Compensatory response of a legume root-nodule system to nodule herbivory by Sitona hispidulus

A laboratory study was conducted to examine the hypothesis that herbivory of nitrogen-fixing root nodules on legumes causes an exact compensatory response in nodule growth. Plants of Medicago sativa (L.) were grown hydroponically in clear plastic growth pouches so that the number and biomass of root nodules could be estimated nondestructively before, and 10 and 18 days after, partial denodulation. For treatments, plants were subjected to 23% denodulation by first-instar larvae of Sitona hispidulus (F.) (a common herbivore of Medicago and Trifolium) or 50% nodule pruning; additional plants were left untreated. Results indicated that nodule herbivory and nodule pruning caused an overcompensatory response in number of nodules. This was also true for number of nodule units (an indirect measure of nodule biomass) per plant at 10 days after denodulation but had changed to an exact compensatory response by day 18. An inverse relationship between change in number of nodule units and initial number of nodules indicated that compensatory nodulation was regulated by a feedback mechanism. Shoot and root biomasses were not affected by denodulation in this study.     

Effect of NO3 on N2 fixation and nitrogenous solutes of xylem in two nodulated West African geocarpic legumes,Kersting's bean (Macrotyloma geocarpum L.) and Bambara groundnut (Vigna subterranea L.)

Nodulation, nitrogen (N₂) fixation and xylem sap composition were examined in sand cultured plants of Bambara groundnut (Vigna subterranea L.) and Kersting's bean (Macrotyloma geocarpum L.) inoculated with Bradyrhizobium strain CB756 and supplied via the roots for a 4 week period from the third week onwards with different levels of (¹⁵N)-nitrate (0–15 mM). The separate contributions of nitrate and N₂ to plant nitrogen were measured by isotope dilution. Increasing levels of nitrate inhibited nodule growth (measured as dry matter or nodule N) of both species parallel with decreased dependence on symbiotically-fixed N. Specific nodule activity (N₂ fixed g nodule dry⁻¹ d⁻¹ of nodules) was reduced progressively with time in V. subterranea at higher (5 or 15 mM) levels of NO₃, but this was not so for M. geocarpum. Root xylem bleeding sap of both species showed ureides (allantoin and allantoic acid) as predominant (>90%) solutes of nitrogen when plants were relying solely on atmospheric N. Levels of ureide and glutamine decreased and those of asparagine and nitrate in xylem increased with increasing level of applied nitrate. Relative levels of xylem ureide-N were positively correlated (R²=0.842 for M. geocarpum and 0.556 for V. subterranea), and the ratio of asparagine to glutamine in xylem exudate negatively correlated (R²=0.955 for M. geocarpum and 0.736 for V. subterranea) with plant reliance on nitrogen fixation. The data indicate that xylem sap analyses might be useful for indirect field assays of nitrogen fixation by the species and that Kersting's bean might offer some potential as a symbiosis in which N₂ fixation is relatively tolerant of soil N.     

Soybean response to nodulation by bradyrhizobia differing in rhizobitoxine phenotype

Rhizobitoxine-producing (RT⁺) strains of Bradyrhizobium japonicum, differing in their abilities to induce foliar chlorosis with ‘Forrest’ soybean (Glycine max [L.] Merr.), were evaluated for effects on short term shoot productivity, nodulation, N₂ fixation, and nodule protein production under greenhouse conditions. Soybeans were singly inoculated with washed suspensions of (Group II) USDA strains 31, 46, 76, 94, 110, 123 or 130. Strains USDA 110 and USDA 123 (Group I/Ia) were included as RT-controls. The plants were cultured in the absence of combined N in horticultural-grade vermiculite for 49 days. Beginning 21 days after planting, plants were evaluated weekly for chlorophyll, leaf protein and biomass accumulation, nodular contents of leghemoglobin, soluble protein and RT, and total shoot N content. Rhizobitoxine was detected in nodules of all RT⁺ strains with the exception of USDA 31. However, only USDA 76 and USDA 94 produced both quantifiable concentrations of RT and symptoms of RT-induced chlorosis. Coincident with moderate to severe chlorosis were reductions in chlorophyll concentrations, shoot and nodule dry weight, leaf protein and total N₂ fixation. During extended periods of severe chlorosis, reductions in Lb and soluble nodular protein were observed. Based on carbon accumulation, all non-chlorotic treatments were statistically more productive than the chlorotic treatments. Similarly, non-chlorotic Group II treatments tended to fix less carbon relative to the RT-Group I/Ia controls, although these differences were not statistically significant. The results of this study suggest that, in the absence of discernable foliar chlorosis, the effect of RT⁺ (Group II) nodulation on short term soybean productivity is minimal. Published as Miscellaneous Paper No. 1439 of the Delaware Agricultural Experiment Station. Published as Miscellaneous Paper No. 1439 of the Delaware Agricultural Experiment Station.     

