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. 2022 Feb 5;42(2):8. doi: 10.1007/s11032-022-01280-1

Characterization and mapping of a downy mildew resistance gene, Pl36, in sunflower (Helianthus annuus L.)

L L Qi 1,✉, X W Cai 2
PMCID: PMC10248693  PMID: 37309323

Abstract

Downy mildew (DM) is one of the most serious diseases in sunflower-growing regions worldwide, often significantly reducing sunflower yields. The causal agent of sunflower DM, the oomycete pathogen Plasmopara halstedii, is highly virulent and aggressive. Studying regional disease spread and virulence evolution in the DM pathogen population is important for the development of new sunflower inbred lines with resistance to the existing DM pathogen. The sunflower line 803–1, as one of nine international differential hosts, has been used in the identification of P. halstedii virulent pathotypes in sunflower since 2000. The DM resistance gene in 803–1 was temporally designated Pl5 + based on allelic analysis but has not been molecularly characterized. In the present study, bulked segregant analysis and genetic mapping confirmed the presence of the Pl gene within a large gene cluster on sunflower chromosome 13 in 803–1, as previously reported. Subsequent saturation mapping in the gene target region with single nucleotide polymorphism (SNP) markers placed this gene at an interval of 3.4 Mb in the XRQ reference genome assembly, a location different from that of Pl5. Therefore, the Pl gene in 803–1 was re-designated Pl36 because it is not allelic with Pl5. Four SNP markers co-segregated with Pl36, and SNP SFW05743 was 1.1 cM proximal to Pl36. The relationship of eight Pl genes in the cluster is discussed based on their origin, map position, and specificity of resistance/susceptibility to DM infection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01280-1.

Keywords: Sunflower, Downy mildew, Differential host, Resistance gene, Mapping

Introduction

Downy mildew (DM) caused by the oomycete pathogen Plasmopara halstedii (Farl.) Berl. et de Toni is the most common disease of sunflower (Helianthus annuus L.) in sunflower-growing regions worldwide. The disease is initiated by soilborne oospores, which can survive in the soil for up to 10 years and serve as primary inoculum. The pathogen infects plants through roots but becomes systemic within the plant, often resulting in plant death. If the seedlings survive, they produce little if any seed when they mature, causing significant yield losses and decreased seed quality (Molinero-Ruiz et al. 2003; Markell et al. 2015). Downy mildew disease has contributed to issues including inadequate plant population and plant placement and is still the major threat to sunflower production.

Seed treatment with the fungicide metalaxyl has been widely used to control DM in the United States (U.S.) from 1985 to 1995. However, as resistance to fungicides occurred in 1995, DM severity has been increased dramatically (Albourie et al. 1998; Gulya et al. 1999; Gulya 2001, 2002). Host resistance has also been used to control DM disease in sunflowers since the 1970s, and resistance to DM is controlled by single dominant genes, designated Pl genes. The pathogenicity of P. halstedii races against the resistance genes (R genes) in sunflower is followed by the gene-for-gene theory, that is, every host R gene corresponds to a pathogen avirulence gene (Flor 1971).

In the 1970s, two P. halstedii races were identified and often referred to by their geographical origin: race 1, the European race, and race 2, the Red River race found in the U.S. (Zimmer 1974). Two additional P. halstedii races, races 3 and 4, were identified in the U.S. in the early 1980s (Carson 1981; Gulya et al. 1982; Gulya and Urs 1985), and subsequently, races 5, 6, and 7 were found in Europe and the U.S., which initiated the need for studies of race identification and resistance evaluation in sunflower (Gulya et al. 1991; Viranyi and Masirevic 1991). The system of assigning sequential numbers to new races used in the 1970s and 1980s lacked a systematic approach to provide easily made comparisons of virulence among races. A new system of race identification was proposed in the late 1990s, which used a set of nine differentials grouped into three subsets of three lines as the core standard differentials to produce a virulent formula with triplet codes (Gulya et al. 1998; Tourvieille de Labrouhe et al. 2000a). Since then, the system has been widely accepted by the international sunflower community, and the differential hosts selected have been used worldwide. The use of additional differential hosts for race identification was also proposed recently by French and U.S. scientists as new virulent races evolved (Tourvieille de Labrouhe et al. 2012; Gilley et al. 2020).

