SCN Resistance Research Articles

Stacking Resistance Alleles from Wild and Domestic Soybean Sources Improves Soybean Cyst Nematode Resistance

ABSTRACTThe soybean cyst nematode (SCN; Heterodera glycines Ichinohe) is the most economically important soybean [Glycine max (L.) Merr.] pathogen in the United States. Field SCN populations are adapting to the narrowly based SCN resistance currently deployed in soybean cultivars. The objective of our research was to measure the effects of combinations of SCN resistance genes or quantitative trait loci (QTL) from the wild soybean (Glycine soja Siebold & Zucc.) PI 468916 and the domesticated soybean accessions PI 88788 and PI 437654. Two populations were developed to test the combinations of QTL and genes. Both populations segregated for the G. soja resistance QTL cqSCN‐006 and cqSCN‐007. Population 1 also segregated for resistance from PI 88788 and Population 2 segregated for resistance from PI 437654. The populations were tested for resistance to three SCN isolates in a greenhouse and with single nucleotide polymorphism (SNP) and simple sequence repeat (SSR) markers. In both populations, the two G. soja resistance alleles significantly increased SCN resistance compared with the alternative alleles. The SCN resistance alleles rhg1 and Rhg4 from PI 437654 and rhg1‐b from PI 88788 also significantly increased resistance compared with the alternative alleles. The two G. soja QTL alleles significantly enhanced the resistance derived from PI 88788. These results show that SCN resistance can be increased through stacking genes and QTL from multiple resistance sources.

Novel quantitative trait loci for broad-based resistance to soybean cyst nematode (Heterodera glycines Ichinohe) in soybean PI 567516C

Soybean cyst nematode (SCN, Heterodera glycines Ichinohe) is the most destructive pest of soybean worldwide. Host plant resistance is an effective approach to control this pest. Plant introduction PI 567516C has been reported to be highly resistant to multiple-HG types of SCN. The objectives of this study were to identify and map novel quantitative trait loci (QTL) for SCN resistance to six HG types (also known as races 1, 2, 3, 5, 14, and LY1). Mapping was conducted using 250 F(2:3) progeny derived from a Magellan (susceptible)×PI 567516C (resistant) cross. F(6:7) recombinant inbred lines (RILs) developed from the F(2:3) progeny were employed to confirm the putative QTL identified. A total of 927 polymorphic simple sequence repeats (SSR) and single nucleotide polymorphism (SNP) markers were genotyped. Following the genetic linkage analysis, permutation tests and composite interval mapping were performed to identify and map QTL. Four QTL were associated with resistance to either multiple- or single-SCN HG types. Two QTL for resistance to multiple-SCN HG types were mapped to Chromosomes 10 and 18 and have not been reported in other SCN resistance sources. New QTL were confirmed by analysis of 250 F(6:7) RILs from the same population. SSR and SNP markers closely associated with these QTL can be useful for the development of near-isogenic lines for fine-mapping and positional cloning of candidate genes for SCN resistance.

A nematode demographics assay in transgenic roots reveals no significant impacts of the Rhg1locus LRR-Kinase on soybean cyst nematode resistance

