Isolation of Rhizobium meliloti nod Gene Inducers from Alfalfa Rhizosphere Soil

Signaling in Legume-Rhizobia Symbiosis

Legumes represent an important source of food protein for human nutrition and animal feed. Therefore, sustainable production of legume crops is an issue of global importance. It is well-known that legume-rhizobia symbiosis allows an increase in the productivity and resilience of legume crops. The efficiency of this mutualistic association strongly depends on precise regulation of the complex interactions between plant and rhizobia. Their molecular dialogue represents a complex multi-staged process, each step of which is critically important for the overall success of the symbiosis. In particular, understanding the details of the molecular mechanisms behind the nodule formation and functioning might give access to new legume cultivars with improved crop productivity. Therefore, here we provide a comprehensive literature overview on the dynamics of the signaling network underlying the development of the legume-rhizobia symbiosis. Thereby, we pay special attention to the new findings in the field, as well as the principal directions of the current and prospective research. For this, here we comprehensively address the principal signaling events involved in the nodule inception, development, functioning, and senescence.

Lignin modification leads to increased nodule numbers in alfalfa

Reduction of lignin levels in the forage legume alfalfa (Medicago sativa) by down-regulation of the monolignol biosynthetic enzyme hydroxycinnamoyl coenzyme A:shikimate hydroxycinnamoyl transferase (HCT) results in strongly increased digestibility and processing ability of lignocellulose. However, these modifications are often also associated with dwarfing and other changes in plant growth. Given the importance of nitrogen fixation for legume growth, we evaluated the impact of constitutively targeted lignin modification on the belowground organs (roots and nodules) of alfalfa plants. HCT down-regulated alfalfa plants exhibit a striking reduction in root growth accompanied by an unexpected increase in nodule numbers when grown in the greenhouse or in the field. This phenotype is associated with increased levels of gibberellins and certain flavonoid compounds in roots. Although HCT down-regulation reduced biomass yields in both the greenhouse and field experiments, the impact on the allocation of nitrogen to shoots or roots was minimal. It is unlikely, therefore, that the altered growth phenotype of reduced-lignin alfalfa is a direct result of changes in nodulation or nitrogen fixation efficiency. Furthermore, HCT down-regulation has no measurable effect on carbon allocation to roots in either greenhouse or 3-year field trials.

Occurrence of flavonoids and nucleosides in agricultural soils

AN ECOLOGICALLY RELEVANT SOIL EXTRACTION PROCEDURE SEPARATED TWO TYPES OF MOLECULES IMPORTANT FOR BACTERIA: flavonoids and small hydrophilic organic compounds. Two flavonoids, identified previously as inducers of nodulation genes in Rhizobium meliloti, were detected in rhizosphere soil from alfalfa (Medicago sativa L.). In addition, biologically significant quantities (micromoles per kilogram) of ribonucleosides and deoxyribonucleosides were found in all soils tested. Long-term wheat (Triticum aestivum L.) plots that had received manure contained elevated amounts of nucleosides, and in a separate experiment, the presence of legumes in a wheat-cropping sequence increased soil nucleosides. Intact bacterial cells accounted for less than 1% of the free nucleosides detected. These results suggest new testable hypotheses for molecular ecologists and differ from those obtained with older, harsher techniques.

Perception and modification of plant flavonoid signals by rhizosphere microorganisms

Flavonoids are a diverse class of polyphenolic compounds that are produced as a result of plant secondary metabolism. They are known to play a multifunctional role in rhizospheric plant-microbe and plant-plant communication. Most familiar is their function as a signal in initiation of the legume-rhizobia symbiosis, but, flavonoids may also be signals in the establishment of arbuscular mycorrhizal symbiosis and are known agents in plant defence and in allelopathic interactions. Flavonoid perception by, and impact on, their microbial targets (e.g. rhizobia, plant pathogens) is relatively well characterized. However, potential impacts on 'non-target' rhizosphere inhabitants ('non-target' is used to distinguish those microorganisms not conventionally known as targets) have not been thoroughly investigated. Thus, this review first summarizes the conventional roles of flavonoids as nod gene inducers, phytoalexins and allelochemicals before exploring questions concerning 'non-target' impacts. We hypothesize that flavonoids act to shape rhizosphere microbial community structure because they represent a potential source of carbon and toxicity and that they impact on rhizosphere function, for example, by accelerating the biodegradation of xenobiotics. We also examine the reverse question, 'how do rhizosphere microbial communities impact on flavonoid signals?' The presence of microorganisms undoubtedly influences the quality and quantity of flavonoids present in the rhizosphere, both through modification of root exudation patterns and microbial catabolism of exudates. Microbial alteration and attenuation of flavonoid signals may have ecological consequences for below-ground plant-microbe and plant-plant interaction. We have a lack of knowledge concerning the composition, concentration and bioavailability of flavonoids actually experienced by microbes in an intact rhizosphere, but this may be addressed through advances in microspectroscopic and biosensor techniques. Through the use of plant mutants defective in flavonoid biosynthesis, we may also start to address the question of the significance of flavonoids in shaping rhizosphere community structure and function.

