Silvia R. Cianzio

A soybean cyst nematode resistance gene points to a new mechanism of plant resistance to pathogens

Soybean (Glycine max (L.) Merr.) is an important crop that provides a sustainable source of protein and oil worldwide. Soybean cyst nematode (Heterodera glycines Ichinohe) is a microscopic roundworm that feeds on the roots of soybean and is a major constraint to soybean production. This nematode causes more than US

Size Distribution and Mineral Nutrients of Soybean Seeds in Response to Drought Stress

1 billion in yield losses annually in the United States alone, making it the most economically important pathogen on soybean. Although planting of resistant cultivars forms the core management strategy for this pathogen, nothing is known about the nature of resistance. Moreover, the increase in virulent populations of this parasite on most known resistance sources necessitates the development of novel approaches for control. Here we report the map-based cloning of a gene at the Rhg4 (for resistance to Heterodera glycines 4) locus, a major quantitative trait locus contributing to resistance to this pathogen. Mutation analysis, gene silencing and transgenic complementation confirm that the gene confers resistance. The gene encodes a serine hydroxymethyltransferase, an enzyme that is ubiquitous in nature and structurally conserved across kingdoms. The enzyme is responsible for interconversion of serine and glycine and is essential for cellular one-carbon metabolism. Alleles of Rhg4 conferring resistance or susceptibility differ by two genetic polymorphisms that alter a key regulatory property of the enzyme. Our discovery reveals an unprecedented plant resistance mechanism against a pathogen. The mechanistic knowledge of the resistance gene can be readily exploited to improve nematode resistance of soybean, an increasingly important global crop.

Mapping genetic loci for iron deficiency chlorosis in soybean

Abstract Accumulation of mineral nutrients in soybean seeds grown under drought stress may have a role in drought tolerance. Because drought stress reduces dry matter accumulation and seed size, drought stress may increase the concentration of mineral nutrients in soybean seeds and may decrease the content of nutrient per seed. The objective was to study the effect of drought on seed size distribution and whether different drought-stress patterns increase mineral nutrient concentration in soybean seeds similar in size. Two experiments were conducted, one in the greenhouse and one in the field. In the greenhouse experiment, three-drought stress treatments were imposed on soybean plants at beginning seed fill (R5) for 23–26 days: well-watered (WW), sudden severe stress (SS), and gradual stress (GS) imposed prior to severe stress. In the field experiment, irrigation and nonirrigation (rainfed) conditions were imposed on soybean plants at R5 for the entire seed filling period. Drought stress decreased seed size in the greenhouse experiment, but not in the field experiment. Within similar seed sizes, drought-stress treatments increased concentrations of phosphorus (P), potassium (K), calcium (Ca), molybdenum (Mo), manganese (Mn), copper (Cu), and zinc (Zn) in seeds above well-watered treatment, but this increase varied between the two experiments. This increase in nutrient concentrations in seeds from drought-stressed plants compared with well-watered plants for seeds similar in sizes indicates that the increase in nutrient concentration in seeds from drought-stressed plants is not necessarily due to a reduction in dry matter accumulation. Accumulation of minerals in seeds might be an important response in drought stress tolerance.

Relationship between seed yield heterosis and molecular marker heterozygosity in soybean

The objective of this study was to map genes controlling iron deficiency chlorosis in two intraspecific soybean [Glycine max (L.) Merrill] populations. Chlorosis symptoms were evaluated by visual scores and spectrometric chlorophyll determinations at the V4 stage (third trifoliolate leaf fully developed) in the field in 1993, and at V2 (first trifoliolate leaf fully developed) and V4 stages in 1994. A total of 89 RFLP and 10 SSR markers in the Pride B216 x A15 population, and 82 RFLP, 14 SSR and 1 morphological I (hilum color) markers in the Anoka x A7 population were used to map quantitative trait loci (QTL) affecting iron deficiency chlorosis. QTL with minor effects were detected on six linkage groups of the Pride B216 x A15 population, suggesting a typical polygene mechanism. In contrast, in the Anoka x A7 population, one QTL contributed an average of 72.7% of the visual score variation and 68.8% of the chlorophyll concentration variation and was mapped on linkage group N. Another QTL for visual score variation, and one for chlorophyll concentration variation were detected on linkage groups A1 and I, respectively. Due to the large LOD score and major genetic effect of the QTL on linkage group N, the quantitative data was reclassified into qualitative data fitting a one major gene model according to the means of the QTL genotypic classes. The major gene was mapped in the same interval of linkage group N using both visual scores and chlorophyll concentrations, thus verifying that one major gene is involved in segregation for iron chlorosis deficiency in the Anoka x A7 population. This study supported a previous hypothesis that two separate genetic mechanisms control iron deficiency in soybean.