BackgroundSoybean cyst nematode (Heterodera glycines, SCN) is the most economically damaging pathogen of soybean (Glycine max) in the U.S. The Rhg1 locus is repeatedly observed as the quantitative trait locus with the greatest impact on SCN resistance. The Glyma18g02680.1 gene at the Rhg1 locus that encodes an apparent leucine-rich repeat transmembrane receptor-kinase (LRR-kinase) has been proposed to be the SCN resistance gene, but its function has not been confirmed. Generation of fertile transgenic soybean lines is difficult but methods have been published that test SCN resistance in transgenic roots generated with Agrobacterium rhizogenes.ResultsWe report use of artificial microRNA (amiRNA) for gene silencing in soybean, refinements to transgenic root SCN resistance assays, and functional tests of the Rhg1 locus LRR-kinase gene. A nematode demographics assay monitored infecting nematode populations for their progress through developmental stages two weeks after inoculation, as a metric for SCN resistance. Significant differences were observed between resistant and susceptible control genotypes. Introduction of the Rhg1 locus LRR-kinase gene (genomic promoter/coding region/terminator; Peking/PI 437654-derived SCN-resistant source), into rhg1- SCN-susceptible plant lines carrying the resistant-source Rhg4+ locus, provided no significant increases in SCN resistance. Use of amiRNA to reduce expression of the LRR-kinase gene from the Rhg1 locus of Fayette (PI 88788 source of Rhg1) also did not detectably alter resistance to SCN. However, silencing of the LRR-kinase gene did have impacts on root development.ConclusionThe nematode demographics assay can expedite testing of transgenic roots for SCN resistance. amiRNAs and the pSM103 vector that drives interchangeable amiRNA constructs through a soybean polyubiqutin promoter (Gmubi), with an intron-GFP marker for detection of transgenic roots, may have widespread use in legume biology. Studies in which expression of the Rhg1 locus LRR-kinase gene from different resistance sources was either reduced or complemented did not reveal significant impacts on SCN resistance.

Development of SNP markers and haplotype analysis of the candidate gene for rhg1, which confers resistance to soybean cyst nematode in soybean

Soybean cyst nematode (SCN; Heterodera glycines Ichinohe) is one of the most destructive pests in the cultivation of soybean (Glycine max (L.) Merr.) worldwide. Markers based on the SCN resistance gene will enable efficient marker-assisted selection (MAS). We sequenced the candidate gene rhg1 in six resistant and two susceptible soybean genotypes and identified 37 SNPs (single nucleotide polymorphisms) among the sequences, of which 11 were in the coding region. Seven of these 11 SNPs led to changes in the amino acid sequence of the gene. The amino acid sequence we obtained differs from the previously published one by a stretch of 26–27 amino acids. Six codominant allele-specific SNP markers based on agarose gel detection were developed and tested in 70 genotypes, among which occurred only nine different haplotypes. Two neutrality tests (Tajima’s D and Fu and Li’s F) were significant for the six SNP loci in the 70 genotypes, which is consistent with intensive directional selection. A strong LD pattern was detected among five SNPs except 2868T > C. Two SNPs (689C > A and 757C > T) formed one haplotype (689C-757C) that was perfectly associated with SCN resistance. The new allele-specific PCR markers located in the alleged sequence of the rhg1 candidate gene, combined with the microsatellite marker BACR-Satt309, will significantly improve the efficiency of MAS during the development of SCN-resistant cultivars.

QTL, additive and epistatic effects for SCN resistance in PI 437654

PI 437654 is a unique accession because of its resistance to nearly all HG types (races) of soybean cyst nematode (Heterodera glycines Ichinohe; SCN). Objectives of this study were to confirm and refine the locations and gene action associated with SCN resistance previously discovered in PI 437654, and to identify new QTLs that may have been missed because of low coverage with genetic markers used in previous studies. Using 205 F(7:9) RILs and 276 SSR and AFLP molecular markers covering 2,406.5 cM of 20 linkage groups (LGs), we confirmed and refined the locations of major SCN resistance QTLs on LG-A2, -B1, and -G previously identified in PI 437654 or other resistant sources. We found that these major QTLs have epistatic effects among them or with other loci for SCN resistance. We also detected some new QTLs with additive or epistatic effects for SCN resistance to different HG types (races) on all LGs except LGs-B2 and -D1b. The QTL on LG-G was associated with resistance to HG types 2.5.7, 1.2.5.7, 0, and 2.7 (races 1, 2, 3, and 5), and it contributed a large proportion of the additive effects. The QTL on LG-A2 was associated with resistance to HG types 2.5.7 and 0 (races 1 and 3). The QTL on LG-B1, associated with resistance to HG types 2.5.7, 0, 2.7 (races 1, 3, and 5), was the similar QTL found in PI 90763 and PI 404198B. In addition to QTL on LGs-A2, -B1 and -G, a novel additive QTL associated with SCN resistance to HG types 0, 2.7, and 1.3.5.6.7 (race 3, 5, and 14) was identified on LG-I flanked by Sat_299 and Sat_189. Several minor QTLs on LGs-C1, D1a, H, and K were also found to be associated with SCN resistance. Confirmation of the new resistance QTL is underway by evaluating another RIL population with a different genetic background.

