The model legume Medicago truncatula A17 is poorly matched for N2 fixation with the sequenced microsymbiont Sinorhizobium meliloti 1021

Medicago truncatula (barrel medic) A17 is currently being sequenced as a model legume, complementing the sequenced root nodule bacterial strain Sinorhizobium meliloti 1021 (Sm1021). In this study, the effectiveness of the Sm1021-M. truncatula symbiosis at fixing N(2) was evaluated. N(2) fixation effectiveness was examined with eight Medicago species and three accessions of M. truncatula with Sm1021 and two other Sinorhizobium strains. Plant shoot dry weights, plant nitrogen content and nodule distribution, morphology and number were analysed. Compared with nitrogen-fed controls, Sm1021 was ineffective or partially effective on all hosts tested (excluding M. sativa), as measured by reduced dry weights and shoot N content. Against an effective strain, Sm1021 on M. truncatula accessions produced more nodules, which were small, pale, more widely distributed on the root system and with fewer infected cells. The Sm1021-M. truncatula symbiosis is poorly matched for N(2) fixation and the strain could possess broader N(2) fixation deficiencies. A possible origin for this reduction in effectiveness is discussed. An alternative sequenced strain, effective at N(2) fixation on M. truncatula A17, is Sinorhizobium medicae WSM419.     

Difference in delta(15)N signatures between nodulated roots and shoots of soybean is indicative of the contribution of symbiotic N(2) fixation to plant N

Symbiotic N(2) fixation has a variable effect on the (15)N abundance of different parts of legumes. Increases in fixation result in (15)N enrichment of nodules, while decreases, in combination with an increased uptake of mineral N, result in (15)N depletion of the root system. The difference between soybean shoot and below-ground delta(15)N (Deltadelta(15)N=delta(15)N(shoot)-delta(15)N(belowground)) was assessed in hydroponic culture over a range of rates of supply of mineral N. The fractional contribution of N(2) fixation to N uptake (%Ndfa) was determined using the natural abundance (NA) technique with ryegrass as a reference plant. Deltadelta(15)N and %Ndfa were highly correlated, and the relationship was tested using the same soybean cultivar grown in pots in N-rich soil. Estimates of %Ndfa derived from the NA method and from the Deltadelta(15)N approach yielded near-identical values. A literature survey showed similar relationships between %Ndfa and Deltadelta(15)N with different growth stages of soybeans grown under glasshouse and field conditions, different cowpea (Vigna unguiculata) cultivars in the field, and tagasaste (Chamaecytisus proliferus) in hydroponic culture. Possible confounding and species-specific (either plant or Rhizobium spp.) influences are discussed. The difference in delta(15)N signatures between nodulated roots and shoots is confirmed as a robust means of quantifying %Ndfa: it is independent of reference plants and offers the possibility of estimating %Ndfa in soils where the isotope composition of mineral N closely matches that of atmospheric N(2).     

Phloem‐derived γ‐aminobutyric acid (GABA) is involved in upregulating nodule N2 fixation efficiency in the model legume Medicago truncatula

Nitrogen fixation in legumes is downregulated through a whole plant N feedback mechanism, for example, when under stress. This mechanism is probably triggered by the impact of shoot‐borne, phloem‐delivered compounds. However, little is known about any whole‐plant mechanism that might upregulate nitrogen fixation, for example, under N deficiency. We induced emerging N‐deficiency through partial excision of nodules from Medicago truncatula plants. Subsequently, the activity and composition of the remaining nodules and shifts in concentration of free amides/amino acids in the phloem were monitored. Furthermore, we mimicked these shifts through artificial feeding of γ‐aminobutyric acid (GABA) into the phloem of undisturbed plants. As a result of increased specific activity of nodules, N₂ fixation per plant recovered almost completely 4–5 d after excision. The concentration of amino acids, sugars and organic acids increased strongly in the upregulated nodules. A concomitant analysis of the phloem revealed a significant increase in GABA concentration. Comparable with the effect of nodule excision, artificial GABA feeding into the phloem resulted in an increased activity and higher concentration of amino acids and organic acids in nodules. It is concluded that GABA might be involved in upregulating nodule activity, possibly because of its constituting part of a putative amino acid cycle between bacteroids and the cytosol.