Effect of pO(2) on Growth and Nodule Functioning of Symbiotic Cowpea (Vigna unguiculata L. Walp.)

Flooding tolerance of forage legumes

We review waterlogging and submergence tolerances of forage (pasture) legumes. Growth reductions from waterlogging in perennial species ranged from >50% for Medicago sativa and Trifolium pratense to <25% for Lotus corniculatus, L. tenuis, and T. fragiferum. For annual species, waterlogging reduced Medicago truncatula by ~50%, whereas Melilotus siculus and T. michelianum were not reduced. Tolerant species have higher root porosity (gas-filled volume in tissues) owing to aerenchyma formation. Plant dry mass (waterlogged relative to control) had a positive (hyperbolic) relationship to root porosity across eight species. Metabolism in hypoxic roots was influenced by internal aeration. Sugars accumulate in M. sativa due to growth inhibition from limited respiration and low energy in roots of low porosity (i.e. 4.5%). In contrast, L. corniculatus, with higher root porosity (i.e. 17.2%) and O2 supply allowing respiration, maintained growth better and sugars did not accumulate. Tolerant legumes form nodules, and internal O2 diffusion along roots can sustain metabolism, including N2 fixation, in submerged nodules. Shoot physiology depends on species tolerance. In M. sativa, photosynthesis soon declines and in the longer term (>10 d) leaves suffer chlorophyll degradation, damage, and N, P, and K deficiencies. In tolerant L. corniculatus and L. tenuis, photosynthesis is maintained longer, shoot N is less affected, and shoot P can even increase during waterlogging. Species also differ in tolerance of partial and complete shoot submergence. Gaps in knowledge include anoxia tolerance of roots, N2 fixation during field waterlogging, and identification of traits conferring the ability to recover after water subsides.

Symbiotic functioning, structural adaptation, and subcellular organization of root nodules from Psoralea pinnata (L.) plants grown naturally under wetland and upland conditions in the Cape Fynbos of South Africa

In the Cape Fynbos of South Africa, Psoralea pinnata (L.) plants occur naturally in both wetland and well-drained soils and yet effectively fix N₂ under the two contrasting conditions. In this study, nodule structure and functioning in P. pinnata plants from the two habitats were evaluated using light and transmission electron microscopy (TEM), as well as the ¹⁵N natural abundance technique. The results showed that, structurally, fully developed P. pinnata nodules were spherical in shape with six components (namely, lenticels, periderm, outer cortex, middle cortex, inner cortex, and a central bacteria-infected medulla region). Morphometric analysis revealed 44 and 84 % increase in cell area and volume of wetland nodules compared to those from upland. The percentage area of nodules occupied by the middle cortex in wetland nodules was twice that of upland nodules. As a result, the size of the medulla region in wetland nodules was significantly reduced compared to upland nodules. Additionally, the average area of medulla occupied by intercellular air spaces in wetland nodules was about five times that of upland nodules (about 431 % increase in wetland over upland nodules). TEM data also showed more bacteroids in symbiosomes of upland nodules when compared to wetland nodules. However, isotopic analysis of above-ground plant parts revealed no differences in symbiotic parameters such as N concentration, ∂¹⁵N and %Ndfa between wetland and upland P. pinnata plants. These results suggest that, under limiting O₂ conditions especially in wetlands, nodules make structural and functional adjustments to meet the O₂ demands of N₂-fixing bacteroids.

Elemental distribution in tissue components of N2-fixing nodules of Psoralea pinnata plants growing naturally in wetland and upland conditions in the Cape Fynbos of South Africa

There is little information on in situ distribution of nutrient elements in N2-fixing nodules. The aim of this study was to quantify elemental distribution in tissue components of N2-fixing nodules harvested from Psoralea pinnata plants grown naturally in wetland and upland conditions in the Cape Fynbos. The data obtained from particle-induced X-ray emission revealed the occurrence of 20 elements (Si, P, S, Cl, K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, As, Br, Rb, Sr, Y, Zr, Mo and Ba) in nodule components. Although, in upland plants, the concentrations of S, Fe, Si, Mn and Cu showed a steady increase from the middle cortex to the medulla region of P. pinnata nodules, in wetland plants, only S, Fe and Mn showed an increase in concentration from the middle cortex to the bacteria-infected medulla of P. pinnata nodules. By contrast, the concentrations of Cl, K, Ca, Zn and Sr decreased from middle cortex to nodule medulla. The alkaline earth, alkali and transition elements Rb, Sr, Y and Zr, never before reported in N2-fixing nodules, were found to occur in root nodules of P. pinnata plants grown in both wetland and upland conditions.

