Utilization of nitrate by bacteroids of Bradyrhizobium japonicum in the soybean root nodule

Systemic regulation of nodule structure and assimilated carbon distribution by nitrate in soybean

BACKGROUND

The nitrate regulates soybean nodulation and nitrogen fixation systemically, mainly in inhibiting nodule growth and reducing nodule nitrogenase activity, but the reason for its inhibition is still inconclusive.

METHODS

The systemic effect of nitrate on nodule structure, function, and carbon distribution in soybean (Glycine max (L.) Merr.) was studied in a dual-root growth system, with both sides inoculated with rhizobia and only one side subjected to nitrate treatment for four days. The non-nodulating side was genetically devoid of the ability to form nodules. Nutrient solutions with nitrogen concentrations of 0, 100, and 200 mg L^-1 were applied as KNO₃ to the non-nodulating side, while the nodulating side received a nitrogen-free nutrient solution. Carbon partitioning in roots and nodules was monitored using ¹³C-labelled CO₂. Other nodule responses were measured via the estimation of the nitrogenase activity and the microscopic observation of nodule ultrastructure.

RESULTS

Elevated concentrations of nitrate applied on the non-nodulating side caused a decrease in the number of bacteroids, fusion of symbiosomes, enlargement of the peribacteroid spaces, and onset of degradation of poly-β-hydroxybutyrate granules, which is a form of carbon storage in bacteroids. These microscopic observations were associated with a strong decrease in the nitrogenase activity of nodules. Furthermore, our data demonstrate that the assimilated carbon is more likely to be allocated to the non-nodulating roots, as follows from the competition for carbon between the symbiotic and non-symbiotic sides of the dual-root system.

CONCLUSION

We propose that there is no carbon competition between roots and nodules when they are indirectly supplied with nitrate, and that the reduction of carbon fluxes to nodules and roots on the nodulating side is the mechanism by which the plant systemically suppresses nodulation under nitrogen-replete conditions.

Nitrogen fixation in transposon mutants from Bradyrhizobium japonicum USDA 110 impaired in nitrate reductase

Tn5 transposon mutagenesis was carried out in Bradyrhizobium japonicum strain USDA 110 to produce defective mutants. From over one thousand clones expressing low levels of nitrate reductase activity as free-living bacteria, approximately five percent had significantly different ratios of nodulation, N2 fixation or nitrate reductase activity compared to the wild strain when determined in bacteroids from soybean nodules. Tn5 insertions were checked previously and mutants were arranged into four different groups. Only one of these groups, designated AN, was less effective at N2 fixation than the wild strain, suggesting a mutation in a domain shared by nitrogenase and NR. The remaining groups of insertions successfully nodulated and were as effective at N2 fixation as the wild strain, but showed diminished ability to reduce nitrate both in nodules and in the isolated bacteroids when assayed in vitro with NADH or methyl viologen as electron donors. PCR amplification demonstrated that Tn5 insertions took place in different genes on each mutant group and the type of mutant (CC) expressing almost no nitrate reductase activity under all treatments seemed to possess transposable elements in two genes. Induction of nitrate reductase activity by nitrate was observed only in those clones expressing a low constitutive activity (AN and AE). Nitrate reductase activity in bacteroids along nodule growth decreased in all groups including the ineffective AN group, whose nodulation was highly inhibited by nitrate at 5 mmol/L N. Host-cultivar interaction seemed to influence the regulation of nitrate reductase activity in bacteroids. Total or partial repression of nitrate reductase activity in bacteroids unaffected by N2 fixation (CC, AJ and AE groups) improved nodule resistance to nitrate and N yields of shoots over those of the wild strain. These observations may suggest that some of the energy supplied to bacteroids was wasted by its constitutive NRA.

Short-term inhibition of legume N2 fixation by nitrate : II. Nitrate effects on nodule oxygen diffusion

