Induction of ammonium assimilation: leguminous roots compared with nodules using a split root system

A split root system for nitrogen uptake, in which one part of the root system was exposed to nitrogen-free nutrient and the other to circulated buffered ammonium, was used to investigate the effects of ammonium per se on the enzyme pathway for its assimilation in nodules and roots of leguminous plants. Plants of Trifolium repens L. cv. Grasslands Huia grown in the system showed similar growth and similar free amino acid content in the NH⁺₄-fed roots and in nodulated plants. Studies of ammonium assimilation using [¹³N]-NH⁺₄, applied to Glycine max [L.] Merr. cv. Amsoy plants, showed the label to be assimilated into amino acids in the NH⁺₄-fed roots and to be transported to the tops before subsequently appearing in the minus-N side of the split root system. Analysis of the xylem sap showed [¹³N]-asparagine to be the principal labelled amino acid component. In these plants, levels of both allantoate and the nodule-specific isoenzyme aspartate aminotransferase-P₂ were at least 10 times higher in the NH⁺₄-fed roots than in the minus-N side of the split root system. These studies strongly suggest that a nodule-type of ammonium assimilation was occurring in the NH⁺₄-fed side of the split root, and that this part of the root was transporting assimilatory products to the tops of the plants in a fashion analogous to that of a nitrogen-fixing nodule. These data implicate the involvement of NH⁺₄ in the induction of its own assimilatory pathway.     

Transport of phosphate from leaves to leguminous root nodules

Summary When P³²-orthophosphate was applied to leaves ofLotus pedunculatus radioactivity was detected in the root nodules. About 50 per cent of the radioactivity was in ATP, ADP and AMP. More than 90 per cent of the P³² compounds were localized in the soluble part of the plant tissue of the nodules. Rhizobial bacteroids contained only 1 per cent of P³² incorporated in the nodules.     

Fine structure of nitrogen-fixing leguminous root nodules from the Canadian Arctic

Effective (nitrogen-fixing) root nodules of Oxytropis maydelliana Trautv., O. arctobia Bunge and Astragulus alpinus L. were collected in the high Arctic tundra and subsequently processed for structural studies. The cylindrically-shaped perennial nodules consisted of the following tissues: nodule cortex, nodule meristem, nodular vascular bundles, an active central region with uninfected and infected cells at various stages of development, and a proximal region of senescent cells. The active central region was dark red-coloured due to the presence of the pigment leghemoglobin. The host cells became infected by the growth of infection threads into cells recently derived from the nodule meristem and the subsequent endocytotic release of rhizobia from unwalled membrane-bound regions of the infection thread. The host plasma membrane adjacent to the unwalled regions of infection thread gave rise to the peribacteroid membrane which surrounded the released bacteria. Thus, nodule development and the basic tissue arrangement of the arctic nodules was similar to that of cylindrically-shaped nodules formed on temperate species of legumes.
The arctic legume nodules are unique in having large numbers of lipid droplets present in the cytoplasm of the nodule cortex and uninfected cells of the central active region. Newly infected cells also have lipid droplets. More developed infected cells lack lipid droplets but often contain amyloplasts. Mature differentiated bacteria were spherically-shaped and contained electron-dense inclusions. Electron-dense material was also present in vesicles formed from dilated endoplasmic reticulum and in the peribacteroid space. The lipid droplets present in the host cytoplasm of the nodule cortex and uninfected cells of the central tissue may be storage products which are used to support nitrogen-fixation in nodules growing under cool temperatures of this harsh environment.     

