R-prime plasmids from Bradyrhizobium japonicum and Rhizobium fredii

The formation of R-prime plasmids was selected in crosses involving soybean microsymbionts with genomic Tn5 insertions and carrying plasmid pJB3JI (with one IS2) copy as donors and Escherichia coli HB101 as recipient. Whereas the parent plasmid was 60 kb, recombinant plasmids between 76 kb and 121 kb were obtained. Restriction and Southern analyses confirmed the mobilization of Tn5 on four R-primes from Bradyrhizobium japonicum I-110 and on an R-prime plasmid from Rhizobium fredii HH303. The largest R-prime plasmid was obtained from the rescue of two symbiotically defective R. fredii mutant strains that required adenosine.Non-standard abbreviation TDP transposon donor pool Scientific article number A-4728 and contribution number 7724 of the Maryland Agricultural Experiment Station     

Conditional lethality involving a cytoplasmic mutant and chlorophyll-deficient malate dehydrogenase mutants in soybean

Summary Conditional lethality in soybean, Glycine max (L.) Merr., occurred in F₂ plants when cytoplasmicchlorophyll mutant Genetic Type T275 was the female parent and when either nuclear mutants T253 or T323 plants were the male parents. Mutant T253 [Mdh1-n (Urbana) y20 (Urbana) k2] is missing two of three mitochondrial malate dehydrogenase isozymes [Mdh1-n (Urbana)] and has yellowish-green leaves [y20 (Urbana)] and a tan-saddle pattern seed coat (k2). Mutant T323 [Mdh1-n (Ames 2) y20 (Ames 2)] also is missing two of three mitochondrial malate dehydrogenase isozymes [Mdh1-n (Ames 2)] and has yellowishgreen leaves [y20 (Ames 2)], but has yellow seed coat (K2). Mutants T275, T253, and T323 are viable both in the field and glasshouse. The genotypes cyt-Y2 Mdh1-n (Urbana) y20 (Urbana) k2/Mdh1-n (Urbana) y20 (Urbana) k2 and cyt-Y2 Mdh1-n (Ames 2) y20 (Ames 2)/Mdh1-n (Ames 2) y20 (Ames 2) are conditional lethals. These genotypes are lethal under field conditions, but plants survive in reduced light under shadecloth in the glasshouse. We do not know if their interaction with cyt-Y2 is due to Mdh1-n, y20, or Mdh1-n y20. The reciprocal cross (cyt-Y2 as male parent) gives viable genotypes. These conditional lethal genotypes should be useful for studies on the interaction between organelle and nuclear genomes.This is journal paper no. J-14777 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA 50011-1010. Project 2985     

Association of RFLP markers with loci conferring broad-based resistance to the soybean cyst nematode (Heterodera glycines)

Soybean cyst nematode (SCN) is a major soybean yield-limiting pest. The present study was conducted to map broad-based SCN resistance loci from the cultivar Hartwig. Two-hundred F₂₃ lines derived from the cross Williams 82 x Hartwig were screened with a fourth-generation SCN inbred and 56 polymorphic molecular markers. Allele states and phenotypes were analyzed using stepwise regression and the model selection was made at P 0.01. Four unlinked RFLP markers (A006, A567, A487, A112) were associated with SCN resistance and the partial coefficient of determinations (R²) were 91%, 1%, 1%, and 1%. We have mapped a new, major SCN resistance locus (A006) and three minor loci (A567, A487, A112). This complete mapping will accelerate the transfer of broad-based resistance without linkage drag and aid in the determination of relationships among various SCN-resistant germplasm sources.     

Molecular markers associated with seed weight in two soybean populations

Seed weight (SW) is a component of soybean, Glycine max (L.) Merr., seed yield, as well as an important trait for food-type soybeans. Two soybean populations, 120 F₄-derived lines of YoungxPI416937 (Pop1) and 111 F₂-derived lines of PI97100xCoker 237 (Pop2), were mapped with RFLP makers to identify quantitative trait loci (QTLs) conditioning SW across environments and populations. The genetic map of Pop1 consisted of 155 loci covering 973 cM, whereas Pop2 involved 153 loci and covered 1600 cM of map distance. For Pop1, the phenotypic data were collected from Plains, GA., Windblow, N.C., and Plymouth, N.C., in 1994. For Pop2, data were collected from Athens, GA., in 1994 and 1995, and Blackville, S.C., in 1995. Based on single-factor analysis of variance (ANOVA), seven and nine independent loci were associated with SW in Pop1 and Pop2, respectively. Together the loci explained 73% of the variability in SW in Pop1 and 74% in Pop2. Transgressive segregation occurred among the progeny in both populations. The marker loci associated with SW were highly consistent across environments and years. Two QTLs on linkage group (LG) F and K were located at similar genomic regions in both populations. The high consistency of QTLs across environments indicates that effective marker-assisted selection is feasible for soybean SW.     

Field evaluation of symbiotic nitrogen fixation by rhizobial strains using15N methodology

Summary Small differences in N₂ fixation by nodulated soybeans (Glycine max. (L.) Merr.), inoculated with various strains ofRhizobium japonicum, were assessed in field experiments using¹⁵N methodology, and compared with yields of plant dry matter and total N. Percentage of plant-N derived from atmospheric N₂ and from fertilizer, and values of %¹⁵N atom excess had lower coefficients of variation than did total N and dry matter yield. Nevertheless the precision of estimates of kg N/ha fixed were sufficient to differentiate only the extremes of the range of strains tested, and there were discrepancies between ranking of strains based on % N derived from fertilizer and on total N yield.     

