Separate loci underlie resistance to root infection and leaf scorch during soybean sudden death syndrome

Soybean [Glycine max (L.) Merr.] cultivars show differences in their resistance to both the leaf scorch and root rot of sudden death syndrome (SDS). The syndrome is caused by root colonization by Fusarium virguliforme (ex. F. solani f. sp. glycines). Root susceptibility combined with reduced leaf scorch resistance has been associated with resistance to Heterodera glycines HG Type 1.3.6.7 (race 14) of the soybean cyst nematode (SCN). In contrast, the rhg1 locus underlying resistance to Hg Type 0 was found clustered with three loci for resistance to SDS leaf scorch and one for root infection. The aims of this study were to compare the inheritance of resistance to leaf scorch and root infection in a population that segregated for resistance to SCN and to identify the underlying quantitative trait loci (QTL). "Hartwig", a cultivar partially resistant to SDS leaf scorch, F. virguliforme root infection and SCN HG Type 1.3.6.7 was crossed with the partially susceptible cultivar "Flyer". Ninety-two F5-derived recombinant inbred lines and 144 markers were used for map development. Four QTL found in earlier studies were confirmed. One contributed resistance to leaf scorch on linkage group (LG) C2 (Satt277; P = 0.004, R2 = 15%). Two on LG G underlay root infection at R8 (Satt038; P = 0.0001 R2 = 28.1%; Satt115; P = 0.003, R2 = 12.9%). The marker Satt038 was linked to rhg1 underlying resistance to SCN Hg Type 0. The fourth QTL was on LG D2 underlying resistance to root infection at R6 (Satt574; P = 0.001, R2 = 10%). That QTL was in an interval previously associated with resistance to both SDS leaf scorch and SCN Hg Type 1.3.6.7. The QTL showed repulsion linkage with resistance to SCN that may explain the relative susceptibility to SDS of some SCN resistant cultivars. One additional QTL was discovered on LG G underlying resistance to SDS leaf scorch measured by disease index (Satt130; P = 0.003, R2 = 13%). The loci and markers will provide tagged alleles with which to improve the breeding of cultivars combining resistances to SDS leaf scorch, root infection and SCN HG Type 1.3.6.7.

Genomic analysis of the rhg1 locus: candidate genes that underlie soybean resistance to the cyst nematode

The rhg1 gene or genes lie at a recessive or co-dominant locus, necessary for resistance to all Hg types of the soybean (Glycine max (L.) Merr.) cyst nematode (Heterodera glycines I.). The aim here was to identify nucleotide changes within a candidate gene found at the rhg1 locus that were capable of altering resistance to Hg types 0 (race 3). A 1.5 +/- 0.25 cM region of chromosome 18 (linkage group G) was shown to encompass rhg1 using recombination events from four near isogenic line populations and nine DNA markers. The DNA markers anchored two bacterial artificial chromosome (BAC) clones 21d9 and 73p6. A single receptor like kinase (RLK; leucine rich repeat-transmembrane-protein kinase) candidate resistance gene was amplified from both BACs using redundant primers. The DNA sequence showed nine alleles of the RLK at Rhg1 in the soybean germplasm. Markers designed to detect alleles showed perfect association between allele 1 and resistance to soybean cyst nematode Hg types 0 in three segregating populations, fifteen additional selected recombination events and twenty-two Plant Introductions. A quantitative trait nucleotide (QTN) [corrected] in the RLK at rhg1 was inferred that alters A87 to V87 in the context of H274 rather than N274. [corrected] Contiguous DNA sequence of 315 kbp of chromosome 18 (about 2 cM) contained additional gene candidates that may modulate resistance to other Hg-types including a variant laccase, a hydrogen-sodium ion antiport and two proteins of unknown function. A molecular basis for recessive and co-dominant resistance that involves interactions among paralagous disease-resistance genes was inferred that would improve methods for developing new nematode-resistant soybean cultivars.