Growth, Yield, and Yield Component Changes among Old and New Soybean Cultivars

Soybean [Glycine max (L.) Merr.] yield has increased at a rate of 25 to 30 kg ha−1 yr−1 due in part to improved genetic gain, and has been further advanced by the addition of resistance to soybean cyst nematode (Heterodera glycines Ichinohe; SCN) in new cultivars. The objective was to determine specific growth changes that explain the yield improvement from old to new cultivars and the further yield improvement gained from the addition of SCN resistance. Studies were conducted at three Iowa locations during 2005 and 2006. Two old and two new SCN‐susceptible, and two new SCN‐resistant cultivars were evaluated for total dry matter (TDM) accumulation and leaf area index (LAI) through the season along with yield and yield components at harvest. New cultivars produced yields superior to older cultivars due to increased crop growth rate (CGR) culminating in greater TDM 105 days after emergence (DAE). Yield was strongly associated with the number of seeds produced m−2 and this yield component accounted for almost all of the yield differences among cultivars. Seeds m−2 was positively related to CGR between 42 and 105 (growth stage R1–R5.5) DAE and to LAI 105 DAE. New SCN‐resistant cultivars produced yields 17 to 19% greater than new susceptible cultivars across three locations. Increased TDM and CGR explained the yield response at the low‐yield location, but not at the high‐yield locations. Apparent harvest index (HI) was similar among all cultivars at each location. Selection for increased yield has indirectly selected for increased TDM and CGR with a similar amount partitioned to seed dry weight. Future yield gains will be made by (i) increasing the amount and the rate of dry matter (DM) and (ii) through the increased production and duration of leaf area.

Locus Interactions Underlie Seed Yield In Soybeans Resistant toHeterodera glycines

The objective of this analysis was to identify linkats in genomic regions underlying seed yield and root SCN resistance QTL. Used were groups of cultivars selected from a single recombinant inbred (RIL) population

Response of Old and New Soybean Cultivars to Heterodera glycines Ichinohe

Soybean [Glycine max (L.) Merr.] cyst nematode (Heterodera glycines Ichinohe; SCN) causes significant yield loss each year. New cultivars (released since 1997) have superior yield compared with older cultivars (released between 1938 and 1983), but responses to SCN have not been evaluated. Studies were established at three locations in Iowa for 2 yr to measure yield loss of 23 cultivars that differed in year of release and SCN resistance in locations with varying SCN population density and Heterodera glycines Type (HG Type). Initial SCN population densities (Pi) were <1000 eggs 100 cm−3 (HG Type 2.7) at Whiting, between 1000 and 4000 eggs 100 cm−3 (HG Type 1.2.5.7 and 2.5.7) at Nevada, and between 4000 and 12,500 eggs 100 cm−3 (HG Type 0 and 7) at De Witt in 2005 and 2006. Analysis of simple effects from year × location interactions consistently showed that new SCN‐resistant cultivars had lower final SCN population densities (Pf) and reproduction factors (Rf) compared with old and new SCN‐susceptible cultivars. Among all locations, seed yield of new SCN‐resistant cultivars was 14% greater than new SCN‐susceptible cultivars and 32% greater than old SCN‐susceptible cultivars. Yield increase was a result of increased seed production (seeds m−2) and not from a change in seed mass. Yield of new SCN‐susceptible cultivars was 18% greater than old SCN‐susceptible cultivars but did not provide greater control of SCN. These data indicate that yield increases with new SCN‐susceptible cultivars are not the result of improved SCN management. Data support the selection of SCN‐resistant cultivars for fields in which SCN has been identified, regardless of Pi or HG Type to increase yield and reduce SCN population densities.

