Root exuded nod-gene inducing signals limit the nodulation capacity of different alfalfa varieties with Rhizobium meliloti

Summary Different nodulation capacities were found among nine different varieties of alfalfa, cultivated in the Central region of Mexico, by Rhizobium meliloti 2011. A correlation between nodulation capacity and foliar dry weight was observed, which points to a genotype dependance on these parameters. A correlation between the nodulation capacity and the R. meliloti nod-gene inducing activity of the root exudates from the different varieties, as measured by -galactosidase induction in a test system consisting of a R. meliloti nodC-lacZ strain incubated with each root exudate, was established. When the root exudate from the best nodulating variety was added to the four poorest nodulating varieties, an increase in nodule formation was observed. We conclude that root exuded nod-gene inducing signals are a symbiotically-limiting component in natural populations of the poorest nodulating varieties of alfalfa.     

Sinorhizobium meliloti chemotaxis to quaternary ammonium compounds is mediated by the chemoreceptor McpX

The bacterium Sinorhizobium meliloti is attracted to seed exudates of its host plant alfalfa (Medicago sativa). Since quaternary ammonium compounds (QACs) are exuded by germinating seeds, we assayed chemotaxis of S. meliloti towards betonicine, choline, glycine betaine, stachydrine and trigonelline. The wild type displayed a positive response to all QACs. Using LC–MS, we determined that each germinating alfalfa seed exuded QACs in the nanogram range. Compared to the closely related nonhost species, spotted medic (Medicago arabica), unique profiles were released. Further assessments of single chemoreceptor deletion strains revealed that an mcpX deletion strain displayed little to no response to these compounds. Differential scanning fluorimetry showed interaction of the isolated periplasmic region of McpX (McpX^PR and McpX_34‐306) with QACs. Isothermal titration calorimetry experiments revealed tight binding to McpX^PR with dissociation constants (K_d) in the nanomolar range for choline and glycine betaine, micromolar K_d for stachydrine and trigonelline and a K_d in the millimolar range for betonicine. Our discovery of S. meliloti chemotaxis to plant‐derived QACs adds another role to this group of compounds, which are known to serve as nutrient sources, osmoprotectants and cell‐to‐cell signalling molecules. This is the first report of a chemoreceptor that mediates QACs taxis through direct binding.     

Charcoal and shrubs modify soil processes in ponderosa pine forests of western Montana

In split-root systems of alfalfa (Medicago sativa L.), already existing nodules or arbuscular mycorrhizal roots suppress further establishment of symbiosis in other root parts, a phenomenon named autoregulation. Roots treated with rhizobial nodulation signals (Nod factors) induce a similar systemic suppression of symbiosis.In order to test the hypothesis that flavonoids play a role in this systemic suppression, split-root systems of alfalfa plants were inoculated on one side of the split-root system with Sinorhizobium meliloti or Glomus mosseae or were treated with Nod factor. HPLC-analysis of alfalfa root extracts from both sides of the split-root system revealed a persistent local and systemic accumulation pattern of some flavonoids associated with the different treatments. The two flavonoids, formononetin and ononin, could be identified to be similarily altered after rhizobial or mycorrhizal inoculation or when treated with Nod factor.Exogenous application of formononetin and ononin partially restored nodulation and mycorrhization pointing towards the involvement of these two secondary compounds in the autoregulation of both symbioses.     

Breeding for better symbiosis

The present review gives a critical assessment of the literature dealing with symbiosis between rhizobia and legumes and between AM fungi and most plants. Associative N₂ fixation (even though strictly speaking not a symbiotic relationship) does have some characteristics of symbiosis due to mutualistic dependence and usefulness of the relationship, and is therefore covered in this review. Nodulation in the rhizobia–legume symbiosis may be limited by an insufficient amount of the nod-gene inducers released from seed and/or roots. However, there is genotypic variation in the germplasm of legume species in all components of the signalling pathway, suggesting a prospect for improving nodulation by selecting and/or transforming legume genotypes for increased exudation of flavonoids and other signalling compounds. Deciphering chromosomal location as well as cloning nod, nif and other genes important in nodulation and N₂ fixation will allow manipulation of the presence and expression of these genes to enhance the symbiotic relationship. Increased efficacy of symbiotic N₂ fixation can be achieved by selecting not only the best host genotypes but by selecting the best combination of host genotype and nodule bacteria. As flavonoids exuded by legume seedlings may not only be nod-gene inducers, but also stimulants for hyphal growth of the AM fungi, selecting and/or transforming plants to increase exudation of these flavonoids may result in a double benefit for mycorrhizal legumes. Mutants unable to sustain mycorrhizal colonisation are instrumental in understanding the colonisation process, which may ultimately pay off in breeding for the more effective symbiosis. In conclusion, targeted efforts to breed genotypes for improved N₂ fixation and mycorrhizal symbiosis will bring benefits in increased yields of crops under a wide range of environmental conditions and will contribute toward sustainability of agricultural ecosystems in which soil-plant-microbe interactions will be better exploited.     