Oxidation and reduction of leghemoglobin in root nodules of leguminous plants

Reactions involving changes that affect the function of leghemoglobin (Lb) are reviewed. The chemical nature of Lb and conditions inside nodules, such as slightly acid pH and the presence of metal ions, chelators, and toxic metabolites (nitrite, superoxide radical, peroxides), are conducive for oxidation of ferrous Lb (Lb²⁺) or its oxygenated form (LbO₂) to nonfunctional ferric Lb (Lb³⁺) and ferryl Lb. Because Lb³⁺ is nearly nonexistent in nodules and undergoes observable reduction in vivo, mechanisms must operate in nodules to maintain Lb in the Lb²⁺ state. Redox reactions of Lb are mediated, for the most part, by activated oxygen species: (a) oxidation of LbO₂ to Lb³⁺ involves superoxide; (b) excess peroxide oxidizes LbO₂ and Lb³⁺ to ferryl Lb and may cause breakdown of heme, release of iron, and generation of hydroxyl radicals (protein radicals may be formed in this process); (c) enzymatic reduction of Lb³⁺ requires active flavin and thiol groups and involves formation of peroxide; and (d) direct reduction of Lb³⁺ by NADH is mediated by superoxide and peroxide. Transition metal ions and certain small molecules of nodules such as flavins may act as intermediate electron carriers between NADH and Lb³⁺, increasing the rate of reaction, which then proceeds via superoxide or flavin radicals, respectively.     

Nitrate-reducing capacity of roots and nodules ofAlnus rubra and roots ofPseudotsuga menziesii

Summary Nitrate-reducing capability was demonstrated for root segments of red alder and, at more than twice that rate, for alder nodules. Root segments of Douglas-fir failed to reduce nitrate despite various treatments designed to induce such activity. The reported response of Douglas-fir to nitrate fertilizer may be ascribed either to microbial assimilation of nitrate ions with subsequent liberation of ammonium in the soil or to nitrate assimilation by fungi that form mycorrhizae with Douglas-fir roots.     

The protective value of the colour and shape of the mountain katydid's antipredator defence

Deimatic behaviour is performed by prey when attacked by predators as part of an antipredator strategy. The behaviour is part of a sequence that consists of several defences, for example they can be preceded by camouflage and followed by a hidden putatively aposematic signal that is only revealed when the deimatic behaviour is performed. When displaying their hidden signal, mountain katydids (Acripeza reticulata) hold their wings vertically, exposing striking red and black stripes with blue spots and oozing an alkaloid-rich chemical defence derived from its Senecio diet. Understanding differences and interactions between deimatism and aposematism has proven problematic, so in this study we isolated the putative aposematic signal of the mountain katydid's antipredator strategy to measure its survival value in the absence of their deimatic behaviour. We manipulated two aspects of the mountain katydid's signal, colour pattern and whole body shape during display. We deployed five kinds of clay models, one negative control and four katydid-like treatments, in 15 grids across part of the mountain katydid's distribution to test the hypothesis that their hidden signal is aposematic. If this hypothesis holds true, we expected that the models, which most closely resembled real katydids would be attacked the least. Instead, we found that models that most closely resembled real katydids were the most likely to be attacked. We suggest several ideas to explain these results, including that the deimatic phase of the katydid's display, the change from a camouflaged state to exposing its hidden signal, may have important protective value.     

Studies of the root nodules of leguminous plants IV: Production of indole acetic acid by a bradyrhizobium sp. from the root nodules of a leguminous shrub,crotalaria retusa L

Bradyrhizobium sp. isolated from the root nodules of a leguminous shrub, Crotalaria retusa L., produced a high amount of indole acetic acid (IAA) from tryptophan in the culture. The bacteria preferred D-isomer to the DL- or L-isomer of tryptophan for the IAA production. The IAA production could be increased up to 153.6% over control by supplementing the medium with arabinose (0.5%), ZnSO₄(0.01 μg/ml), KNO₃ (0.1%), thiamine-HCl (0.01 μg/ml) and EDTA (5 μg/ml). The possible role of the rhizobial production of IAA with the rhizobia-legume symbiosis is discussed.     

The influence of shape and colour cue classes on facial health perception

Facial appearance signals information about an individual, and one trait in particular is vitally important for social interaction and mate choice decisions: physical health. Facial cues to health can be divided into two broad classes - facial shape, which is linked to previous health and is relatively fixed; and facial colouration, which changes over the short-term, reflecting current health. These cue classes in themselves give insight into the kinds of health condition valued by human observers when making social inferences. Here, using novel and generalizable methods, the influence of these cue classes on health perception and their link to a measure of general health are examined. Study One employs a Brunswik lens model approach, finding that observers utilise exclusively shape cues to judge health, and that of these shape cues, only averageness is related to a measure of self-reported general health. Study Two shows that when averageness and carotenoid colouration are varied together, both make separable contributions to perceived health, but that averageness explains a larger proportion of variance. Taken together, these results indicate that humans may have evolved to favour cues to previous condition when judging health, because they are more valid. However, the findings also suggest that the role of facial appearance in perceiving health is more complex than previously thought, with different cues potentially reflecting specific aspects of physiological health.     

