Function Of Root Nodules Research Articles

Calcium in the Life Cycle of Legume Root Nodules.

The present review highlights both the fundamental questions of calcium localization, compartmentation, and its participation in symbiosome signaling cascades during nodule formation and functioning. Apparently, the main link of such signaling is the calmodulin…calcium- and calmodulin-dependent protein kinases…CYCLOPS…NIN…target genes cascade. The minimum threshold level of calcium as a signaling agent in the presence of intracellular reserves determines the possibility of oligotrophy and ultraoligotrophy in relation to this element. During the functioning of root nodules, the Ca2+-ATPases activity maintains homeostasis of low calcium concentrations in the cytosol of nodule parenchyma cells. Disturbation of this homeostasis can trigger the root nodule senescence. The same reasons determine the increase in the effectiveness of symbiosis with the help of seed priming with sources of calcium. Examples of calcium response polymorphism in components of nitrogen fixing simbiosis important in practical terms are shown.

Effect of 24-epibrassinolide on Bradyrhizobium japonicum in pure culture and in symbiosis with Glycine max

Brassinosteroids are a group of plant steroid hormones that regulate a wide range of physiological responses, including cell elongation, photomorphogenesis, xylem differentiation, seed germination, etc. It has been proven that these hormones contribute to the formation of legume-rhizobia symbiotic systems. Applied research indicates the potentially complex role of brassinosteroids in the formation of nodules with both positive and negative effects. Therefore, research aimed at studying their role in the processes of symbiosis formation and functioning remains relevant. The aim of the study was to investigate the effect of 24-epibrassinolide on growth indicators of Bradyrhizobium japonicum in pure culture and realization of their nitrogen-fixing potential in symbiosis with soybean plants. It was found that the B. japonicum PC08 strain did not show a pronounced response to the application of concentrations 5 · 10—4, 1 · 10—5 g/L of 24-epibrassinolide.Both positive and negative effects of the tested substance on the growth dynamics this strain were weakly expressed or completely absent. The addition of this phytohormone to the growth medium of the rhizobial strain B. japonicum T21-2 led to stimulation of the growth of rhizobial cells. It was found that 24-epibrassinolide provided an increase in the number of root nodules, regardless of its concentration and the method of application. It had a significant effect on the growth of the root system of soybean plants, while its use as a component of the inoculation suspension in concentration 1 · 10—5 g/L was more effective. As a result of our research, changes in the dynamics of rhizobia growth under the influence of 24-epibrassinolide were revealed. The effect of its application was largely determined by the strain of B. japonicum that we used. We believe that the reaction to the use of 24-epibrassinolide depends on the properties and origin of the strain. The positive effect of 24-epibrasinolide on the number of formed root nodules at the late stages of the formation of the symbiotic apparatus was confirmed. Recorded changes in nitrogen-fixing activity of symbiotic systems under the influence of 24-epibrasinolide indicate that brassinosteroids have an effect on the functioning of root nodules.

Nitrogen fixation symbiosis and salt tolerance of the boreal woody species Elaeagnus commutata

Soil salinity is a major abiotic stress that reduces the growth, survival and physiological functions of most plants. In the boreal forest, elevated levels of NaCl pose reclamation challenges for habitats disturbed by the oil sand industry. The objectives of this study were to determine how salinity affects the nitrogen fixation capability of the native boreal N-fixing shrub, wolf willow (Elaeagnus commutata), as well as to examine how salinity affects its growth and physiology. Seedlings of E. commutata were exposed to 0, 50 and 100 mM NaCl in a hydroponic system and physiological parameters were measured after 1, 4, 8 and 12 weeks of treatment. All plants survived the salinity treatments. Gas exchange parameters including leaf photosynthesis, stomatal conductance and transpiration were only reduced at 100 mM NaCl. In addition, plants showed signs of photosynthetic recovery after 4 weeks. Increasing salinity also led to moderate declines in plant shoot and root growth. While N and K content in plant leaf and root tissues were not reduced by higher salinity, salinity did reduce Ca content in plant leaf tissues. Root nodule function was largely inhibited at 100 mM NaCl, resulting from the absence of nitrogen-fixing vesicles within these nodules. Our data suggested that E. commutata seedlings can survive and grow under up to 100 mM NaCl while maintaining moderate levels of physiological functions, and reduced ability to fix nitrogen, limiting improvements to soil fertility in restoration applications.

Study of phenanthrene utilizing bacterial consortia associated with cowpea (Vigna unguiculata) root nodules.

Many legumes have been selected as model plants to degrade organic contaminants with their special associated rhizosphere microbes in soil. However, the function of root nodules during microbe-assisted phytoremediation is not clear. A pot study was conducted to examine phenanthrene (PHE) utilizing bacteria associated with root nodules and the effects of cowpea root nodules on phytoremediation in two different types of soils (freshly contaminated soil and aged contaminated soil). Cowpea nodules in freshly-contaminated soil showed less damage in comparison to the aged-contaminated soil, both morphologically and ultra-structurally by scanning electron microscopy. The study of polycyclic aromatic hydrocarbon (PAH) attenuation conducted by high performance liquid chromatography revealed that more PAH was eliminated from liquid culture around nodulated roots than nodule-free roots. PAH sublimation and denaturation gradient gel electrophoresis were applied to analyze the capability and diversity of PAH degrading bacteria from the following four parts of rhizo-microzone: bulk soil, root surface, nodule surface and nodule inside. The results indicated that the surface and inside of cowpea root nodules were colonized with bacterial consortia that utilized PHE. Our results demonstrated that root nodules not only fixed nitrogen, but also enriched PAH-utilizing microorganisms both inside and outside of the nodules. Legume nodules may have biotechnological values for PAH degradation.