Soybean Cultivar and Planting Date Response to Soil Fumigation

Soybean [Glycine max (L.) Merr.] seed yield will be maximized by planting the last week of April or the first week of May in Iowa. Soybean cyst nematode (Heterodera glycines Ichinohe; SCN) is present in the majority of Iowa fields and may influence cultivar responses to planting date. This study was conducted to determine whether specific cultivars need to be selected based on planting date. The effects of soil fumigation, planting date, and cultivar selection were evaluated at three sites in Iowa near De Witt, Nevada, and Whiting during 2005 and 2006. These sites varied in rainfall, soil type, and SCN population densities. Yields of the first two planting dates were increased an average of 10% by fumigation and yields of the SCN‐susceptible cultivar NK‐S32‐G5 and the SCN‐resistant cultivar P91M90 increased 11%. Cultivars with the PI88788 source of SCN resistance produced greater yields than other sources of SCN resistance in the nonfumigated treatment but yields were equal among all sources of SCN resistance in the fumigated treatment. Cultivars responded similarly to planting date, with maximum yields at early dates followed by a consistent decline as planting was delayed. The combination of SCN‐resistant cultivars and early planting resulted in the greatest and most consistent yields across diverse production environments. Cultivar selection should focus on high‐yielding, adapted cultivars with resistance to SCN when SCN is present in a field, and these cultivars should be selected for both early and late planting dates.

QTL associated with horizontal resistance to soybean cyst nematode in Glycine soja PI464925B

Soybean cyst nematode (Heterodera glycines Ichinohe; SCN) is the primary disease responsible for yield loss of soybean [Glycine max (L.) Merr.]. Resistant cultivars are an effective management tool; however, the sources currently available have common resistant genes. Glycine soja Sieb. and Zucc., the wild ancestor of domesticated soybean, represents a diverse germplasm pool with known SCN resistance. The objectives of this research were to: (1) determine the genetic variation and inheritance of SCN resistance in a G. max ('S08-80') x G. soja (PI464925B) F (4:5) recombinant inbred line (RIL) population; and (2) identify and evaluate quantitative trait loci (QTL) associated with SCN resistance. Transgressive segregation for resistance was observed, although neither parent was resistant to the Chatham and Ruthven SCN isolates. Broad sense heritability was 0.81 for the Ruthven and 0.91 for the Chatham isolate. Root dry weight was a significant covariate that influenced cyst counts. One RIL [female index (FI) = 5.2 +/- 1.11] was identified as resistant to the Chatham isolate (FI < 10). Seventeen and three RILs infected with Chatham and Ruthven isolates, respectively, had mean adjusted cyst counts of zero. Unique and novel QTL, which derived resistance from G. soja, were identified on linkage groups I, K, and O, and individually explained 8, 7 and 5% (LOD = 2.1-2.7) of the total phenotypic variation, respectively. Significant epistatic interactions were found between pairs of SSR markers that individually may or may not have been associated with SCN resistance, which explained between 10 and 15% of the total phenotypic variation. Best-fit regression models explained 21 and 31% of the total phenotypic variation in the RIL population to the Chatham and Ruthven isolates, respectively. The results of this study help to improve the understanding of the genetic control of SCN resistance in soybean caused by minor genes resulting in horizontal resistance. The incorporation of the novel resistance QTL from G. soja could increase the durability of SCN-resistance in soybean cultivars, especially if major gene resistance breaks down.