Divergent selection for dinitrogen fixation and yield in soybean

Summary Nitrogen fixation is generally considered to be a major parameter of productivity in soybean (Glycine max). The aim of the investigations reported here was to analyse the genetic behaviour of this trait in view of its possible use as an indirect criterion of selection for productivity. Divergent selection for nitrogen fixation rate was carried out on F₂ populations obtained from crosses between high-yielding cultivars that are well adapted to French climatic conditions. The genetic component of nitrogen fixation and yield was isolated through the analysis of (1) the nitrogen fixation potentials of the genotypes under controlled conditions and (2) the field yields under favourable conditions. Divergent selection resulted in two groups of genotypes whose nitrogen fixation abilities are significantly different. The F₆ filial progeny obtained by single seed descent from the two groups displayed significantly different abilities for nitrogen fixation and for field productivity. The gain achieved for the nitrogen fixation activity with respect to the mean value of the parents ranged from 20% to 33% for the positive selection, depending on the crosses. The occurrence of positive and significant correlations between the level of nitrogen fixation activity in F₂ plants and N₂ fixation or yield in the F₆ generation corroborates the relatively high heritability of this trait and suggests its possible use as an indirect selection criterion for yield.     

The growth of soybean under free air [CO<Subscript>2</Subscript>] enrichment (FACE) stimulates photosynthesis while decreasing in vivo Rubisco capacity

Down-regulation of light-saturated photosynthesis (A_sat) at elevated atmospheric CO₂ concentration, [CO₂], has been demonstrated for many C₃ species and is often associated with inability to utilize additional photosynthate and/or nitrogen limitation. In soybean, a nitrogen-fixing species, both limitations are less likely than in crops lacking an N-fixing symbiont. Prior studies have used controlled environment or field enclosures where the artificial environment can modify responses to [CO₂]. A soybean free air [CO₂] enrichment (FACE) facility has provided the first opportunity to analyze the effects of elevated [CO₂] on photosynthesis under fully open-air conditions. Potential ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation (V_c,max) and electron transport through photosystem II (J_max) were determined from the responses of A_sat to intercellular [CO₂] (C_i) throughout two growing seasons. Mesophyll conductance to CO₂ (g_m) was determined from the responses of A_sat and whole chain electron transport (J) to light. Elevated [CO₂] increased A_sat by 15–20% even though there was a small, statistically significant, decrease in V_c,max. This differs from previous studies in that V_c,max/J_max decreased, inferring a shift in resource investment away from Rubisco. This raised the C_i at which the transition from Rubisco-limited to ribulose-1,5-bisphosphate regeneration-limited photosynthesis occurred. The decrease in V_c,max was not the result of a change in g_m, which was unchanged by elevated [CO₂]. This first analysis of limitations to soybean photosynthesis under fully open-air conditions reveals important differences to prior studies that have used enclosures to elevate [CO₂], most significantly a smaller response of A_sat and an apparent shift in resources away from Rubisco relative to capacity for electron transport.Abbreviations FACE Free air [CO₂] enrichment - Rubisco Ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP Ribulose-1,5-bisphosphate - SoyFACE Soybean free air [CO₂] enrichment - VPD Vapor pressure deficit     

Assessment of selenium bioavailability from naturally produced high-selenium soy foods in selenium-deficient rats

We assessed the bioavailability of selenium (Se) from a protein isolate and tofu (bean curd) prepared from naturally produced high-Se soybeans. The Se concentrations of the soybeans, the protein isolate and tofu were 5.2 ± 0.2, 11.4 ± 0.1 and 7.4 ± 0.1 mg/kg, respectively. Male weanling Sprague–Dawley rats were depleted of Se by feeding them a 30% Torula yeast-based diet (4.1 μg Se/kg) for 56 days, and then they were replenished with Se for an additional 50 days by feeding them the same diet containing 14, 24 or 30 μg Se/kg from the protein isolate or 13, 23 or 31 μg Se/kg from tofu, respectively. l-Selenomethionine (SeMet) was used as a reference. Selenium bioavailability was determined on the basis of the restoration of Se-dependent enzyme activities and tissue Se concentrations in Se-depleted rats, comparing those responses for the protein isolate and tofu to those for SeMet by using a slope-ratio method. Dietary supplementation with the protein isolate or tofu resulted in linear or log-linear, dose-dependent increases in glutathione peroxidase activities in blood and liver and in thioredoxin reductase activity in liver. Furthermore, supplementation with the protein isolate or tofu resulted in linear or log-linear, dose-dependent increases in the Se concentrations of plasma, liver, muscle and kidneys. These results indicated an overall bioavailability of approximately 101% for Se from the protein isolate and 94% from tofu, relative to SeMet. We conclude that Se from naturally produced high-Se soybeans is highly bioavailable in this model and that high-Se soybeans may be a good dietary source of Se.     

Cultivar-related differences in the distribution of cell-wall-bound thionins in compatible and incompatible interactions between barley and powdery mildew

Leaf-specific thionins of barley (Hordeum vulgare L.) have been identified as a novel class of cell-wall proteins toxic to plant-pathogenic fungi and possibly involved in the defence mechanism of plants. The distribution of these polypeptides has been studied in the host-pathogen system of barley and Erisyphe graminis DC.f.sp. hordei Marchal (powdery mildew). Immunogold-labelling of thionins in several barley cultivars indicates that resistance or susceptibility may be attributed to the presence or absence of thionins at the penetration site in walls and papillae of epidermal leaf cells.All of the leaf-specific thionin genes are confined to the distal end of the short arm of chromosome 6 of barley. None of the genes for cultivarspecific resistance to powdery mildew which have previously been mapped on barley chromosomes are found close to this locus.