Performance and prospects of Rag genes for management of soybean aphid

Abstract In soybean [Glycine max (L.) Merr.] heterosis has been reported for seed yield. Molecular markers may be useful to select diverse parents for the expression of heterosis and yield improvement. The objective of this study was to determine if molecular markers could be used to predict yield heterosis in soybean. From each Maturity Group (MG) II and III, 21 genotypes were selected on the basis of high yield (HY), different geographic origin (GO), and isozyme loci (ISO) and for diversity in restriction fragment length polymorphisms (RFLP), and crosses were made within MGs and selection criteria groups to obtain 6 F1 hybrids per group. The 21 parents and the 24 F1 hybrids of each MG were evaluated for yield in replicated tests at two locations in 2 years, and midparent heterosis (MPH) and high-parent heterosis (HPH) estimates were calculated. On the basis of hybrid performance during the first year, 12 parents (3 per selection criteria group) were chosen in each MG to conduct a second RFLP analysis using 129 probes. Genetic distances (GDM) for pairs of the 12 genotypes were calculated with this RFLP information and correlated with MPH and HPH estimates. Significant MPH averages for seed yield were observed in the combined analysis of variance in each of the four selection criteria groups of MG II, and in the HY, ISO, and GO of MG III. Significant HPH averages were observed only in the ISO and GO groups of MG II. The greatest frequency of F1 hybrids with significant MPH was observed in the ISO and GO groups of both MGs. For HPH, the greatest frequency was observed in the ISO group of both MGs. In both MGs, the ISO group had the largest absolute MPH value; the RFLP group had generally the smallest. The observations indicated that the expression of heterosis in seed yield might be associated with diversity in the isozyme loci present in the parents. For the genotypes included in the second RFLP analysis, correlations of GDMs with MPH and HPH values on an entry-mean basis were low and not significant, indicating that heterosis in yield may not be associated with genetic diversity at the molecular level as determined by RFLPs. The results suggest that in soybean, parent selection on the basis of RFLPs and isozyme loci to exploit heterosis in seed yield may not be feasible. There was no association between genetic distance estimated by the RFLP analysis and seed yield heterosis, and in spite of the observed relationship between isozyme loci and heterosis for yield, the practicality of using the isozyme markers to select parents may be limited because of the reduced number of assayable isozyme loci in soybean.

Molecular characterization of iron deficiency chlorosis in soybean

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an invasive insect pest of soybean [Glycine max (L.) Merr. (Fabaceae)] in North America, and it has led to extensive insecticide use in northern soybean‐growing regions there. Host plant resistance is one potential alternative strategy for managing soybean aphid. Several Rag genes that show antibiosis and antixenosis to soybean aphid have been recently identified in soybean, and field‐testing and commercial release of resistant soybean lines have followed. In this article, we review results of field tests with soybean lines containing Rag genes in North America, then present results from a coordinated regional test across several field sites in the north‐central USA, and finally discuss prospects for use of Rag genes to manage soybean aphids. Field tests conducted independently at multiple sites showed that soybean aphid populations peaked in late summer on lines with Rag1 or Rag2 and reached economically injurious levels on susceptible lines, whereas lines with a pyramid of Rag1 + Rag2 held soybean aphid populations below economic levels. In the regional test, aphid populations were generally suppressed by lines containing one of the Rag genes. Aphids reached putative economic levels on Rag1 lines for some site years, but yield loss was moderated, indicating that Rag1 may confer tolerance to soybean aphid in addition to antibiosis and antixenosis. Moreover, no yield penalty has been found for lines with Rag1, Rag2, or pyramids. Results suggest that use of aphid‐resistant soybean lines with Rag genes may be viable for managing soybean aphids. However, virulent biotypes of soybean aphid were identified before release of aphid‐resistant soybean, and thus a strategy for optimal deployment of aphid‐resistant soybean is needed to ensure sustainability of this technology.

Possible identification of quantitative trait loci affecting iron efficiency in soybean

Abstract Iron deficiency chlorosis (IDC) of soybean occurs on calcareous soils when a cultivar is unable to utilize the iron (Fe) in the soil. The objectives of this study were to map gene(s) for Fe chlorosis resistance in two Glycine max x G. max populations, and to evaluate the use of marker‐assisted selection in breeding for improved Fe chlorosis resistance in soybean. Chlorosis symptoms were evaluated by visual scores and spectrometric chlorophyll determinations at the V4 stage (third trifoliate leaf fully developed) in each of two years. Ninety restriction fragment length polymorphism (RFLP) and 10 simple sequence repeat (SSR) markers in the Pride B216 x A15 population, and 82 RFLP, 14 SSR and one morphological (hilum color) markers in the Anoka × A7 population were used to map quantitative trait loci (QTL) affecting IDC. In the Pride population, 120 random F2 plants were used, 92 were used in the Anoka population. In the Pride population, QTL with minor effects were detected in three linkage groups for visual scores, and for chlorophyll concentration, for a total of five different linkage groups for both traits, indicating a polygene mechanisms for IDC. In the Anoka population, two QTL were each mapped for visual scores and chlorophyll concentration. One of the QTL had major effect and was mapped in the same interval of linkage group N using both visual scores and chlorophyll concentrations, verifying that one major gene is involved in segregation for IDC in this population. Different genetic linkage groups in soybean have been identified by letters, and as such will be used throughout the paper. Two QTL on linkage groups I and N were common to the Pride and Anoka populations and were considered for use in marker‐assisted selection. A thorough analysis conducted in both populations, however, indicated the impossibility of using these markers for marker‐assisted selection. In no cases were both of the QTL‐flanking markers present in one population also present in the other population. On the basis of these results we concluded the markers identified in this study can not be used in marker‐assisted selection because of a general lack of common markers between the two populations.