Glycine soja PI 468916 SCN Resistance Loci's Associated Effects on Soybean Seed Yield and Other Agronomic Traits

The soybean cyst nematode (SCN, Heterodera glycines Riggs and Niblack) is the most important soybean [Glycine max (L.) Merr.] pathogen in the USA and its control relies on genetic resistance. When resistance genes from exotic sources are transferred into elite cultivars, deleterious alleles are frequently transferred with resistance through genetic linkages. Two SCN resistance loci have been identified in G. soja Sieb. and Zucc. PI 468916, and the effect these loci have on yield and other agronomic traits is not known. The objective of this study was to determine the effect of each G. soja SCN resistance gene on yield and other agronomic traits in elite soybean backgrounds. These effects were tested in two populations each segregating for SCN resistance derived from G. soja, with one population also segregating for the SCN resistance loci, rhg1, derived from G. max PI 88788. Each population was analyzed for genetic markers linked to the SCN resistance loci and field tested in multiple environments with low to high SCN infestations. The G. soja and rhg1 SCN resistance alleles either had no effect or significantly (P &lt; 0.05) enhanced yield compared with the susceptible alleles. The SCN resistance alleles were also associated with increasing days to maturity, plant height, and lodging scores. Deployment of the G. soja SCN resistance loci will increase the genetic diversity for SCN resistance and will provide breeders with an alternative source of SCN resistance that is not associated with reduced yield.

Localization of Two Loci that Confer Resistance to Soybean Cyst Nematode from Glycine soja PI 468916

Two quantitative trait loci (QTL) that confer resistance to the soybean cyst nematode (SCN, Heterodera glycines Riggs and Niblack) isolate PA3 (HG Type 0, Race 3) were previously mapped in a population of F2–derived lines developed from a cross with the Glycine soja Sieb. and Zucc. plant introduction (PI) 468916. These two QTL represent new resistance genes as they reside in genetic locations that differ from where SCN‐resistance QTL in soybean [Glycine max (L). Merr.] cultivars map. The objectives of this research were to confirm the G. soja SCN‐resistance QTL in a domestic soybean background and to localize, or better define, the positions of these QTL. Additional molecular markers were identified within the regions containing the SCN‐resistance QTL by using amplified fragment length polymorphism (AFLP) markers and bulked segregant analysis (BSA). Each QTL was localized by developing and testing several backcross populations that segregated for different genomic regions where the resistance QTL map. The SCN‐resistance QTL on LG E was localized to a 13.2‐cM interval with an average marker distance of 0.6 cM. The SCN‐resistance QTL on LG G was localized to an 18.5‐cM interval with an average marker distance of 3.1 cM. The AFLP‐BSA technique proved to be beneficial for adding molecular markers to the two regions from G. soja that confer resistance to SCN. These closely linked markers will assist in map‐based cloning of the G. soja PI 468916 SCN resistance genes which could potentially uncover genes involved in the soybean–SCN interaction and aid with their continued exploration.

Localization of a Quantitative Trait Locus Providing Brown Stem Rot Resistance in the Soybean Cultivar Bell

Many soybean [Glycine max (L.) Merr.] genotypes that carry resistance to soybean cyst nematode (Heterodera glycines Ichinohe) (SCN) from plant introduction (PI) 88788 also carry resistance to brown stem rot (BSR) caused by the soilborne fungus Phialophora gregata (Allington &amp; Chamberlain) W. Gams f. sp. sojae Kobayashi, Yamamoto, Negishi, and Ogoshi. The objectives of our research were to map and localize BSR resistance quantitative trait loci (QTL) from Bell, a BSR resistant cultivar with SCN resistance from PI 88788. Initial mapping was done with a population of 93 F4–derived lines developed from a cross between Bell and the SCN and BSR susceptible cultivar, Colfax. Lines were evaluated for BSR resistance in two field environments, a greenhouse, and with genetic markers from linkage group (LG) J. To confirm and further localize a resistance QTL, three near isogenic line (NIL) populations were created using an F4–derived line from the Bell × Colfax population. In the F4 population, markers on LG J were significantly (P &lt; 0.001) associated with BSR resistance in both the field (R 2 = 45%) and greenhouse (R 2 = 51%). Data from the NIL populations indicates the QTL is near the closely linked markers 21E22.sp1, 21E22.sp2, and 35E22.sp1, which is the same genetic region where Rbs1, Rbs2, and Rbs3, the three named BSR resistance genes, were previously mapped. In addition, the genetic region is tightly linked to a previously identified SCN resistance QTL from PI 88788. These results explain why many SCN resistant cultivars also carry BSR resistance and should assist breeders when selecting for resistance to both diseases.