Nitrate regulates rhizobial and mycorrhizal symbiosis in common bean (Phaseolus vulgaris L.)

Nitrogen-limited conditions are considered to be a prerequisite for legume-rhizobial symbiosis, but the effects of nitrate-rich conditions on symbiotic status remain poorly understood. We addressed this issue by examining rhizobial (Rhizobim tropici) and arbusclar mycorrhizal (Glomus intraradices) symbiosis in Phaseolus vulgaris L. cv. Negro Jamapa under nitrate pre-incubation and continuous nitrate conditions. Our results indicate that nitrate pre-incubation, independent of the concentration, did not affect nodule development. However, the continuous supply of nitrate at high concentrations impaired nodule maturation and nodule numbers. Low nitrate conditions, in addition to positively regulating nodule number, biomass, and nitrogenase activity, also extended the span of nitrogen-fixing activity. By contrast, for arbuscular mycorrhizae, continuous 10 and 50 mmol/L nitrate increased the percent root length colonization, concomitantly reduced arbuscule size, and enhanced ammonia transport without affecting phosphate transport. Therefore, in this manuscript, we have proposed the importance of nitrate as a positive regulator in promoting both rhizobial and mycorrhizal symbiosis in the common bean.

Subcellular organization of N2-fixing nodules of cowpea (Vigna unguiculata) supplied with silicon

Provision of silicon (0, 0.048, 0.096, 0.24, 0.48, and 0.96 g/l) in the form of silicic acid (H4SiO4) to nodulated cowpea plants (Vignia unguiculata [L.] Walp.) grown in liquid culture resulted in considerable changes in the internal organization of nodule structure. Compared to the control plants which received no added silicate, bacteroid numbers increased significantly (P < or = 0.05) at silicate concentrations of both 0.096 and 0.48 g/l. The number of symbiosomes also increased by 3.2-fold at the silicate concentration of 0.96 g/l compared to the control. In contrast, the size of bacteroids and symbiosomes decreased significantly (P < 0.05) inside nodules of silicate-treated plants. The peribacteroid space was also decreased considerably (P < 0.05) with the application of 0.096 and 0.96 g of silicate per liter to plants. However, the size of intercellular spaces adjacent to infected and uninfected interstitial cells within the nodule medulla increased significantly (P < or = 0.05) at 0.096 g of silicate per liter followed by a sharply marked (P < or = 0.05) decrease with each subsequent increase in silicate application. The result was a large decrease (P < 0.05) in the area of bacteria-infected tissue occupied by intercellular space at the highest silicate concentration, which was caused by a significant (P < or = 0.05) increase in cell wall thickness. Our findings show that the positive effects of silicon on N2 fixation might actually be due to an increased number of bacteroids and symbiosomes.

Effects of oxygen on nodule physiology and expression of nodulins in alfalfa

Early nodulin 2 (ENOD2) transcripts and protein are specifically found in the inner cortex of legume nodules, a location that coincides with the site of a barrier to O2 diffusion. The extracellular glycoprotein that binds the monoclonal antibody MAC236 has also been localized to this site. Thus, it has been proposed that these proteins function in the regulation of nodule permeability to O2 diffusion. It would then be expected that the levels of ENOD2 mRNA/protein and MAC236 antigen would differ in nodules with different permeabilities to O2. We examined the expression of ENOD2 and other nodule-expressed genes in Rhizobium meliloti-induced alfalfa nodules grown under 8, 20, or 50% O2. Although there was a change in the amount of MAC236 glycoprotein, the levels of ENOD2 mRNA and protein did not differ significantly among nodules grown at the different [O2], suggesting that neither ENOD2 transcription nor synthesis is involved in the long-term regulation of nodule permeability. Moreover, although nodules from all treatments reduced their permeability to O2 as the partial pressure of O2 (pO2) was increased to 100%, the levels of extractable ENOD2 and MAC236 proteins did not differ from those measured at the growth pO2, further suggesting that if these proteins are involved in a short-term regulation of the diffusion barrier, they must be involved in a way that does not require increased transcription or protein synthesis.