Phosphorus requirements and nitrogen accumulation by three mungbean (Vigna radiata (L) Welzek) cultivars

A promising approach for overcoming poor crop yields in phosphorus (P)-deficient soils is to exploit the genetic variation among plants to grow under low P conditions. We examined the P requirements of three mungbean cultivars, T-77, MI-5 and E-72, using four P rates, 0, 30, 60 and 90 mg P kg^-1 soil (designated P₀, P₁, P₂ and P₃, respectively). Nodulation was highest in T-77, and unlike the other cultivars, nodule numbers were not increased by P application. Similarly, growth of T-77 was the highest, and was not influenced by P rates. In contrast shoot yields of MI-5 and E-72 at P₀ were only 76 and 65%, respectively, of the maximum obtained under P application. Nodule dry weight and the amount of N fixed (Ndfa) in each cultivar was enhanced by P application, with T-77 generally giving the lowest response, and accumulating the highest Ndfa. The data suggest a higher P requirement for N₂ fixation (especially for T-77) than for growth. All plants increased their P uptake as P rates increased, with T-77 accumulating the highest amount of P at each P level. Differences in the physiological P use efficiency, PPUE (g shoot mg^-1 P) among genotypes were generally not significant, neither were there any consistent trends as P rates changed. The ability to absorb P therefore appeared to be more important than PPUE in enhancing growth. We conclude from our data that it is possible by selection to obtain plants capable of good growth and high N₂ fixation in soils of low P; cultivar T-77 is a good example.     

Alfalfa root nodule phosphoenolpyruvate carboxylase: characterization of the cDNA and expression in effective and plant-controlled ineffective nodules

Phosphoenolpyruvate carboxylase (PEPC) plays a key role in N₂ fixation and ammonia assimilation in legume root nodules. The enzyme can comprise up to 2% of the soluble protein in root nodules. We report here the isolation and characterization of a cDNA encoding the nodule-enhanced form of PEPC. Initially, a 2945 bp partial-length cDNA was selected by screening an effective alfalfa nodule cDNA library with antibodies prepared against root nodule PEPC. The nucleotide sequence encoding the N-terminal region of the protein was obtained by primer-extension cDNA synthesis and PCR amplification. The complete amino acid sequence of alfalfa PEPC was deduced from these cDNA sequences and shown to bear striking similarity to other plant PEPCs. Southern blots of alfalfa genomic DNA indicate that nodule PEPC is a member of a small gene family. During the development of effective root nodules, nodule PEPC activity increases to a level that is 10- to 15-fold greater than that in root and leaf tissue. This increase appears to be the result of increases in amount of enzyme protein and PEPC mRNA. Ineffective nodules have substantially less PEPC mRNA, enzyme protein and activity than do effective nodules. Maximum expression of root nodule PEPC appears to be related to two signals. The first signal is associated with nodule initiation while the second signal is associated with nodule effectiveness. Regulation of root nodule PEPC activity may also involve post-translational processes affecting enzyme activity and/or degradation.     

2,4-D诱导固氮螺菌和慢生型大豆根瘤菌在小麦上的结瘤作用及其效果

分别在水培和砂培条件下进行了2,4-D诱导固氮螺菌和慢生型大豆根瘤菌在小麦根上的结瘤试验,结果表明2,4-D能诱发它们在小麦根系上形成“类根瘤”,扫描电镜结果证明只有个别细菌进入小麦根瘤细胞内,在细胞间隙有较多的细菌。用乙炔还原法仅检测到接种大豆根瘤菌的小麦根瘤有微量的固氮酶活性,但在盆栽植株的生长方面,看不到2,4-D,2,4-D+固氮螺菌或2,4-D+根瘤菌对小麦生长的促进作用。     

Use of Arabidopsis thaliana and Pseudomonas syringae in the Study of Plant Disease Resistance and Tolerance

The interaction between Arabidopsis thaliana and the bacterium Pseudomonas syringae is being developed as a model experimental system for plant pathology research. Race-specific ("gene-for-gene") resistance has been demonstrated for this interaction, and pathogen genes that determine avirulence have been isolated and characterized. Because certain lines of both Arabidopsis and soybean are resistant to bacteria carrying the avirulence genes avrRpt2 and avrB, extremely similar pathogen recognition mechanisms are apparently present in these two plant species. Isogenic bacterial strains that differ by the presence of single avirulence genes are being used to analyze plant resistance. Plant resistance genes have been identified in crosses between resistant and susceptible lines. The extensive map-based cloning tools available in Arabidopsis are being used to isolate these resistance genes. In a related project, ethylene-insensitive Arabidopsis mutants are being used to examine the role of ethylene in disease development. Ethylene apparently mediates symptom formation in susceptible plants and is not required for resistance, suggesting possible strategies for enhancement of disease tolerance in crops.