The nine core differentials selected include HA 304, RHA 265, and RHA 274 (D1-D3); PMI3, PM17, and 803–1 (D4-D6); and HA-R4, QHP1, and RHA 335 (D7-D9), with HA 304 as a universal susceptible line. They have been selected as core differentials because of their public availability, fixed inbred lines, consistent reaction to DM, and a better understanding of their resistance (Tourvieille de Labrouhe et al. 2000a). The DM R genes were later identified by molecular mapping in seven differentials, except for 803–1 (Vear et al. 1997; Bert et al. 2001; Gedil et al. 2001; Liu et al. 2012; Gascuel et al. 2015; Pecrix et al. 2018a). Analysis of DM resistance relationship in a test cross between 803–1 and XRQ that carries the gene Pl5 revealed no segregation in the test cross population, indicating that the R gene in 803–1 may be in the same region as Pl5. However, segregation studies could not distinguish closely linked genes in the small population size. Additionally, 803–1 and XRQ exhibited different specificities for P. halstedii races; therefore, the R gene in 803–1 was temporally designated Pl5 + (Vear et al. 2008; Gascuel et al. 2015; Pecrix et al. 2018b). The present study aimed to molecularly map the DM R gene in 803–1 and perform comparative analysis with the other genes mapped to the same region.

Materials and methods

Mapping population

A cross was made between HA 89 and 803–1 in 2011, and the F2 population was developed from a selected F1 plant. The F2-derived F3 families were used for DM phenotypic evaluation. HA 89 (PI 599,773) is a sunflower maintainer line, susceptible to DM. Line 803–1 was selected from a breeding population involving H. tuberosus in the cross, and its DM resistance originated from H. tuberosus, a sunflower wild perennial species (Tourvieille de Labrouhe et al. 2000a).

Phenotypic evaluation of the F2:3 population

P. halstedii race 734 as a new virulent race was chosen to evaluate seedlings of the F2:3 families for resistance to DM (Gulya et al. 2011). Thirty seedlings of each 140 F2:3 family along with HA 89 and 803–1 were inoculated with P. halstedii race 734 in the greenhouse in the spring of 2013 to assess the phenotypic variations following the method described by Qi et al. (2015). The presence of white sporulation on the cotyledons and true leaves in seedling was considered susceptible, while resistant seedling was no sporulation.

Bulk segregant analysis and genetic mapping

Genomic DNA of 140 F2 plants, as well as HA 89 and 803–1 parent lines, was isolated from the lyophilized tissues collected at the four-leaf stage using the Qiagen plant kit following the manufacturer’s instructions (Qiagen, Valencia, CA, USA). The DNA concentration was measured using a NanoDrop 2000 spectrophotometer (Thermo Fisher Scientific, Wilmington, DE, USA).

Two parents, HA 89 and 803–1, were first screened for polymorphisms with 860 simple sequence repeat (SSR) markers selected from the sunflower SSR maps (Tang et al. 2002, 2003; Yu et al. 2003). The conditions of polymerase chain reaction (PCR) for SSR primers were described byQi et al. (2011). Chromosome location of the Pl gene inherited from 803–1 was determined by bulked segregant analysis (BSA, Michelmore et al. 1991). Two bulks were created based on F2:3 family phenotypic evaluation, susceptible (S) bulk with pooled DNA from 10 F2 plants susceptible to DM, and resistant (R) bulk with pooled DNA from 10 F2 plants homozygously resistant to DM. The SSR markers showing polymorphism between the parents were selected to screen two bulks, and subsequently, the entire F2 population of 140 plants was genotyped by the markers which differentiated the R bulk from the S bulk in BSA.

Segregation distortion of markers and DM trait in the F2 population was checked by the chi-square (χ2) test. Phenotypes and segregation data of DNA markers from the population were combined to develop a genetic map using JoinMap 4.1 with a regression mapping algorithm and Kosambi’s mapping function (Van Ooijen 2006).

Marker saturation in the target region

To saturate the gene target region, an additional 42 single nucleotide polymorphism (SNP) markers were selected from the sunflower published SNP maps with 12 NSA SNPs and 30 SFW SNPs (Table S2, Bowers et al. 2012; Talukder et al. 2014). Genotyping of the parental lines and the F2 population with NSA SNPs was conducted by BioDiagnostics, Inc. (River Falls, WI, USA), while genotyping of the SFW SNPs was performed in the laboratory following the methods described by Qi et al. (2015) and Long et al. (2017). The SNP PCR primers are listed in Table S3. Size segregation of SNP PCR products was conducted in an IR2 4300/4200 DNA Analyzer (LI-COR, Lincoln, NE, USA).