Adaptation of Nodulated Soybean (Glycine max L. Merr.) to Growth in Rhizospheres Containing Nonambient pO(2)

Nodulated soybean (Glycine max L. Merr. cv White Eye inoculated with Bradyrhizobium japonicum strain CB 1809) plants were cultured in the absence of combined N from 8 to 28 days with their root systems maintained continuously in 1, 2.5, 5, 10, 20, 40, 60, or 80% O(2) (volume/volume) in N(2). Plant dry matter yield was unaffected by partial pressure of oxygen (pO(2)) and N(2) fixation showed a broad plateau of maximum activity from 2.5 to 40 or 60% O(2). Slight inhibition of nitrogenase activity occurred at 1% O(2) and as much as 50% inhibition occurred at 80% O(2). Low pO(2) (less than 10%) decreased nodule mass on plants, but this was compensated for by those nodules having higher specific nitrogenase activities. Synthesis and export of ureides in xylem was maintained at a high level (70-95% of total soluble N in exudate) over the range of pO(2) used. Measurements of nitrogenase (EC 1.7.99.2) activity by acetylene reduction indicated that adaptation of nodules to low pO(2) was largely due to changes in ventilation characteristics and involved increased permeability to gases in those grown in subambient pO(2) and decreased permeability in those from plants cultured with their roots in pO(2) greater than ambient. A range of structural alterations in nodules resulting from low pO(2) were identified. These included increased frequency of lenticels, decreased nodule size, increased volume of cortex relative to the infected central tissue of the nodule, as well as changes in the size and frequency of extracellular voids in all tissues. In nodules grown in air, the inner cortex differentiated a layer of four or five cells which formed a band, 40 to 50 micrometers thick, lacking extracellular voids. This was reduced in nodules grown in low pO(2) comprising one or two cell layers and being 10 to 20 micrometers thick in those from 1% O(2). Long-term adaptation to different external pO(2) involved changes which modify diffusive resistance and are additional to adjustments in the variable diffusion barrier.

Effect of pO(2) on the Formation and Status of Leghemoglobin in Nodules of Cowpea and Soybean

Nodulated cowpea (Vigna unguiculata [L.] Walp. cv Vita 3: Bradyrhizobium strain CB756) and soybean (Glycine max [L.] Merr. cv White Eye: Bradyrhizobium strain CB1809) were grown with their root systems maintained in a flowing gas stream containing a range of pO(2) (1-80%, v/v) in N(2) for up to 28 days after planting. At the extremes of sub- and supra-ambient pO(2), the levels of leghemoglobin (Lb) in nodules were reduced. However, neither the proportional composition of Lb component proteins (eight in soybean, three in cowpea) nor their oxidation state was affected by pO(2). Short-term changes in pO(2) (transferring plants grown with sub- or supra-ambient pO(2) in the rhizosphere to air or vice versa) caused a significant decline in Lb content and, in cowpea but not soybean, where pO(2) was increased, a higher percentage of oxidation of Lb. Combining data on changes in Lb level of cowpea nodules grown in sub-ambient pO(2) with those for their structural adaptation to an under supply of O(2) indicated that, despite the nodules having a lower level of Lb, the amount per infected cell was increased by up to twofold and per bacteroid up to fivefold (in those from 1% O(2)) compared to those grown in air. Progressive decline in pO(2) resulted in a progressive increase on this basis, indicating a close relationship between Lb content and the adaptation of nodule functioning to external O(2) level.