Genomic analysis of a region encompassing QRfs1 and QRfs2: genes that underlie soybean resistance to sudden death syndrome

Candidate genes were identified for two loci, QRfs2 providing resistance to the leaf scorch called soybean (Glycine max (L.) Merr.) sudden death syndrome (SDS) and QRfs1 providing resistance to root infection by the causal pathogen Fusarium solani f.sp. glycines. The 7.5 +/- 0.5 cM region of chromosome 18 (linkage group G) was shown to encompass a cluster of resistance loci using recombination events from 4 near-isogenic line populations and 9 DNA markers. The DNA markers anchored 9 physical map contigs (7 are shown on the soybean Gbrowse, 2 are unpublished), 45 BAC end sequences (41 in Gbrowse), and contiguous DNA sequences of 315, 127, and 110 kbp. Gene density was high at 1 gene per 7 kbp only around the already sequenced regions. Three to 4 gene-rich islands were inferred to be distributed across the entire 7.5 cM or 3.5 Mbp showing that genes are clustered in the soybean genome. Candidate resistance genes were identified and a molecular basis for interactions among the disease resistance genes in the cluster inferred.

Definition of Soybean Genomic Regions That Control Seed Phytoestrogen Amounts

Soybean seeds contain large amounts of isoflavones or phytoestrogens such as genistein, daidzein, and glycitein that display biological effects when ingested by humans and animals. In seeds, the total amount, and amount of each type, of isoflavone varies by 5 fold between cultivars and locations. Isoflavone content and quality are one key to the biological effects of soy foods, dietary supplements, and nutraceuticals. Previously we had identified 6 loci (QTL) controlling isoflavone content using 150 DNA markers. This study aimed to identify and delimit loci underlying heritable variation in isoflavone content with additional DNA markers. We used a recombinant inbred line (RIL) population ( $n=100$ ) derived from the cross of “Essex” by “Forrest,” two cultivars that contrast for isoflavone content. Seed isoflavone content of each RIL was determined by HPLC and compared against 240 polymorphic microsatellite markers by one-way analysis of variance. Two QTL that underlie seed isoflavone content were newly discovered. The additional markers confirmed and refined the positions of the six QTL already reported. The first new region anchored by the marker BARC-Satt063 was significantly associated with genistein ( $P=0.009$, $Rcirc;2=29.5\%$ ) and daidzein ( $P=0.007$, $Rcirc;2=17.0\%$ ). The region is located on linkage group B2 and derived the beneficial allele from Essex. The second new region defined by the marker BARC-Satt129 was significantly associated with total glycitein ( $P=0.0005$, $Rcirc;2=32.0\%$ ). The region is located on linkage group D1a+Q and also derived the beneficial allele from Essex. Jointly the eight loci can explain the heritable variation in isoflavone content. The loci may be used to stabilize seed isoflavone content by selection and to isolate the underlying genes.

Resistance locus pyramids alter transcript abundance in soybean roots inoculated with Fusarium solani f.sp. glycines

Soybean Sudden Death Syndrome (SDS) is caused by Fusarium solani f.sp. glycines (Fsg). Six quantitative trait loci (QTLs), each conferring partial resistance to SDS, have been discovered in an Essex x Forrest recombinant inbred line (RIL) population, but their mode of action is not clear. This study aimed to identify genes (ESTs) whose mRNA transcripts were altered in abundance in soybean roots following inoculation of Fsg. Roots of the soybean variety Forrest (four resistance alleles) were inoculated with Fsg, and 14 days later RNA sequences that were differentially expressed relative to uninoculated roots were enriched using suppression subtraction and differential display. The abundance of these RNAs was quantified in inoculated and non-inoculated roots by macroarray hybridizations. A unigene set of 135 ESTs was identified and used in a further macroarray analysis. The abundance of 28 cDNA fragments was increased more than two-fold in inoculated compared to uninoculated roots of RIL 23 (six resistance alleles). In Forrest and Essex (two resistance alleles), the level of only one mRNA was increased two-fold in inoculated roots compared to the uninoculated roots. In Essex most of the mRNAs analyzed decreased in abundance (61/135 showed a two-fold decrease), while in Forrest most mRNA abundances did not change. Among the 28 cDNAs that revealed a two-fold or higher increase in mRNA abundance in RIL 23, 14% code for proteins known to be involved in plant defense, 21% in metabolism, 14% in cell structure and 4% in transport. Unannotated ESTs accounted for 43% of the genes, and 4% of the sequences were previously unknown. The plant defense-related genes that showed a differential response to Fsg inoculation suggested a role for the phenylproponoid pathway in soybean defense against Fsg. In Essex, genes involved in plant defense, cell wall synthesis, ethylene synthesis and metabolism were expressed at lower levels in inoculated roots. The difference in response between the 2-, 4- and 6-gene pyramids suggests that QTLs for SDS resistance serve to delay symptoms or confer resistance by maintaining or increasing the expression of specific genes after inoculation/infection.