Results

Targeting of the Pl gene in 803–1

The phenotypic evaluation indicated that HA 89 was consistently susceptible to P. halstedii race 734 in the greenhouse, while 803–1 was resistant to infection. Segregation in the F2:3 population fit a 1:2:1 ratio (χ2 = 0.63, df = 2, P = 0.70), with 33 F3 families classified as homozygous resistance, 68 segregating, and 39 homozygous susceptible, indicating that the resistance in 803–1 was mediated by a major gene. The DM-resistant genotypes of the F2 plants were determined based on the homozygosity or segregation of the respective F2:3 families.

In the initial screen of polymorphism between two parents with 860 SSR markers, 283 (32.9%) showed polymorphisms. Subsequently, these polymorphic SSR markers were used to screen the S and R bulks. Eight SSR markers located on sunflower chromosome 13 were associated with the resistance in the R bulk, and these positive markers were used to genotype the F2 population. The resultant genetic map consisting of eight SSR markers and one Pl gene was 17.4 cM in length using the genotypic and phenotypic data from the F2 and F2:3 populations (Fig. 1a). Recombination mapping showed that the six of eight SSR markers, HT333, ORS191, ORS 45, ORS799, ORS581, and HT382, represent a coherent linkage block around the DM resistance gene covering a region of 7.6 cM genetic distance. SSRs ORS581 (0.1 cM) and HT382 (5.2 cM) were the closest markers flanking the Pl gene (Fig. 1a). Based on the map position and the resistance spectra (see below “Comparative analysis of Pl36 and other Pl genes on chromosome 13”), the Pl gene in 803–1 was renamed Pl36, which is not an allele of Pl5.

Fig. 1.

Fig. 1

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Genetic maps of chromosome 13. a SSR map of Pl36, b saturation map of Pl36, and c gene cluster map (data of Pl5, Pl21, Pl22, Pl31, and Pl32 from Pecrix et al. (2018b); Pl8 and Pl34 data from Qi et al. (2017) and Talukder et al. (2019), respectively)

Saturation mapping and the candidate genes in the Pl36 interval

In the above SSR map, Pl36 was genetically mapped to a region of 5.3 cM in sunflower chromosome 13. To saturate the Pl36 gene region, an additional 42 SNP markers previously mapped to the target region were selected from the published SNP genetic maps (Table S2). Polymorphism survey in the parents of HA 89 and 803–1 identified 14 polymorphic SNPs. Genotyping of the F2 population with these polymorphic markers indicated that all of them were mapped to the target region, reducing the Pl36 interval from 5.3 to 1.3 cM (Fig. 1b). Four SNP markers, SFW01497, SFW05453, SFW05832, and SFW08875, co-segregated with Pl36, and SFW05743 was 1.1 cM proximal to Pl36.

The sequence alignment of the Pl36 flanking markers of SFW08875 and SFW05743 against the XRQr1.0 genome assembly indicated that Pl36 locates on a region of 3.4 Mb between 193,131,235 bp and 196,521,026 bp. A total of 108 high-confidence genes in this region were identified from the XRQ reference genome. Among them, 25 genes were related to disease resistance function with nucleotide-binding and leucine-rich repeat domains (Table 1).

Table 1.

Plant disease defense-related genes discovered in the 3.4 Mb Pl36 region between 193,131,235 bp and 196,521,026 bp in the XRQ genome

Gene name Physical position (bp) Length (bp) Description
Start End
HanXRQChr13g0425281 193,174,218 193,180,316 6099 Putative leucine-rich repeat protein kinase family protein
HanXRQChr13g0425301 193,203,299 193,216,053 12,755 Putative leucine-rich repeat domain, L domain-like
HanXRQChr13g0425381 193,311,123 193,372,996 61,874 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425411 193,382,936 193,385,531 2596 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425431 193,450,684 193,477,269 26,586 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425451 193,537,771 193,540,778 3008 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425641 194,083,996 194,085,196 1201 Putative leucine-rich repeat domain, L domain-like
HanXRQChr13g0425651 194,238,872 194,241,638 2767 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425731 194,394,511 194,397,550 3040 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425741 194,397,931 194,401,664 3734 Putative leucine-rich repeat domain, L domain-like
HanXRQChr13g0425771 194,457,496 194,464,792 7297 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425821 194,535,874 194,538,691 2818 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425841 194,590,568 194,593,334 2767 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425851 194,725,998 194,753,531 27,534 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425891 194,800,201 194,803,684 3484 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425931 195,196,820 195,210,745 13,926 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0425941 195,250,038 195,252,703 2666 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0426101 195,969,735 195,974,818 5084 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0426111 196,050,445 196,051,986 1542 Putative leucine-rich repeat domain, L domain-like
HanXRQChr13g0426131 196,069,820 196,071,361 1542 Putative leucine-rich repeat domain, L domain-like
HanXRQChr13g0426171 196,094,778 196,097,083 2306 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0426181 196,130,584 196,131,246 663 Putative leucine-rich repeat domain, L domain-like
HanXRQChr13g0426201 196,153,203 196,158,637 5435 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0426301 196,420,201 196,422,631 2431 Putative NB-ARC; P-loop containing nucleoside triphosphate hydrolase; Leucine-rich repeat domain, L domain-like
HanXRQChr13g0426321 196,434,138 196,437,021 2884 Putative leucine-rich repeat domain, L domain-like
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Comparative analysis of Pl36 and other Pl genes on chromosome 13