Chemotaxis of Rhizobium meliloti towards Nodulation Gene-Inducing Compounds from Alfalfa Roots

Luteolin, a flavone present in seed exudates of alfalfa, induces nodulation genes (nod) in Rhizobium meliloti and also serves as a biochemically specific chemoattractant for the bacterium. The present work shows that R. meliloti RCR2011 is capable of very similar chemotactic responses towards 4′,7-dihydroxyflavone, 4′,7-Dihydroxyflavanone, and 4,4′-dihydroxy-2-methoxychalcone, the three principal nod gene inducers secreted by alfalfa roots. Chemotactic responses to the root-secreted nod inducers in capillary assays were usually two- to four-fold above background and, for the flavone and flavonone, occurred at concentrations lower than those required for half-maximal induction of the nodABC genes. Complementation experiments indicated that the lack of chemotactic responsiveness to luteolin seen in nodD1 and nodA mutants of R. meliloti was not due to mutations in the nod genes, as previously thought. Thus, while nod gene induction and flavonoid chemotaxis have the same biochemical specificity, these two functions appear to have independent receptors or transduction pathways. The wild-type strain was found to suffer selective, spontaneous loss of chemotaxis towards flavonoids during laboratory subculture.     

Microscopic studies of cell divisions induced in alfalfa roots by Rhizobium meliloti

We have used spot-inoculation and new cytological procedures to observe the earliest events stimulated in alfalfa (Medicago sativa L.) roots by Rhizobium meliloti. Roots were inoculated with 1–10 nl of concentrated bacteria, fixed in paraformaldehyde, and after embedding and sectioning stained with a combination of acridine orange and DAPI (4-6-diamidino-2-phenylindole hydrochloride). Normal R. meliloti provoke cell dedifferentiation and mitosis in the inner cortex of the root within 21–24 h after inoculation. This activation of root cells spreads progressively, leading to nodule formation. In contrast, the R. meliloti nodA and nodC mutants do not stimulate any activation or mitosis. Thus the primary and earliest effect of Rhizobium nod gene action is plant cellular activation. A rapid, whole-mount visualization by lactic acid shows that the pattern of nodule form varies widely. Some R. meliloti strains were found to be capable of stimulating on alfalfa roots both normal nodules and a hybrid structure intermediate between a nodule and a lateral root.     

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 5·10³ 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 (10³ cells/plant) were co-inoculated with 10⁶ 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.Abbreviations EH emergent root hairs - kb kilobase - RDU relative distance unit - RT root tip This is journal article No. 188-87 of the Ohio Agricultural Research and Development Center     

Adsorption ofRhizobium meliloti to alfalfa roots: Dependence on divalent cations and pH

Adsorption ofRhizobium meliloti L5-30 in low numbers to alfalfa (Medicago sativa L.) roots was dependent on the presence of divalent cations, and required neutral pH. Adsorption was proportional to Ca and/or Mg concentrations up to 1.5 mM. Ca was not substituted by Sr, Ba or Mn. Adsorption was abolished and viability decreased at pH6. When lowering pH, higher Ca concentrations were required to attain similar adsorption levels, indicating a marked interactive effect between Ca and H ions. Pretreatment of the roots with Ca and low pH did not affect subsequent adsorption of the bacteria. However, Ca pretreatment ofR. meliloti sustained further adsorption at low Ca levels and low pH substantially affected their ability to adsorb. Low pH appears to affect the stability of binding causing desorption of the previously bound bacteria. The presence of saturating concentrations of heterologousR. leguminosarum bv.trifolii A118, did not prevent the expression of divalent cations and pH requirements, as well as their interaction. Our results suggest that rhizobial binding to the root surface already shows the Ca and pH dependence of alfalfa nodulation, which was generally associated to some event prior to rhizobial penetration of root hairs.     