Inhibition of Glutamine Synthetase by Phosphinothricin Leads to Transcriptome Reprograming in Root Nodules of Medicago truncatula

Glutamine synthetase (GS) is a vital enzyme for the assimilation of ammonia into amino acids in higher plants. In legumes, GS plays a crucial role in the assimilation of the ammonium released by nitrogen-fixing bacteria in root nodules, constituting an important metabolic knob controlling the nitrogen (N) assimilatory pathways. To identify new regulators of nodule metabolism, we profiled the transcriptome of Medicago truncatula nodules impaired in N assimilation by specifically inhibiting GS activity using phosphinothricin (PPT). Global transcript expression of nodules collected before and after PPT addition (4, 8, and 24 h) was assessed using Affymetrix M. truncatula GeneChip arrays. Hundreds of genes were regulated at the three time points, illustrating the dramatic alterations in cell metabolism that are imposed on the nodules upon GS inhibition. The data indicate that GS inhibition triggers a fast plant defense response, induces premature nodule senescence, and promotes loss of root nodule identity. Consecutive metabolic changes were identified at the three time points analyzed. The results point to a fast repression of asparagine synthesis and of the glycolytic pathway and to the synthesis of glutamate via reactions alternative to the GS/GOGAT cycle. Several genes potentially involved in the molecular surveillance for internal organic N availability are identified and a number of transporters potentially important for nodule functioning are pinpointed. The data provided by this study contributes to the mapping of regulatory and metabolic networks involved in root nodule functioning and highlight candidate modulators for functional analysis.

Jasmonate biosynthesis in legume and actinorhizal nodules

Jasmonic acid (JA) is a plant signalling compound that has been implicated in the regulation of mutualistic symbioses. In order to understand the spatial distribution of JA biosynthetic capacity in nodules of two actinorhizal species, Casaurina glauca and Datisca glomerata, and one legume, Medicago truncatula, we determined the localization of allene oxide cyclase (AOC) which catalyses a committed step in JA biosynthesis. In all nodule types analysed, AOC was detected exclusively in uninfected cells. The levels of JA were compared in the roots and nodules of the three plant species. The nodules and noninoculated roots of the two actinorhizal species, and the root systems of M. truncatula, noninoculated or nodulated with wild-type Sinorhizobium meliloti or with mutants unable to fix nitrogen, did not show significant differences in JA levels. However, JA levels in all plant organs examined increased significantly on mechanical disturbance. To study whether JA played a regulatory role in the nodules of M. truncatula, composite plants containing roots expressing an MtAOC1-sense or MtAOC1-RNAi construct were inoculated with S. meliloti. Neither an increase nor reduction in AOC levels resulted in altered nodule formation. These data suggest that jasmonates are not involved in the development and function of root nodules.

Differential Effects of Combined N Sources on Early Steps of the Nod Factor–Dependent Transduction Pathway in Lotus japonicus

The development of nitrogen-fixing nodules in legumes is induced by perception of lipochitin-oligosaccharide signals secreted by a bacterial symbiont. Nitrogen (N) starvation is a prerequisite for the formation, development, and function of root nodules, and high levels of combined N in the form of nitrate or ammonium can completely abolish nodule formation. We distinguished between nitrate and ammonium inhibitory effects by identifying when and where these combined N sources interfere with the Nod-factor-induced pathway. Furthermore, we present a small-scale analysis of the expression profile, under different N conditions, of recently identified genes involved in the Nod-factor-induced pathway. In the presence of high levels of nitrate or ammonium, the NIN gene fails to be induced 24 h after the addition of Nod factor compared with plants grown under N-free conditions. This induction is restored in the hypernodulating nitrate-tolerant har1-3 mutant only in the presence of 10 and 20 mM KNO3. These results were confirmed in Lotus plants inoculated with Mesorhizobium loti. NIN plays a key role in the nodule organogenesis program and its downregulation may represent a crucial event in the nitrate-dependent pathway leading to the inhibition of nodule organogenesis.

Long- and Short-term Effects of Nitrogen on Formation and Function of Root Nodules on Alnus maritima

Symbiotic associations between Alnus maritima (Marsh.) Muhl. ex Nutt. (seaside alder) and the actinomycete Frankia result in root nodules in which atmospheric nitrogen (N) is fixed. This has led to interest in producing seaside alders with minimal N fertilizer and in using the species on low-N soils. Our objectives were to determine how applied N influences nodulation and to characterize how short-term changes in root-zone N affect the function of established nodules. Seaside alders native to the Delmarva Peninsula (Alnus maritima subsp. maritima) were grown in perlite inoculated with soil from roots of indigenous plants. Plants were treated with N-free Hoagland solution supplemented with ammonium nitrate at 0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, 2, 4, and 8 mm for 10 weeks. Nodulation decreased as applied N increased. While plants treated with ammonium nitrate at 4 and 8 mm formed nearly no nodules, 0.5 mM resulted in vigorous, healthy plants that formed, on average, 70 nodules. In a second experiment, a nodulated population of seaside alders was established by treating seedlings with 0.5-mm ammonium nitrate in otherwise N-free Hoagland solution for 6 weeks. Plants then were provided ammonium nitrate at 0.5, 2, or 4 mm for two weeks. Acetylene-reduction assays showed that ammonium nitrate at 4 mm suppressed nodule activity. Daily irrigation with N-free solution subsequently led to a rapid depletion of root-zone N and a concomitant resurgence of nodule activity among plants previously provided 2- and 4-mm ammonium nitrate. These results provide useful information on how to manage fertility to optimize nodulation and show suppression of nodule activity caused by N fertilization can be temporary if excess N is leached from the root zone.