The gene cluster on the lower end of chromosome 13 harbors seven DM genes reported previously, including Pl5, Pl8, Pl21, Pl22, Pl31, Pl32, and Pl34 (Table S1). To determine the relationships of Pl36 with these genes, we analyzed the map position and reaction to P. halstedii infection for each of the genes. Linkage analysis of DNA markers associated with the genes revealed that eight Pl genes were located in a region of approximately 15 Mb between 181 and 196 Mb on the assembly of the reference genome XRQr1.0, with Pl22 in the 181–183 Mb region, Pl5/Pl21 in the 186 Mb region, Pl31/ Pl32 in the 188–196 Mb region, and Pl8/Pl34/Pl36 in the 193–196 Mb region (Fig. 1c). A set of 18 SNP markers was used for Pl8/Pl34/Pl36 genetic mapping, and the results revealed that only four markers were common in the three maps (Table 2). Five sunflower lines, PM17 (Pl5), RHA 340 (Pl8), HAS42 (Pl31), HAS54 (Pl32), and 803–1 (Pl36), were tested with more than 17 P. halstedii races, and the results showed that Pl5 was susceptible to P. halstedii races 330, 334, 730, and 774, Pl8 to 730, and Pl36 to 770 and 774. Virulence to Pl31 and Pl32 was not found in any of the tested races (Table 3, Gascuel et al. 2015; Pecrix et al. 2018b; Gilley et al. 2020). Gilley et al. (2020) reported that RHA 428 (Pl34) was susceptible to 16 of 67 P. halstedii isolates tested. Taken together, Pl36 in 803–1 is different from seven other genes mapped on chromosome 13.

Table 2.

Map positions of SNP markers linked to the Pl8, Pl34, and Pl36 on sunflower chromosome 13

SNP marker Genetic position (cM) Physical position in XRQr1.0 (bp)
Pl8 map* Pl34 map** Pl36 map Start End
NSA _001379 - 11.3 10.8 181,040,776 181,040,397
SFW08145 0.0 - 11.5 190,991,361 190,991,254
SFW06095 0.3 12.2 11.9 190,803,806 190,803,688
SFW05832 - 12.2 12.2 190,685,508 190,685,615
SFW01499 - 12.2 - 190,716,677 190,716,796
SFW08283 - 12.2 12.0 190,750,047 190,749,927
SFW05792 - - 12.0 190,880,273 190,880,360
SFW05240 0.8 12.2 - 190,827,363 190,827,244
SFW05453 - 12.2 12.2 193,062,603 193,062,484
SFW01497 1.3 12.2 12.2 193,089,467 193,089,349
SFW09223 - 12.5 - 193,017,796 193,017,881
SFW08875 1.3 12.5 12.2 193,131,235 193,131,123
NSA _000423 1.3 - - 193,596,862 193,597,149
Pl8 1.7 - - - -
Pl34 - 12.8 - - -
Pl36 - - 12.2 - -
SFW04317 - - 13.5 196,474,077 196,473,983
SFW05743 - - 13.3 196,521,145 196,521,026
SFW06597 3.0 - 14.3 197,229,773 197,229,654
NSA _002430 - - 17.2 - -
NSA _002251 3.4 16.2 17.2 - -
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*Taken from Qi et al. (2017)

**Taken from Talukder et al. (2019); the common markers in three maps are bold, and the markers presented on only one map are in italics

Table 3.

