG. At potentials below −0.5 MPa, strain 31 had a lower specific growth rate than strain 17 and an irregular cell morphology. In contrast, neither growth nor cell morphology of either strain was affected significantly over the same range of water potentials created by a permeating solute, NaCl. Despite the superior growth of strain 17 at the low water potentials imposed by PEG, upshock of water-stressed cells (−1.0 MPa; PEG) into normal YEM (−0.15 MPa) resulted in a faster recovery of growth by strain 31 than by strain 17. Different responses of the two strains to a water potential increase were also revealed in nodulation studies. Strain 31 required significantly fewer days to nodulate alfalfa than strain 17 did when the strains were transferred from YEM with PEG at −1.0 MPa onto the roots of alfalfa seedlings in plant growth medium (−0.1 MPa). The addition of supplemental calcium (0.1 mM) to growth medium with PEG (−1.0 MPa) reduced the differences between strains in their responses to water stress. The severe growth restriction and morphological abnormalities shown by strain 31 were corrected, and the prolonged recovery time shown by water-stressed cells (−1.0 MPa; PEG) of strain 17 upon transfer to normal YEM was shortened. The latter strain also nodulated earlier and more rapidly after growth in PEG medium at −1.0 MPa in the presence of supplemental calcium ions. These results indicate that the efficacy of osmoregulation can vary among strains of the same species and that the mechanism of osmoregulation may differ depending on the nature of the water stress.

Nodule formation is stimulated by the ethylene inhibitor aminoethoxyvinylglycine

Previous researchers found that formation and function of nitrogen-fixing nodules on legume roots were severely inhibited by addition of exogenous ethylene. Nodule formation by Rhizobium meliloti on Medicago sativa was stimulated twofold when the ethylene biosynthesis inhibitor aminoethoxyvinylglycine (AVG) was added with the inoculum. Stimulation of nodule formation by AVG showed a similar concentration dependence as inhibition of ethylene biosynthesis, suggesting that the primary action of AVG is the inhibition of ethylene biosynthesis. When AVG was added 2 to 3 days after inoculation, the number of nodules formed was still increased. On a per plant basis, however, the average nitrogen fixation was unchanged by AVG treatment and was independent of nodule number.

Nodulation of Glycine max by Six Bradyrhizobium japonicum Strains with Different Competitive Abilities

The root nodule locations of six Bradyrhizobium japonicum strains were examined to determine if there were any differences which might explain their varying competitiveness for nodule occupancy on Glycine max. When five strains were added to soybeans in plastic growth pouches in equal proportions with a reference strain (U.S. Department of Agriculture, strain 110), North Carolina strain 1028 and strain 110 were the most competitive for nodule occupancy, followed by U.S. Department of Agriculture strains 122, 76, and 31 and Brazil strain 587. Among all strains, nodule double occupancy was 17% at a high inoculum level (10 7 CFU pouch −1 ) and 2% at a low inoculum level (10 4 CFU pouch −1 ). The less competitive strains increased their nodule representation by an increase in the doubly occupied nodules at the high inoculum level. Among all strains, the number of taproot and lateral root nodules was inversely related at both the high and low inoculum levels ( r = −0.62 and −0.69, respectively; P = 0.0001). This inverse relationship appeared to be a result of the plant host control of bacterial infection. Among each of the six strains, greater than 95% of the taproot nodules formed at the high inoculum density were located on 25% of the taproot length, the nodules centering on the position of the root tip at the time of inoculation. No differences among the six strains were observed in nodule initiation rates as measured by taproot nodule position. Taproot nodules were formed in the symbiosis before lateral root nodules. One of the poorly competitive strains (strain 76) occupied three times as many taproot nodules as lateral root nodules when competing with strain 110 (nodules were harvested from 4-week-old plants). Among these six wild-type strains of B. japonicum, competitive ability evidently is not related to nodule initiation rates.

Competition by Bradyrhizobium Strains for Nodulation of the Nonlegume Parasponia andersonii

Bradyrhizobium strains isolated from the nonlegume Parasponia spp. formed a group of strains that were highly competitive for nodulation of P. andersonii when paired with strains isolated from legumes. Strains from legumes, including those of similar effectiveness to NGR231 and CP283, were not able to form nodules as single occupants on P. andersonii in the presence of Parasponia strains. However, NGR86, an isolate from Macroptilium lathyroides , jointly occupied one-third of the nodules formed with each of the three strains isolated from Parasponia spp. Time taken for nodules to appear may have influenced the outcome of competition, since CP283 and all isolates from legumes were slow to nodulate P. andersonii . Among the Parasponia strains, competitiveness for nodulation of P. andersonii was not associated with effectiveness of nitrogen fixation. The highly effective strain CP299 was a poor competitor when paired with the least effective strain NGR231. CP283 was the least competitive of the Parasponia strains but was still able to dominate nodules when paired with legume isolates. Dual occupancy was high, up to 67% when the inoculum contained CP299 and CP273. Both the Muc + and Muc - types of CP283 form a symbiosis of similar effectiveness and were similarly competitive at high inoculation densities, but the Muc - form was more competitive at low inoculum densities. Both forms frequently occupied the same nodule. Bradyrhizobium strains isolated from Parasponia spp. may have specific genetic information that favor their ability to competitively and effectively infect plants in the genus Parasponia (Ulmaceae) outside the Leguminosae.