Association between Soybean Cyst Nematode Resistance Loci and Yield in Soybean

Breeding for resistance to soybean cyst nematode (Heterodera glycines Ichinohe; SCN) is an important objective of soybean [Glycine max (L.) Merr.] breeders. There is concern, however, that SCN resistant soybean cultivars yield less than SCN susceptible cultivars under low SCN pressure. This is supported by the report that two quantitative trait loci (QTL) conferring yield depression are linked to the major SCN resistance gene rhg1 on linkage group (LG) G in the resistant plant introduction (PI) 209332. The objective of our study was to test for the association of SCN resistance with yield and other agronomic traits in populations developed from soybean cultivars with resistance derived from PI 88788. We tested five populations of near isogenic lines (NIL) segregating for genetic markers Satt309 or CTA linked to rhg1 and two populations of NILs segregating for Satt431 linked to the resistance QTL cqSCN‐003 on LG J. Population sizes ranged from 23 to 44 NILs, and the populations were yield tested in two years across four locations that had low SCN incidence in previous years. Near isogenic lines predicted to carry the SCN resistance allele yielded significantly (p &lt; 0.05) less than NILs predicted to carry the susceptibility allele in one out of five populations segregating for Satt309 (118 kg ha−1) and in one out of two populations segregating for Satt431 (76 kg ha−1). Significant differences between the two classes of NILs were also observed for plant maturity, plant lodging and plant height in some populations; however, these differences were small in magnitude. Molecular marker data from the NIL populations for the genetic regions flanking the segregating resistance genes suggest the presence of a yield depression QTL distal to rhg1 on LG G and another yield depression QTL linked or pleiotropic to cqSCN‐003 on LG J. Confirmation and refinement of our results will enable soybean breeders to use marker‐assisted breeding to select against yield depressing regions and develop SCN resistant cultivars that compete with SCN susceptible cultivars even under low SCN pressure.

’Forrest’ resistance to the soybean cyst nematode is bigenic: saturation mapping of the Rhg1and Rhg4 loci

Field resistance to cyst nematode (SCN) race 3 (Heterodera glycines I.) in soybean [Glycine max (L.) Merr.] cv ’Forrest’ is conditioned by two QTLs: the underlying genes are presumed to include Rhg1 on linkage group G and Rhg4 on linkage group A2. A population of recombinant inbred lines (RILs) and two populations of near-isogenic lines (NILs) derived from a cross of Forrest×Essex were used to map the loci affecting resistance to SCN. Bulked segregant analysis, with 512 AFLP primer combinations and microsatellite markers, produced a high-density genetic map for the intervals carrying Rhg1 and Rhg4. The two QTLs involved in resistance to SCN were strongly associated with the AFLP marker EATGMCGA87 (P=0.0001, R2=24.5%) on linkage group G, and the AFLP marker ECCGMAAC405 (P=0.0001, R2 =26.2%) on linkage group A2. Two- way analysis of variance showed epistasic interaction (P=0.0001, R2 =16%) between the two loci controlling SCN resistance in Essex×Forrest recombinant inbred lines. Considering the two loci as qualitative genes and the resistance as female index FI <5%, jointly the two loci explained over 98% of the resistance. The locations of the two QTLs were confirmed in the NILs populations. Therefore SCN resistance in Forrest×Essex is bigenic. High-efficiency marker-assisted selection can be performed using the markers to develop cultivars with stable resistance to SCN.