The Biological Activity of the Sinorhizobium meliloti Glucan

The study of the effect of periplasmic glucan isolated from the root-nodule bacterium Sinorhizobium meliloti CXM1-188 on the symbiosis of another strain (441) of the same root-nodule bacterium with alfalfa plants showed that this effect depends on the treatment procedure. The pretreatment of alfalfa seedlings with glucan followed by their bacterization with S. meliloti 441 insignificantly influenced the nodulation parameters of symbiosis (the number of root nodules and their nitrogen-fixing activity) but induced a statistically significant increase in the efficiency of symbiosis (expressed as the masses of the alfalfa overground parts and roots). At the same time, the pretreatment of S. meliloti 441 cells with glucan brought about a considerable decrease in the nodulation parameters of symbiosis (the number of root nodules and their nitrogen-fixing activity decreased by 2.5–11 and 7 times, respectively). These data suggest that the stimulating effect of rhizobia on host plants may be due not only to symbiotrophic nitrogen fixation but also to other factors. Depending on the experimental conditions, the treatment of alfalfa plants with glucan and their bacterization with rhizobial cells enhanced the activity of peroxidase in the alfalfa roots and leaves by 10–39 and 12–27%, respectively.     

Flavonoids Released Naturally from Alfalfa Seeds Enhance Growth Rate of Rhizobium meliloti

Alfalfa (Medicago sativa L.) releases different flavonoids from seeds and roots. Imbibing seeds discharge 3′,4′,5,7-substituted flavonoids; roots exude 5-deoxy molecules. Many, but not all, of these flavonoids induce nodulation (nod) genes in Rhizobium meliloti. The dominant flavonoid released from alfalfa seeds is identified here as quercetin-3-O-galactoside, a molecule that does not induce nod genes. Low concentrations (1-10 micromolar) of this compound, as well as luteolin-7-O-glucoside, another major flavonoid released from germinating seeds, and the aglycones, quercetin and luteolin, increase growth rate of R. meliloti in a defined minimal medium. Tests show that the 5,7-dihydroxyl substitution pattern on those molecules was primarily responsible for the growth effect, thus explaining how 5-deoxy flavonoids in root exudates fail to enhance growth of R. meliloti. Luteolin increases growth by a mechanism separate from its capacity to induce rhizobial nod genes, because it still enhanced growth rate of R. meliloti lacking functional copies of the three known nodD genes. Quercetin and luteolin also increased growth rate of Pseudomonas putida. They had no effect on growth rate of Bacillus subtilis or Agrobacterium tumefaciens, but they slowed growth of two fungal pathogens of alfalfa. These results suggest that alfalfa can create ecochemical zones for controlling soil microbes by releasing structurally different flavonoids from seeds and roots.     

Use of a promoter-specific probe to identify two loci from the Rhizobium meliloti nodulation regulon

The nodulation regulon of Rhizobium meliloti AK631 includes several operons (nodABC, hsnABC, hsnD, efn locus) which have in common a consensus promoter sequence called the nod box. A synthetic nod box probe was used to identify two additional nod boxes, n4 and n5, which were subcloned for study. By constructing lac fusions, we show that n4 and n5 sponsor induction of downstream regions as previously shown for n1-nodABC and n2-hsnABC. Using site-directed Tn5 mutagenesis, we find that the n5 locus plays a significant role in nodulation of alfalfa and sweetclover, whereas the n4 locus is important for alfalfa, but not for sweetclover. Hybridization data suggest that the n5 locus is conserved among Rhizobium species. In contrast, the n4 locus seems to be unique to Rhizobium meliloti strains, in agreement with the host-specific phenotype of n4 locus mutants. Thus, the use of a promoter probe allows us to identify nodulation genes which may be overlooked by standard methods such as random Tn5 mutagenesis.     