Pleiotropic soybean mutants defective in both urease isozymes

Abstract Iron (Fe)‐deficiency in soybean [Glycine max (L.) Merr.] may cause yield reductions when certain genotypes are planted on 4 calcareous soil. This research was conducted to map quantitative trait loci (QTL) for Fe‐efficiency in a set of lines (set 1), and to test these associations in another set of lines of the same population (tester set). The population was formed by crossing a Glycine max experimental line (Fe‐inefficient) and a C. soja plant introduction (Fe‐efficient). Set 1 was used to construct a restriction fragment length polymorphism (RFLP) linkage map using 272 markers for the analysis. The tester set was developed to check the results obtained from set 1. Iron‐efficiency was measured using 13 F2‐derived lines of both sets in field plantings on calcareous soils. Set 1 and and the tester set were evaluated over two and one years, respectively. The lines were grown each year at two locations per year, with three replications per location in Iowa. Three markers were significantly (P<0.01)...

Identification of Candidate Genes Underlying an Iron Efficiency Quantitative Trait Locus in Soybean

SummaryTwo new soybean [Glycine max (L.) Merr. cv. Williams] loci, designated Eu2 and Eu3, were identified in which ethyl methanesulfonate (EMS)-induced mutation eliminated urease activity. These loci showed no linkage to each other or to the “Sun-Eul” locus described in the accompanying paper (Meyer-Bothling and Polacco 1987). Unlike sun (seed urease-null) mutations those at Eu2 and Eu3 affected both urease isozymes: the embryo-specific (seed) and the ubiquitous (leaf) urease. The eu2/eu2 mutant had no leaf activity and 0.6% normal seed activity. Two mutant Eu3 alleles were recovered, eu3-e1 and Eu3-e3. The eu3-e1/eu3-e1 genotype lacked both activities while Eu3-e3/Eu3-e3 had coordinately reduced leaf (0.1%) and seed (0.1%) activities. Only the Eu3-e3 mutation showed partial dominance, yielding about 5%–10% normal activity for each urease in the heterozygous state. Each homozygous mutant contained normal levels of embryo-specific urease mRNA and protein subunit, both of normal size. However, urease polymerization was aberrant in all three mutants. In all cases where urease could be measured, it was found to be temperature sensitive and, in addition, the embryospecific urease of Eu3-e3/Eu3-e3 had an altered pH dependence. These mutants may be defective in a urease maturation function common to both isozymes as suggested by the normal levels of urease gene product, coordinately (or nearly so) reduced urease isozyme activities, temperature sensitivity in both ureases (Eu3-e3) and the non-linkage of Eu2 and Eu3 to the locus encoding embryo-specific urease (Sun-Eul). Ubiquitous urease activity is reduced in mutant seed coat and callus culture as well as in leaf and cotyledon tissue. No mutant callus utilized urea (5 to 10 nM) as sole nitrogen source. However, all mutant cell lines tolerated normally toxic levels of urea (25 to 250 mM) added to medium containing KNO3/NH4NO3 as nitrogen source. Urea thus may be used in cell culture as a selection agent for phenotypes either lacking or regaining an active ubiquitous urease.

A new mutant class of soybean lacks urease in leaves but not in leaf-derived callus or in roots

Prevalent on calcareous soils in the United States and abroad, iron deficiency is among the most common and severe nutritional stresses in plants. In soybean (Glycine max) commercial plantings, the identification and use of iron-efficient genotypes has proven to be the best form of managing this soil-related plant stress. Previous studies conducted in soybean identified a significant iron efficiency quantitative trait locus (QTL) explaining more than 70% of the phenotypic variation for the trait. In this research, we identified candidate genes underlying this QTL through molecular breeding, mapping, and transcriptome sequencing. Introgression mapping was performed using two related near-isogenic lines in which a region located on soybean chromosome 3 required for iron efficiency was identified. The region corresponds to the previously reported iron efficiency QTL. The location was further confirmed through QTL mapping conducted in this study. Transcriptome sequencing and quantitative real-time-polymerase chain reaction identified two genes encoding transcription factors within the region that were significantly induced in soybean roots under iron stress. The two induced transcription factors were identified as homologs of the subgroup lb basic helix-loop-helix (bHLH) genes that are known to regulate the strategy I response in Arabidopsis (Arabidopsis thaliana). Resequencing of these differentially expressed genes unveiled a significant deletion within a predicted dimerization domain. We hypothesize that this deletion disrupts the Fe-DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT)/bHLH heterodimer that has been shown to induce known iron acquisition genes.