Morphological and structural adaptation of nodules of cowpea to functioning under sub- and supra-ambient oxygen pressure

Nodules of cowpea (Vigna unguiculata (L.) Walp. cv. Vita 3:Bradyrhizobium CB 756) from 28-d-old plants cultured for 23 d with their root systems maintained in O2 levels from 1 to 80% (v/v, in N2) in the external gas phase showed a range of structural changes which have been interpreted in relation to an over- or under-supply of O2. A response to the partial pressure of O2 in the gas phase (pO2) was noted with respect to nodule size, lenticel development, the relative distributions of cortical and infected central tissue, the differentiation of cortex, especially the inner cortex, the frequency and size of infected and uninfected interstitial cells, the volume of extracellular spaces both in cortex and infected tissue, and in the frequency of bacteroids. As a consequence of these changes the surface area of inner cortex relative to the nitrogenase-containing units of fixing tissue (infected cells or bacteroids) was increased by as much as 20-fold. Effectiveness of bacteroid functioning increased from 0.10 ± 0.02 · 10(-9) μmol acetylene reduced per bacteroid in air-grown nodules to 0.9 ± 0.16 · 10(-9) (same units) per bacteroid in those cultured in 1% O2.

Effect of oxygen pressure on synthesis and export of nitrogenous solutes by nodules of cowpea

Nodules of cowpea plants (Vigna unguiculata (L.) Walp. cv. Vita 3 :Bradyrhizobium CB756) cultured for periods of 23 d with their root systems maintained in atmospheres containing a range of partial pressures of O2 (pO2; 1-80%, v/v, in N2) formed and exported ureides (allantoin and allantoic acid) as the major products of fixation at all pO2 tested. In sub-ambient pO2 (1 and 2.5%) nodules contained specific activities of uricase (urate: O2 oxidoreductase; EC 1.7.3.3) and allantoinase (allantoin hydrolyase; EC 3.5.2.5) as much as sevenfold higher than in those from air. On a cell basis, uninfected cells in nodules from 1% O2 contained around five times the level of uricase. Except for NAD: glutamate synthase (EC 1.4.1.14), which was reduced in sub-ambient O2, the activities of other enzymes of ureide synthesis were relatively unaffected by pO2. Short-term effects of pO2 on assimilation of fixed nitrogen were measured in nodules of air-grown plants exposed to subambient pO2 (1, 2.5 or 5%, v/v in N2) and(15)N2. Despite a fall in total(15)N2 fixation, ureide synthesis and export was maintained at a high level except in 1% O2 where formation was halved. The data indicate that in addition to the structural and diffusional adaptations of cowpea nodules which allow the balance between O2 supply and demand to be maintained over a wide range of pO2, nodules also show evidence of biochemical adaptations which maintain and enhance normal pathways for the assimilation of fixed nitrogen.

Effect of pO(2) during Growth on the Gaseous Diffusional Properties of Nodules of Cowpea (Vigna unguiculata L. Walp.)

Adaptations of nodules of cowpea (Vigna unguiculata L. Walp. cv Vita 3: Bradyrhizobium CB 756) to growth in pO(2) ranging from 1 to 80% O(2) (volume/volume) involved both readily reversible mechanisms of adjustment and more stable alterations which together resulted in nodules with widely ranging resistance to diffusion of gases. Those grown in subambient pO(2) (1-5% O(2) were altered such that rapid diffusional adjustment was unable to prevent irreversible loss of nitrogenase on their transfer to higher levels of O(2). Those cultured in 80% had adapted to over-supply of O(2) such that their transfer to lower levels of O(2) limited both nitrogenase and respiratory CO(2) release. There was also some evidence for ;protective respiration.' Measurement of diffusional properties based on gas exchange kinetics indicated that gaseous permeability values for nodules from 5 to 40% O(2) were relatively constant around 20 x 10(-3) millimeters per second, while those for nodules from 1% O(2) were as high as 67.7 x 10(-3) millimeter per second and from 80% as low as 6.8 x 10(-3) millimeters per second. Estimates of the thickness of the diffusion barrier ranged from 7.5 micrometers for nodules from 1% O(2) to 71.9 micrometers in those from 80% O(2).