The Rhizobium meliloti early nodulation genes (nodABC) are nitrogen-regulated: Isolation of a mutant strain with efficient nodulation capacity on alfalfa in the presence of ammonium

Summary The presence of combined nitrogen in the soil suppresses the formation of nitrogen-fixing root nodules by Rhizobium. We demonstrate that bacterial genes determining early nodulation functions (nodABC) as well as the regulatory gene nodD3 are under nitrogen (NH ₄ ⁺ ) control. Our results suggest that the gene product of nodD3 has a role in mediating the ammonia regulation of early nod genes. The general nitrogen regulatory (ntr) system as well as a chromosomal locus mutated in Rhizobium meliloti were also found to be involved in the regulation of nod gene expression. A R. meliloti mutant with altered sensitivity to ammonia regulation was isolated, capable of more efficient nodulation of alfalfa than the wild-type strain in the presence of 2 mM ammonium sulfate.     

Major flavonoids in uninoculated and inoculated roots of Vicia sativa subsp. nigra are four conjugates of the nodulation gene-inhibitor kaempferol

Inoculation of Vicia sativa subsp. nigra (V. sativa) roots with Rhizobium leguminosarum biovar. viciae (R.l. viciae) bacteria substantially increases the ability of V. sativa to induce rhizobial nodulation (nod) genes. This increase is caused by the additional release of flavanones and chalcones which all induce the nod genes of R.l. viciae (K. Recourt et al., Plant Mol Biol 16: 841–852). In this paper, we describe the analyses of the flavonoids present in roots of V. sativa. Independent of inoculation with R.l. viciae, these roots contain four 3-O-glycosides of the flavonol kaempferol. These flavonoids appeared not capable of inducing the nod genes of R.l. viciae but instead are moderately active in inhibiting the activated state of those nod genes. Roots of 7-day-old V. sativa seedlings did not show any kaempferol-glycosidase activity consistent with the observation that kaempferol is not released upon inoculation with R.l. viciae. It is therefore most likely that inoculation with infective (nodulating) R.l. viciae bacteria results in de novo flavonoid biosynthesis and not in liberation of flavonoids from a pre-existing pool.     

Release and Modification of nod-Gene-Inducing Flavonoids from Alfalfa Seeds

Traces of luteolin, an important rhizobial nod gene inducer in Rhizobium meliloti, are released by alfalfa (Medicago sativa L.) seeds, but most luteolin in the seed exudate is conjugated as luteolin-7-O-glucoside (L7G). Processes affecting the production of luteolin from L7G in seed exudate are poorly understood. Results from this study establish that (a) seed coats are the primary source of flavonoids, including L7G, in seed exudate; (b) these flavonoids exist in seeds before imbibition; and (c) both the host plant and the symbiotic R. meliloti probably can hydrolyze L7G to luteolin. Glycolytic cleavage of L7G is promoted by glucosidase activity released from sterile seeds during the first 4 hours of imbibition. Thus, L7G from imbibing alfalfa seeds may serve as a source of the nod-gene-inducing luteolin and thereby facilitate root nodulation by R. meliloti.     

Inoculation of Vicia sativa subsp. nigra roots with Rhizobium leguminosarum biovar viciae results in release of nod gene activating flavanones and chalcones

Flavonoids released by roots of Vicia sativa subsp. nigra (V. sativa) activate nodulation genes of the homologous bacterium Rhizobium leguminosarum biovar viciae (R. l. viciae). Inoculation of V. sativa roots with infective R. l. viciae bacteria largely increases the nod gene-inducing ability of V. sativa root exudate (A.A.N. van Brussel et al., J Bact 172: 5394–5401). The present study showed that, in contrast to sterile roots and roots inoculated with R. l. viciae cured of its Sym plasmid, roots inoculated with R. l. viciae harboring its Sym plasmid released additional nod gene-inducing flavonoids. Using ¹H-NMR, the structures of the major inducers released by inoculated roots, 6 flavanones and 2 chalcones, were elucidated. Roots extracts of (un)inoculated V. sativa contain 4 major non-inducing, most likely glycosylated, flavonoids. Therefore, the released flavonoids may either derive from the root flavonoids or inoculation with R. l. viciae activates de novo flavonoid biosynthesis.