超结瘤大豆根瘤的亚显微结构

电镜观察表明,超结瘤大豆未受侵染的宿主细胞中有一明显增大的细胞核。幼年美菌体为椭圆形,里面有个拟核区,正常类菌体有完整的周膜和PHB颗粒。受侵染的寄主细胞中出现类似无效根瘤的异常现象：少数类菌体退化或溶解,还有空周膜及裸露的类菌体,这可能是超结瘤大豆固氮活性较低的原因。     

Regulation of Nodule Efficiency by the Undisturbed Soybean Plant1

Acetylene reduction activity (ARA) has been widely used to estimatecomparative rates of nitrogen fixation in soybean [Glycine max(L.) Merr.]. To use the non-destructive closed-container ARAassay to study plant-mediated regulation of N₂ fixation, itwas first necessary to determine the validity of the ARA procedure.Thus, well-nodulated, hydroponically-grown soybean plants wereindividually assayed for ARA twice weekly for 5 weeks. Plantswere subsequently analysed for total nitrogen by the Kjeldahlprocedure and the values obtained were compared with the N-accumulationvalues estimated by ARA. For the 168 plants examined, ARA measurementsunderestimated N₂ fixation by approximately 50%. However, N₂fixation was underestimated by approximately 80% in plants witha very low ARA whereas N₂ fixation was underestimated by only20% in plants with a high ARA. Because of the accuracy and reliabilityof the Kjeldahl measurements, these results show that nodulatedsoybean plants can exhibit different efficiencies in acetylenereduction. Thus, vigorous, well-nodulated, hydroponically-grownsoybean plants readily control nodule efficiency and the N fixationprocess.     

Soybean (Glycine max) Nodule Physical Traits Associated with Permeability Responses to Oxygen

Nodule permeability (P) controls the amount of O2 entering the nodule and is an important determinant of N2 fixation. Modulation of water volume in the intercellular spaces of the nodule cortex was hypothesized to change the effective thickness of a diffusion barrier and account for changes in P. This hypothesis was examined by evaluating physical traits of nodules that may affect P. The first test of the hypothesis was to determine whether alterations in P may result in changing both the density and the air space content of nodules as the water content of intercellular spaces was varied. Density of nodules exposed to 21 kPa O2 increased as the time following detachment from the plant increased from 5 to 60 min. Nodules from soybean (Glycine max [L.] Merr.) plants shaded for 48 h had a lower fractional air space content than nodules from control plants. Nodule detachment and prolonged shading decreased P, and the increase in density and decrease in fractional air space content associated with decreased P in these treatments supports the proposed hypothesis. The second test of the hypothesis was to determine whether nodules released water easily in response to water potential gradients. The intrinsic capacitance of nodules determined by pressure-volume analysis was 0.29 MPa-1 and indicated that the tissue can release relatively large amounts of water from the symplast with only small changes in total nodule water potential. Estimates of the bulk modulus of elasticity ranged from 0.91 to 2.60 MPa and indicated a high degree of elasticity. It was concluded that the physical properties of nodules were consistent with P modulation by the release and uptake of intercellular water in the nodule cortex.     

Short-Term Nitrate Effects on Hydroponically-Grown Soybean cv. Bragg and its Supernodulating Mutant: II. DISTRIBUTION AND RESPIRATION OF RECENTLY-FIXED 13C-LABELLED PHOTOSYNTHATE

Fully symbiotic or nitrate treated (3 d, 4·0 mol m^–3)soybean (Glycine max [L.] Merr.) cv. Bragg and a nitrate tolerantsupernodulating soybean mutant nts 1007 were exposed to ¹³Cenriched CO₂ for a period of 10 h. During this period and forthe subsequent 24 h, continuous measurements of ¹³CO₂ and ¹²CO₂evolution of their root systems were undertaken. Three harvestsduring the experiment allowed determinations of the distributionof recently fixed carbon in different plant organs. These measurementsindicated higher dependence of N₂ fixation in nts 1007 on recentlyfixed carbon (RFC) by showing elevated RFC concentrations innodules as well as their augmented respiration. Root respirationof both genotypes was generally more reliant on stored carbon. Nitrate induced in all measured parameters a clear responsein the mutant analogous to the wild type, but quantitative differencesremained throughout. Nodule respiratory activity, the relativespecific activity (RSA), and the utilization of RFC were substantiallyreduced, but remained higher in nts 1007 than in Bragg, whilethe demand of roots for RFC increased in Bragg more than inthe supernodulator. The elevated carbon requirement of the nodulecomplement of the mutant and a high dependence on recently fixedcarbon could be attributed to higher nodule growth and maintenancecosts of the supernodulating genotype and were not associatedwith augmented nitrogen fixation activity. This less efficientutilization of carbon and the associated almost parasitic characterof the nodule complement of nts 1007 is considered to be thecause of reduced growth of the mutant. No evidence was foundfor a physiologically based nitrate tolerance in terms of nitrogenfixation. Key words: Glycine max, nitrate, N₂fixation, respiration, carbon partitioning, steady-state labelling     