Feedback regulation of nodule formation in alfalfa

When high dosages of wild-type Rhizobium meliloti RCR2011 were inoculated at two different times, 24 h apart, onto either the primary roots of alfalfa (Medicago sativa L.) seedlings or onto lateral roots on opposite sides of a split-root system, the number of nodules generated by the second inoculum was much smaller than the number generated by the first inoculum. These results provide evidence that alfalfa has an active, systemic mechanism for feedback control of nodulation. Non-nodulating mutants and delayed, weakly nodulating mutants did not elicit a discernable suppression of nodulation by subsequently inoculated wild-type cells. An appreciable number of Rhizobium infections thus seem required to elicit the suppressive response. Mutants in nodulation regions IIb and IIa nodulated extensively in the initially susceptible region of the root, but nodule initiation by these mutants was 100-1000 times less efficient, respectively, than the parent. Nodules formed by these mutants emerged 1 d later than normal. The IIb mutants elicited a relatively strong suppression of nodulation in younger parts of the root, but region-IIa mutants elicited only a weak response. These results indicate that elicitation of the regulatory response need not be proportional to nodule formation and imply that genes in region IIa play an important role in elicitation. At high dosages, the region-II mutants induced the development of thick, short roots in a considerably higher percentage of plants than the wild-type bacteria. Nodules generated by wild-type isolates and region-II mutants did not emerge in strict acropetal sequence, probably because some infections developed more slowly than others. Prior exposure of the root to non-nodulating mutants resulted in nodulation by the parent in regions of the root otherwise too mature to be susceptible, indicating that exposure to these mutants may affect the sequence of root development.

Enhanced nodule initiation on alfalfa by wild-typeRhizobium meliloti co-inoculated withnod gene mutants and other bacteria

Nodule formation on alfalfa (Medicago sativa L.) roots was determined at different inoculum dosages for wild-typeRhizobium meliloti strain RCR2011 and for various mutant derivatives with altered nodulation behavior. The number of nodules formed on the whole length of the primary roots was essentially constant regardless of initial inoculum dosage or subsequent bacterial multiplication, indicative of homeostatic regulation of total nodule number. In contrast, the number of nodules formed in just the initially susceptible region of these roots was sigmoidally dependent on the number of wild-type bacteria added, increasing rapidly at dosages above 5·10(3) bacteria/plant. This behavior indicates the possible existence of a threshold barrier to nodule initiation in the host which the bacteria must overcome. When low dosages of the parent (10(3) cells/plant) were co-inoculated with 10(6) cells/plant of mutants lacking functionalnodA, nodC, nodE, nodF ornodH genes, nodule initiation was increased 10- to 30-fold. Analysis of nodule occupancy indicated that these mutants were able to help the parent (wild-type) strain initiate nodules without themselves occupying the nodules. Co-inoculation withR. trifolii orAgrobacterium tumefaciens cured of its Ti plasmid also markedly stimulated nodule initiation by theR. meliloti parent strain. Introduction of a segment of the symbiotic megaplasmid fromR. meliloti intoA. tumefaciens abolished this stimulation.Bradyrhizobium japonicum and a chromosomal Tn5 nod(-) mutant ofR. meliloti did not significantly stimulate nodule initiation when co-inoculated with wild-typeR. meliloti. These results indicate that certainnod gene mutants and members of theRhizobiaceae may produce extracellular "signals" that supplement the ability of wild-typeR. meliloti cells to induce crucial responses in the host.

Induction of pathogenic-like responses in the legume Macroptilium atropurpureum by a transposon-induced mutant of the fast-growing, broad-host-range Rhizobium strain NGR234

Mutant strain ANU2861, a transposon Tn5 mutant of the fast-growing, broad-host-range Rhizobium strain ANU280 (NGR234 Smr Rfr) overproduces polysaccharide, is an ade auxotroph, and induces poorly developed nodules on Leucaena leucocephala and Lablab purpureus (H.C. Chen, M. Batley, J.W. Redmond, and B.G. Rolfe, J. Plant Physiol. 120:331-349, 1985). Strain ANU2861 cannot form nodules on Macroptilium atropurpureum Urb. (siratro) or on Desmodium intortum and D. uncinatum and the nonlegume Parasponia. The parent strain, ANU280, effectively nodulates all these legume species except Parasponia, on which it forms ineffective nodules. Ultrastructural examination of infection sites on the legume siratro showed that mutant strain ANU2861 caused root hair curling (Hac+ phenotype), some cortical cell division (Noi+), but no infection threads (Inf-). Localized cellular responses, known to occur in phytopathological interactions, were observed in electron micrographs of the epidermal tissue at or near the infection zone after inoculation with strain ANU2861 but not the wild-type parental strain. These include (i) the rapid (within 20 h) accumulation of osmiophilic droplets attached to membranes at potential sites of strain ANU2861 penetration and (after 48 h) in the epidermal cells in the immediate region of the curled root hairs, and (ii) localized cell death of the epidermal cells. In addition, strain ANU2861 can initiate a systemic response in split-root siratro plants which prevents the successful nodulation of strain ANU280. A 6.3-kilobase fragment of wild-type genomic DNA, which includes the site of Tn5 insertion in strain ANU2861, was cloned and introduced to strain ANU2861. All the phenotypic defects of the mutant strain were corrected by the introduction of this DNA fragment. This indicates that the original Tn5 insertion is responsible for the phenotype.