Identification of a new major QTL associated with resistance to soybean cyst nematode (Heterodera glycines)

Resistance of soybean [Glycine max (L.) Merr.] to cyst nematode (SCN) (Heterodera glycines Ichinohe), one of the most destructive pathogens affecting soybean, involves a complex genetic system. The identification of QTLs associated with SCN resistance may contribute to the understanding of such system. The objective of this work was to identify and map QTLs for resistance to SCN Race 14 with the aid of molecular markers. BC3F2:3 and F2:3 populations, both derived from an original cross between resistant cv. Hartwig and the susceptible line BR-92–31983 were screened for resistance to SCN Race 14. Four microsatellite (Satt082, Sat_001, Satt574 and Satt301) and four RAPD markers (OPAA-11795, OPAE-08837, OPR-07548 and OPY-072030) were identified in the BC3F2:3 population using the bulked segregant analysis (BSA) technique. These markers were amplified in 183 F2:3 families and mapped to a locus that accounts for more than 40% of the resistance to SCN Race 14. Selection efficiency based on these markers was similar to that obtained with the conventional method. In the case of the microsalellite markers, which identify homozygous resistant genotypes, the efficiency was even higher. This new QTL has been mapped to the soybean linkage group D2 and, in conjunction with other QTLs already identified for SCN resistance, will certainly contribute to our understanding of the genetic basis of resistance of this important disease in soybean.

Genetic analysis of resistance to soybean cyst nematode in PI 438489B.

Soybean (Glycine max L. Merr.) plant introduction PI 438489B is a unique source that has resistance to all known populations of soybean cyst nematode (Heterodera glycines Ichinohe, SCN). This PI line also has many desirable agronomic characteristics, which makes it an attractive source of SCN resistance for use in a soybean breeding program. However, characterization of SCN resistance genes in this PI line have not been fully researched. In this study, we investigated the inheritance of resistance to populations of SCN races 1, 2, 3, 5, and 14 in PI 438489B. PI 438489B was crossed to the susceptible cultivar ‘Hamilton’ to generate F1 hybrids. The random F2 plants and F3 lines were evaluated in the greenhouse for reaction to these five populations of SCN races. Resistance to SCN races 1, 3, and 5 were mostly conditioned by three genes (Rhg Rhg rhg). Resistance to race 2 was controlled by four genes (Rhg rhg rgh rgh). Three recessive genes were most likely involved in giving resistance to race 14. We further concluded that resistance to different populations of SCN races may share some common genes or pleiotropic effects may be involved.

Expression of Soybean Cyst Nematode Resistance in Transgenic Hairy Roots of Soybean

ABSTRACTTransgenic hairy roots of soybean [Glycine max (L.) Merrill] induced by Agrobacterium rhizogenes support the complete life cycle of soybean cyst nematode (SCN, Heterodera glycines Ichinohe) in vitro. However, expression of SCN resistance in hairy soybean roots has not been investigated. A transgenic hairy root system would be useful in developing an assay for candidate SCN resistance genes. The objectives of this study were to characterize transgene expression in SCN‐infected hairy soybean roots and to evaluate a transgenic hairy root system for investigations of resistance to SCN. Seedling cotyledons of the SCN‐susceptible cultivars, Agassiz and Parker, and SCN‐resistant Bell and Faribault were infected with A. rhizogenes strain K599 transformed with T‐DNA binary vectors containing the gusA gene fused to promoters from either the cauliflower mosaic virus (CaMV 35S), Arabidopsis thaliana phenylalanine ammonia lyase (PAL), or bean (Phaseolus vulgaris L.) chalcone synthase‐8 (CHS) genes. Nine days after inoculating transgenic hairy roots with sterile J2 nematodes, CHS‐regulated β‐glucuronidase (GUS) staining at infection sites increased in hairy roots of resistant Faribault and decreased in susceptible Agassiz. PAL‐regulated GUS staining was absent at infection sites in hairy roots of resistant cultivars, but was increased in infection sites in susceptible cultivars. Thirty‐five days after inoculation with SCN, the mean number of cysts formed on hairy roots of the resistant cultivars was about 14% of the mean number of cysts formed on hairy roots of the susceptible cultivars, indicating that the SCN resistance phenotypes were preserved in transgenic hairy roots. These results indicated that the transgenic hairy soybean root system will be useful for investigating differential transgene expression during nematode infection and evaluation of candidate SCN resistance genes.