A comparison was made of changes in nitrogenase (N2ase; EC 1.18.6.1) activity, oxygen diffusion resistance and NO 3 (-) metabolism in symbioses ofPhaseolus vulgaris L. andVigna radiata (L.) Wilczek during a 3-d exposure to 10 mM NO 3 (-) . Bacteroids fromPhaseolus nodules lacked nitrate reductase (NR;EC 1.7.99.4) but those fromVigna nodules had elevated amounts of the enzyme. The nodule cytosol of both species contained assimilatory NR (EC 1.6.6.1). Both symbioses showed a C2H2-induced decline in N2ase activity, the extent of which remained constant with NO 3 (-) exposure forPhaseolus but became greater forVigna. Nitrate application for 3 d reduced maximum (pre-decline) rates of C2H2-reduction activity by 83% and 36% inPhaseolus andVigna, respectively. Nitrogenase-linked respiration (NLR) closely paralleled N2ase activity as the carbon costs of N2ase were not significantly altered by NO 3 (-) . The relationship between NLR and increases in external O2 concentration from 21 to 60% was used to characterize the oxygen diffusion resistance (R) of nodules from both species. In absolute terms the minimum R ofPhaseolus nodules increased with NO 3 (-) , whereas the ability to adjust this R in response to O2 was lost after 2d. ForVigna nodules the increase in minimum R was much smaller and the adjustment ability was retained for the 3-d period of NO 3 (-) exposure. Bacteroids ofVigna and the cytosol of both species contained NR prior to NO 3 (-) exposure, and activities increased 1.5- to 2-fold during the treatment period. Despite this, NO 2 (-) was not detected in nodules ofPhaseolus, and showed only a very small accumulation in the cytosol ofVigna nodules. It is proposed that nodules have a two-stage response to applied NO 3 (-) . In the first stage NO 3 (-) is restricted to the nodule cortex and causes a reversible increase in R. In the second stage NO 3 (-) may enter the infected region and toxic amounts of NO 2 (-) can be generated in nodules having high bacteroid andor cytosol NR activities. This NO 2 (-) can irreversibly damage the nodules and accelerate their senescence.

Short-term inhibition of legume N2 fixation by nitrate : I. Nitrate effects on nitrate-reductase activities of bacteroids and nodule cytosol

The hypothesis of NO2 (-) toxicity as the causative factor of NO3 (-) inhibition of nitrogenase (N2ase; EC 1.18.6.1) activity has been evaluated using a short-term exposure (3 d) of several legumes. Treatment of plants with 10 mM NO3 (-) induced nitrate reductase (NR) from bacteroids (EC 1.7.99.4) and nodule cytosol (EC 1.6.6.1) in most species. Regardless of the levels of both enzymes, significant accumulation of NO2 (-) did not occur in nodules. Dissection of nodules into cortical and infected regions, and subsequent NO2 (-) assays in conditions that suppressed enzyme activities, indicated that, in the short-term, bacteroid NR does not generate NO2 (-) in vivo. This is probably because NO3 (-) access is restricted to the nodule cortex. Accumulation of NO2 (-) at levels that are damaging for N2ase and leghaemoglobin were only observed when a delay occurred between dissection and assaying of nodules. It is concluded that NO2 (-) is not responsible for the initial NO3 (-)-induced decline of N2ase activity, and that toxic amounts of NO2 (-) only build up in nodules following longer exposures to NO3 (-), when this anion is actively reduced by bacteroid and cytosol enzymes.

Which steps are essential for the formation of functional legume nodules?

Nodulation is reviewed in terms of the phenotypes proposed by Vincent (1980). Individual legumes may be infectible by one or more of the three bacterial genera (collectively known as rhizobia) Rhizobium, Bradyrhizobium, or Azorhizobium. The type of infection process by which rhizobia gain entry is largely governed by the host genotype. In addition to the widely studied root-hair pathway, infections may be associated with lateral root emergence or occur between root epidermal cells. The exact chemical and physical nature of the root hair/epidermal cell wall is likely to be a critical factor in determining whether infections can proceed. In addition to differing with species, wall composition may be influenced by soil chemical (e.g. Ca²⁺ ) and biotic factors (e.g. bacteria). Rhizobial features essential for infection include particular surface polysaccharides and the induction of nodulation genes by plant root exudates. Neither of these is likely to be a major barrier to the extension of nodulation to new hosts. Dissemination of rhizobia within developing nodules may be intercellular, via infection threads or by division of a small number of infected cells. All functional symbioses eventually have 'intracellular' bacteria, in the sense that rhizobia are geographically located within the boundary of the host cell walls. However, they remain extracellular in the sense that they are always confined by a membrane which is largely of host cell origin. In some genera they are also surrounded by infection thread walls, probably modified forms of 'invasive' infection thread walls, which allow differentiation of rhizobia into the nitrogen-fixing form. Thus, natural, functional, symbioses may (a) never involve a stage in which bacteria are confined within tubular infection threads or (b) never release bacteria from infection threads. These features are determined by host genotype. The one feature of legume nodules so far found never to vary is the stem-like character of a peripheral vascular system. This contrasts with the central vascular system of actinorhizas and the rhizobial-induced nodules on the Ulmaceous genus Parasponia. Although of great intrinsic interest, this character is unlikely to present an insurmountable barrier to the extension of nodulation to new species. Other features, such as the ability to produce haemoglobin are now known to the in the genetic makeup of many higher plants. The discovery of the wide range of nodule structures occurring in nature, together with work on mutant rhizobia which may bypass critical stages in the nodulation process, suggest various ways in which the extension of nodulation to non-nodulated legumes and to other (initially at least, dicotyledonous) plants may be engineered. CONTENTS Summary 129 I. Introduction 130 II. The symbionts 130 III. Stages in nodulation 132 IV. Stems and nodules 143 V. Prospects for finding/making new symbioses 144 VI. Conclusions 145 Acknowledgements 147 References 147.