Computational investigation of small RNAs in the establishment of root nodules and arbuscular mycorrhiza in leguminous plants

Many small RNAs have been confirmed to play important roles in the development of root nodules and arbuscular mycorrhiza. In this study, we carried out the identification of certain small RNAs in leguminous plants(Medicago truncatula, soybean, peanut and common bean), such as miRNAs, tRFs and srRNAs, as well as the computational investigation of their regulations. Thirty miRNAs were predicted to be involved in establishing root nodules and mycorrhiza, and 12 of them were novel in common bean and peanut. The generation of tRFs in M. truncatula was not associated with tRNA gene frequencies and codon usage. Six tRFs exhibited different expressions in mycorrhiza and root nodules. Moreover, srRNA~(5.8S) in M. truncatula was generated from the regions with relatively low conservation at the rRNA 3′ terminal. The protein-protein interactions between the proteins encoded by the target genes of miRNAs, tRFs and srRNAs were computed. The regulation of these three types of sRNAs in the symbiosis between leguminous plants and microorganisms is not a single regulation of certain signaling or metabolic pathways but a global regulation for the plants to own growth or specific events in symbiosis.     

Control of nodule number by the phytohormone abscisic Acid in the roots of two leguminous species

The effects of the phytohormone abscisic acid (ABA) on plant growth and root nodule formation were analyzed in Trifolium repense (white clover) and Lotus japonicus, which form indeterminate and determinate nodules, respectively. In T. repense, although the number of nodules formed after inoculation with Rhizobium leguminosarum bv. trifolii strain 4S (wild type) was slightly affected by exogenous ABA, those formed by strain H1(pC4S8), which forms ineffective nodules, were dramatically reduced 28 days after inoculation (DAI). At 14 and 21 DAI, the number of nodules formed with the wild-type strain was decreased by exogenous ABA. In L. japonicus, the number of nodules was also reduced by ABA treatment. Thus, exogenous ABA inhibits root nodule formation after inoculation with rhizobia. Observation of root hair deformation revealed that ABA blocked the step between root hair swelling and curling. When the ABA concentration in plants was decreased by using abamine, a specific inhibitor of 9-cis-epoxycarotenoid dioxygenase, the number of nodules on lateral roots of abamine-treated L. japonicus increased dramatically, indicating that lower-than-normal concentrations of endogenous ABA enhance nodule formation. We hypothesize that the ABA concentration controls the number of root nodules.     

Routes of pyruvate synthesis in phosphorus-deficient lupin roots and nodules

Here, nodulated lupins (Lupinus angustifolius (cv Wonga)) were hydroponically grown at low phosphate (LP) or adequate phosphate (HP). Routes of pyruvate synthesis were assessed in phosphorus (P)-starved roots and nodules, because P-starvation can enhance metabolism of phosphoenolpyruvate (PEP) via the nonadenylate-requiring PEP carboxylase (PEPc) route. Since nodules and roots may not experience the same degree of P stress, it was postulated that decreases in metabolic inorganic phosphorus (Pi) of either organ, should favour more pyruvate being synthesized from PEPc-derived malate. Compared with HP roots, the LP roots had a 50% decline in Pi concentrations and 55% higher ADP : ATP ratios. However, LP nodules maintained constant Pi levels and unchanged ADP : ATP ratios, relative to HP nodules. The LP roots had greater PEP metabolism via PEPc and synthesized more pyruvate from PEPc-derived malate. In nodules, P supply did not influence PEPc activities or levels of malate-derived pyruvate. These results indicate that nodules were more efficient than roots in maintaining optimal metabolic Pi and adenylate levels during LP supply. This caused an increase in PEPc-derived pyruvate synthesis in LP roots, but not in LP nodules.