Selecting Soybean Cultivars for Dual Resistance to Soybean Cyst Nematode and Sudden Death Syndrome Using Two DNA Markers

DNA markers for genes conditioning resistance to soybean [Glycine max (L.) Merr.] root infection by [Fusarium solani (Mart.) Sacc. f. sp. glycine (Burk.)] (Rfs1), sudden death syndrome (SDS), and soybean cyst nematode (SCN; Heterodera glycines Ichinohe; Rhg4 and rhg1) were previously identified in ‘Essex’ × ‘Forrest’. This study tests the effectiveness of those markers in selecting for disease resistance among recombinant inbred lines from ‘Flyer’ × ‘Hartwig’. A total of 535 among 739 lines were scored by two markers, providing four genotypes. A stratified random sample of 50 lines was evaluated for SDS by F. solani root infection severity at two locations and SCN race 3 index of parasitism in the greenhouse. Selection with BLT65 identified 281 among 671 lines with the genomic region that underlies Rhg4‐derived SCN resistance. Selection with Satt038 identified 230 among 613 lines containing the genomic region that underlies resistance to SDS (rfs1) and rhg1 ‐derived SCN resistance. A total of 93 out of 535 lines had genomic regions that underlie resistance to both SDS and SCN in Essex × Forrest. Segregation of both markers was not random (P ≤ 0.05). Infection severity means for genotypes with the Hartwig allele at Satt038 (28–29%) were lower (P = 0.0001, R2 = 28%) than with the Flyer allele (31–42%); irrespective of maturity group. BLT65 was not associated with infection severity. Mean SCN index of parasitism was lower (P ≤ 0.05) only for genotypes carrying the Hartwig allele at both Satt038 and BLT65. Therefore, alleles conferring resistance to SDS and SCN in Essex × Forrest are transferable to other populations.

Rotation and Fertilization Effects on Corn and Soybean Yields and Soybean Cyst Nematode Populations in a No‐Tillage System

AbstractRotations have long been used to improve crop yields. No‐tillage production acreage is increasing, and production information on rotation effects on yields and soybean cyst nematode populations is needed for this system. Field experiments were initiated in 1985 and continued through 1992 to evaluate P‐K fertilization effects on yield of a corn‐soybean [Zea mays L.‐Glycine max (L.) Merr.] rotation and on populations of soybean cyst nematode (Heterodera glycines Ichinohe) (SCN). The research was conducted on a Loring silt loam soil (a fine‐silty, mixed, active, thermic Oxyaquic Fragiudalf), using no‐tillage management. The experimental design was a split‐plot for corn evaluation and split‐split‐plot for soybean evaluation. Individual treatments were replicated five times. Main plots were broadcast P‐K rates of 0–0, 15–28, 29–56, 44–84, and 59–112 kg ha−1. The split‐plots were continuous corn, continuous soybean, a corn‐soybean rotation, and a soybean‐corn rotation. The split‐split‐plots were two soybean cultivars possessing different levels of SCN resistance. Yields of both crops were increased by the fertilization. In this study, corn yields were increased 14% and soybean yields increased 11% with the rotation. Rotating the two cultivars produced similar yields and nonrotated yields of resistant TN 4‐86 were higher than susceptible Essex, indicating an effect of SCN. The primary benefit of corn in the rotation was to reduce SCN populations. However, within Essex rotations the SCN population recovery was rapid, while populations remained relatively low in the TN 4‐86 rotations. The SCN populations were greater when rotations were fertilized with the two lowest P‐K rates, compared with populations within the unfertilized check and the highest P‐K rate. Soybean producers should consider a no‐tillage system of rotating corn with SCN‐resistant soybean cultivars, fertilized at high P and K rates for yield improvement and reduced SCN populations.