Common loci underlie field resistance to soybean sudden death syndrome in Forrest, Pyramid, Essex, and Douglas

Soybean [ Glycine max (L.) Merr.] sudden death syndrome (SDS) caused by Fusarium solani f. sp. glycines results in severe yield losses. Resistant cultivars offer the most-effective protection against yield losses but resistant cultivars such as 'Forrest' and 'Pyramid' vary in the nature of their response to SDS. Loci underlying SDS resistance in 'Essex'&emsp14;x&emsp14;Forrest are well defined. Our objectives were to identify and characterize loci and alleles that underlie field resistance to SDS in Pyramidx'Douglas'. SDS disease incidence and disease severity were determined in replicated field trials in six environments over 4 years. One hundred and twelve polymorphic DNA markers were compared with SDS disease response among 90 recombinant inbred lines from the cross PyramidxDouglas. Two quantitative trait loci (QTLs) for resistance to SDS derived their beneficial alleles from Pyramid, identified on linkage group G by BARC-Satt163 (261-bp allele, P=0.0005, R(2)=16.0%) and linkage group N by BARC-Satt080 (230-bp allele, P=0.0009, R(2)=15.6%). Beneficial alleles of both QTLs were previously identified in Forrest. A QTL for re- sistance to SDS on linkage group C2 identified by BARC-Satt307 (292-bp allele, P=0.0008, R(2)=13.6%) derived the beneficial allele from Douglas. A beneficial allele of this QTL was previously identified in Essex. Recombinant inbred lines that carry the beneficial alleles for all three QTLs for resistance to SDS were significantly ( P</=0.05) more resistant than other recombinant inbred lines. Among these recombinant inbred lines resistance to SDS was environmentally stable. Therefore, gene pyramiding will be an effective method for developing cultivars with stable resistance to SDS.

Quantitative trait loci in Two Soybean Recombinant Inbred Line Populations Segregating for Yield and Disease Resistance

Molecular makers linked to quantitative trait loci (QTL) can assist soybean [Glycine max (L.) Merr.] breeders to combine traits of low heritability, such as yield, with disease resistance. The objective of this study was to identify markers linked to yield QTL in two recombinant inbred line (RIL) populations ['Essex' x 'Forrest' (ExF; n = 100) and 'Flyer' x 'Hartwig' (FxH; n = 94)] that also segregate for soybean cyst nematode (SCN) resistance genes (rhg1 and Rhg4). Each population was yield tested in four environments between 1996 and 1999. The resistant parents produced lower yields. Heritability of yield across four environments was 47% for ExF and 57% for FxH. Yield was normally distributed in both populations. High yielding, SCN resistant transgressive segregants were not observed. In the ExF RIL population, 134 microsatellite markers were compared against yield by ANOVA and MAPMAKER QTL. Regions associated with yield were identified by SATT294 on linkage group (LG.) C1 (P = 0.006, R(2) = 10%), SATT440 on LG. I (P = 0.007, R(2) = 10%), and SATT337 on LG. K (P = 0.004, R(2) = 10%). Essex provided the beneficial allele at SATT337. Mean yields among FxH RILs were compared against 33 microsatellite markers from LG. K. In addition 136 markers from randomly selected LGs were compared with extreme phenotypes by bulk segregant analysis. Two regions on LG. K (20 cM apart) associated with yield were identified by SATT326 (P = 0.0004, R(2) = 15%) and SATT539 (P = 0.0008, R(2) = 14%). Flyer provided both beneficial alleles. Both populations revealed a yield QTL in the interval (5 cM) between SATT337 and SATT326. These populations may share a common allele for yield in this region, given that about 40% of Flyer genome derived from Essex.