Conservation of a Symbiotic DNA Region in Soybean Root Nodule Bacteria

Bradyrhizobium japonicum USDA 3I1b110 contains a DNA region in which symbiotic genes and many repeated sequences are closely linked. Hybridization analysis revealed that this region was highly conserved in some B. japonicum strains (USDA 24, USDA 122, USDA 123, ATCC 10324, 61A24) but not in others (USDA 76, 61A76, 61A101). The genomic presence of multiple copies of one of the repeated sequences (RSα) appeared to be specifically characteristic for soybean root nodule bacteria, including the fast-growing Rhizobium fredii, which carries most of these RSα copies on the symbiotic plasmid.     

Temperature-Sensitive Initiation of Chromosome Replication in a Mutant of Bacillus subtilis

A mutant of Bacillus subtilis Ts37 has been isolated in which deoxyribonucleic acid (DNA) synthesis is inhibited at high temperature. The results presented here indicate that the process of initiation of DNA replication is temperature sensitive in this mutant. After shifting to 45 C, DNA increases 40 to 50% before synthesis ceases; an inhibition of protein synthesis permits an equivalent amount of DNA to be synthesized. A density shift experiment coupled with a marker frequency analysis shows that DNA synthesized at 45 C is highly enriched in the markers situated at the end of the chromosome. Transforming DNA extracted from a culture which has been incubated at 45 C exhibits the relative transforming efficiency for origin and terminus markers characteristic of completed chromosomes. After a shift back from 45 C to 30 C, reinitiation appears to occur always in the same region of the bacterial chromosome; in addition, replication as well as cell division is synchronized.     

Temperature-Sensitive Initiation of Chromosome Replication in a Mutant of Escherichia coli

The properties of Escherichia coli mutant D2-47LT indicate that it is temperature-sensitive for a protein required for the initiation of chromosome replication. The results of several different experiments are consistent with this hypothesis, and no support was found for the alternate hypotheses tested. Although the strain is usually unable to initiate replication at 42 C, some of the initiation proteins are apparently synthesized at the restrictive temperature. This can cause initiation on partially replicated, but not completed, chromosomes. It appears that the temperature-sensitive protein is required for initiation on completed chromosomes.     

Light Microscopy Study of Nodule Initiation in Pisum sativum L. cv Sparkle and in Its Low-Nodulating Mutant E2 (sym 5)

We compared nodule initiation in lateral roots of Pisum sativum (L.) cv Sparkle and in a low-nodulating mutant E2 (sym 5). In Sparkle, about 25% of the infections terminated in the epidermis, a similar number stopped in the cortex, and 50% resulted in the formation of a nodule meristem or an emerged nodule. The mutant E2 (sym 5) was infected as often as was the parent, and it formed a normal infection thread. In the mutant, cell divisions rarely occurred in advance of the infection thread, and few nodule primordia were produced. Growing the mutant at a low root temperature or adding Ag⁺ to the substrate increased the number of cell divisions and nodule primordia. We conclude that, in the E2 line, the infection process is arrested in the cortex, at the stage of initial cell divisions before the establishment of a nodule primordium.     

Phosphorylation-Dependent Regulation of G-Protein Cycle during Nodule Formation in Soybean

Signaling pathways mediated by heterotrimeric G-protein complexes comprising Gα, Gβ, and Gγ subunits and their regulatory RGS (Regulator of G-protein Signaling) protein are conserved in all eukaryotes. We have shown that the specific Gβ and Gγ proteins of a soybean (Glycine max) heterotrimeric G-protein complex are involved in regulation of nodulation. We now demonstrate the role of Nod factor receptor 1 (NFR1)-mediated phosphorylation in regulation of the G-protein cycle during nodulation in soybean. We also show that during nodulation, the G-protein cycle is regulated by the activity of RGS proteins. Lower or higher expression of RGS proteins results in fewer or more nodules, respectively. NFR1 interacts with RGS proteins and phosphorylates them. Analysis of phosphorylated RGS protein identifies specific amino acids that, when phosphorylated, result in significantly higher GTPase accelerating activity. These data point to phosphorylation-based regulation of G-protein signaling during nodule development. We propose that active NFR1 receptors phosphorylate and activate RGS proteins, which help maintain the Gα proteins in their inactive, trimeric conformation, resulting in successful nodule development. Alternatively, RGS proteins might also have a direct role in regulating nodulation because overexpression of their phospho-mimic version leads to partial restoration of nodule formation in nod49 mutants.     