The genetic and biochemical basis for nodulation of legumes by rhizobia

Soil bacteria of the genera Azorhizobium, Bradyrhizobium, and Rhizobium are collectively termed rhizobia. They share the ability to penetrate legume roots and elicit morphological responses that lead to the appearance of nodules. Bacteria within these symbiotic structures fix atmosphere nitrogen and thus are of immense ecological and agricultural significance. Although modern genetic analysis of rhizobia began less than 20 years ago, dozens of nodulation genes have now been identified, some in multiple species of rhizobia. These genetic advances have led to the discovery of a host surveillance system encoded by nodD and to the identification of Nod factor signals. These derivatives of oligochitin are synthesized by the protein products of nodABC, nodFE, NodPQ, and other nodulation genes; they provoke symbiotic responses on the part of the host and have generated immense interest in recent years. The symbiotic functions of other nodulation genes are nonetheless uncertain, and there remain significant gaps in our knowledge of several large groups of rhizobia with interesting biological properties. This review focuses on the nodulation genes of rhizobia, with particular emphasis on the concept of biological specificity of symbiosis with legume host plants.

The Effects of Heavy Metals and Root Immersion on Isoflavonoid Metabolism in Alfalfa (Medicago sativa L.)

Modest increases in the concentration of medicarpin, 6-fold in leaves and 4-fold in roots, were observed in alfalfa (Medicago sativa L.) seedlings treated with 1 mM metal salts for 72 h. However, medicarpin-3-O-glucoside-6"-O-malonate (MGM) and formononetin-7-O-glucoside-6"-O-malonate (FGM) levels were up to 50-fold lower in metal-treated compared to control roots. Approximately 10% of the "missing" conjugates could be accounted for in the root treatment solution, where FGM and MGM transiently accumulated prior to their hydrolysis. Time-course studies revealed that total isoflavonoid content (roots plus solution) increased slightly after CuCl2 treatment, whereas the levels of FGM and MGM increased rapidly in alfalfa roots immersed in water. This increase was reduced by aeration. The phenylalanine ammonia-lyase inhibitor L-[alpha]-aminooxy-[beta]-phenylpropionic acid was used to show that immersion of the roots reduced conjugate rates of degradation, which explains their accumulation. In contrast, conjugate rates of degradation were elevated in CuCl2-treated roots, with 50% of the increase being due to hydrolysis. Up to 90% of formononetin and medicarpin produced in response to CuCl2 treatment arose via conjugate hydrolysis. Our results demonstrate that both immersion/anaerobiosis and abiotic elicitation modify isoflavonoid metabolism in alfalfa, and that metal-stimulated accumulation of phytoalexins may arise through the release from preformed stores rather than de novo synthesis.

Isolation of chalcone synthase and chalcone isomerase cDNAs from alfalfa (Medicago sativa L.): highest transcript levels occur in young roots and root tips

Flavonoids are involved in several different interactions between plants and microorganisms. In the Rhizobium-legume symbiosis, they play an important role as inducers of rhizobial nodulation (nod) genes. We have identified from an alfalfa cDNA library four clones for chalcone synthase (CHS) and two clones for chalcone isomerase (CHI); CHS and CHI are key enzymes in flavonoid biosynthesis. In Medicago sp., CHS is encoded by 8-12 genes, and CHI is encoded by 1-2 genes. Here we report the DNA sequence of these clones as well as their relatedness to other legume CHS and CHI clones. In addition, we report on the expression patterns of two CHS gene family members as well as the CHI gene in M. sativa cv. Iroquois. While CHS and CHI transcript levels are high in root tips and entire young roots, they are low in effective nodules elicited by wild-type strains of Rhizobium meliloti and very low in aerial portions of the plant (stems, leaves, flowers). However, wounding the cotyledons results in a rapid increase in transcript levels of both chalcone synthase and chalcone isomerase genes in these organs.

Alfalfa (Medicago sativa L.) Root Exudates Contain Isoflavonoids in the Presence of Rhizobium meliloti

Root exudates of alfalfa (Medicago sativa L.) inoculated with symbiotic Rhizobium meliloti bacteria contained three isoflavonoids that were not found in exudates of uninoculated plants. Data from proton nuclear magnetic resonance, mass spectrometry, and ultraviolet-visible absorbance analyses indicated that root exudates of inoculated plants contained aglycone and glycoside forms of the phytoalexin medicarpin and a formononetin-7-O-(6"-O-malonylglycoside), a conjugated form of the medicarpin precursor formononetin. The medicarpin molecules did not induce nod gene transcription in R. meliloti, but the formononetin-7-O-(6"-O-malonylglycoside) induced nod genes regulated by both NodD1 and NodD2 proteins in R. meliloti. Hydrolysis of either the malonyl or the glycosyl linkage from the formononetin conjugate eliminated nod gene-inducing activity. The nod gene-inducing activity of crude root exudates was increased 200 and 65% upon inoculation with R. meliloti or R. leguminosarum bv phaseoli, respectively. When root exudate from uninoculated alfalfa was incubated with R. meliloti, high performance liquid chromatography analyses showed no evidence that bacterial metabolism produced medicarpin. These results indicate that alfalfa responds to symbiotic R. meliloti by exuding a phytoalexin normally elicited by pathogens and that the microsymbiont can use a precursor of the phytoalexin as a signal for inducing symbiotic nod genes.