Proto-Oncogenic Role of Mutant IDH2 in Leukemia Initiation and Maintenance

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In Vivo Regulatory Phosphorylation of Soybean Nodule Phosphoenolpyruvate Carboxylase

In this report we provide evidence that cytosolic phosphoenolpyruvate carboxylase (PEPC) in soybean (Glycine max L.) root nodules is regulated in vivo by a seryl-phosphorylation cycle, as with the C4, Crassulacean acid metabolism, and C3 leaf isoforms. Pretreatment of parent plants by stem girdling for 5 or 14 h caused a significant decrease in the apparent phosphorylation state of nodule PEPC, as indicated by the 50% inhibition constant (L-malate) and specific activity values assayed at suboptimal conditions, whereas short-term darkness alone was without effect. However, extended (26 h) darkness led to the formation of a relatively dephosphorylated nodule PEPC, an effect that was reversed by illuminating the darkened plants for 3 h. This reversal of the apparent phosphorylation state in the light was prevented by concomitant stem girdling. In contrast, the optimal activity of nodule PEPC and its protein level showed little or no change in all pretreated plants. These results suggest that the phosphorylation state of PEPC in soybean root nodules is possibly modulated by photosynthate transported recently from the shoots. In situ [32P]orthophosphate labeling, immunoprecipitation, and phosphoamino acid analyses confirmed directly that PEPC in detached intact soybean nodules is phosphorylated on a serine residue(s).     

Nodule Nitrate Reductase as a Source of Reduced Nitrogen in Soybean, Glycine max

Hydroponically grown soybeans were fed ¹⁵N-enriched NaNO₃ at nine reproductive stages of development. The stem exudates contained excess ¹⁵N in the fully reduced nitrogen fraction. The soybean nodules had high nitrate reductase activity, whereas the roots had no detectable nitrate reductase activity. Based on these results, we concluded that the nodule nitrate reductase system has the potential of contributing significantly to the nitrogen economy of the plant.     

Inhibition of Nodule Development in Soybean by Nitrate or Reduced Nitrogen

Imsande, J. 1986. Inhibition of nodule development in soybeanby nitrate or reduced nitrogen.—J. exp. Bot. 37: 348–355. Nodulation of hydroponically grown soybean plants [Glycine max(L.) Merr.] is inhibited by continuous growth in the presenceof 4· mol m^–3 KNO₃ The presence of 4·0 molm^–3 ‘starter nitrate’ for 3-6 d during noduledevelopment, however, subsequently stimulates nodule dry weightaccumulation and nitrogenase activity. These stimulations occureven though 4· mol m^–3 nitrate temporarily delaysnodule development, i.e. the late steps of nodule developmentare reversibly inhibited by a short-term exposure to 4·0mol m^–3 nitrate. On the other hand, treatment with 4·0mol m^–3 nitrate in excess of 14 d significantly reducesnodule dry weight Thus, extended growth in the presence of 4·0mol m^–3 KNO₃ seems to block both early and late stepsof nodule development. Nodulation of hydroponically grown soybeansis also inhibited by continuous growth in the presence of 2·0mol m^–3 (NH₄)₂SO₄ This inhibition is not caused by acidityof the growth medium. On the other hand, nodule development6 d after inoculation with Rhizoblum japonicum is not delayedby a 7-d exposure to 2·0 mol m^–3 (NH₄)₂SO₄ butis partially inhibited by a prolonged exposure to (NH₄)₂SO₄Because repression of nodulation by 4·0 mol m^–3KNO₃ is more severe than that by 2·0 mol m^–3 (NH₄)₂SO₄and because ammonium taken up by the soybean plant is not activelyoxidized to nitrate, it is suggested that there are at leasttwo mechanisms by which nitrate utilization represses noduleformation in soybean. Key words: Glycine max, nitrogen, nitrogen fixation, nodulation