Reaction of sunflower genotypes with Pl genes in chromosome 13 to five P. halstedii races

Genotype Pl gene P. halstedii races Reference
330 334 730 770 774
PM17 Pl5 S S S NA S Gascuel et al. (2015)
RHA 340 Pl8 NA NA S R R Gilley et al. (2020)
803–1 Pl36 R R R S S Gascuel et al. (2015), Geilly et al. (2020)
HAS42 Pl31 R R R NA R Pecrix et al. (2018b)
HAS54 Pl32 R R R NA R Pecrix et al. (2018b)
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NA Not available

Discussion

Studying regional disease spread and virulence evolution in the P. halstedii pathogen population is important for the development of new sunflower inbred lines with resistance to the existing P. halstedii pathogen. A set of standard differentials with known genes is a prerequisite to identify virulence phenotypes. As one of the universal standard differentials used in the world, the Pl gene in 803–1 was temporally designated Pl5 + based on an allelic analysis; however, the nature and precise position on chromosomes of this gene remain unknown. In the present study, we genetically mapped the Pl gene in 803–1 to sunflower chromosome 13 and revealed that this gene is not allelic to Pl5, re-designated Pl36. With the evolutionary changes that occur in the P. halstedii population and the new Pl gene discovered in sunflower, it is necessary to add new differential hosts to maintain the relevance of race determination, as proposed by Tourvieille de Labrouhe et al. (2012) and Gilley et al. (2020).

The Pl genes in sunflower are often organized as clusters on sunflower chromosomes 1, 4, 8, and 13 (Table S1, Ma et al. 2017; Pecrix et al. 2018b, Talukder et al. 2019), which is considered a quick adaptive response to pathogens via recombination events as reported in other crops (Hulbert et al. 2001; Meyers et al. 2003). The sub-cluster II at the lower end of sunflower chromosome 13 is the largest gene cluster in the sunflower genome, which harbors seven Pl genes previously mapped, Pl5, Pl8, Pl21, Pl22, Pl31, Pl32, and Pl34, and six rust genes, R4, R13a, R13b, and R16-R18 (Fig. 1c; Pericx et al. 2018a and b, Talukder et al. 2019; Qi et al. 2021). Pl36 in 803–1 is located within this gene cluster. Cultivated sunflower has low genetic diversity due to its recent evolutionary history (Blackman et al. 2011). DM resistance in sunflower has traditionally relied upon the use of the Pl genes introduced from wild sunflower species and the selection of germplasm with seedling resistance genes. Eight Pl genes in the cluster can be traced to their wild origin; Pl5, Pl22, and Pl36 were originally from H. tuberosus, Pl8 was from H. argophyllus, and Pl21, Pl31, Pl32, and Pl34 were from wild H. annuus (Table S1). Based on map position, Pl36 mapped to the 193.6–196.5 Mb interval is not an allele of Pl5 in the 181.7–183.4 Mb region, as well as Pl22 in the 186.4–186.5 Mb region, although they are from the same wild species (Fig. 1c).

Recent advancements in sunflower genomics, especially the accessibility of complete genome sequences of HA412-HO and XRQ and concomitant availability of sequence-based SNP markers, have greatly facilitated detailed genetic studies of DM resistance in sunflower (Badouin et al. 2017; Ma et al. 2019, 2020). Based on the marker position in the reference genome, comparative analysis can be performed to determine the physical location of the markers linked to genes. Pl36 is close to Pl8 and Pl34 in a 4.0 Mb interval between SNP markers SFW08875 and SFW06597 in the XRQr1.0 assembly (Table 2). Among the five SNPs mapped to this region, only SFW08875 was mapped to three maps, and SFW06597 was mapped to the Pl8 and Pl36 maps. NSA_000423 was mapped only to the Pl8 map and SFW04317 and SFW05413 to the Pl36 map. These SNPs are located at different positions in the XRQr1.0 assembly, suggesting that the three genes are not alleles (Table 2). Additionally, three genes exhibited diverse specificities of susceptibility/resistance when infected with different P. halstedii races (Gilley et al 2020), supporting that they are different genes in the cluster.

Extensive use of race-specific Pl genes in sunflower production increased genetic variation in the P. halstedii population through mutation. In the 1970s, only two P. halstedii races were reported (Zimmer 1974). However, currently, 44 P. halstedii races have been recorded worldwide (Trojanová et al. 2017). With 37 Pl genes (Pl1-Pl36, PlArg) reported from sunflower and its wild species, 31 have been mapped to sunflower chromosomes (Table S1). However, mass cultivation of sunflower hybrids with particularly single dominant gene resistance favors pathogen population developing virulence strains due to strong selection pressures, such as was found for the genes Pl6 and Pl7 (Tourvieille de Labrouhe et al. 2000b; Gulya 2007; Gulya et al. 2011). Among 31 mapped Pl genes, seven, PlArg, Pl15, Pl17-Pl20, and Pl35, confer resistance to all P. halstedii races found in North America, and 10, Pl23-Pl32, confer resistance to all P. halstedii races identified in Europe, most of which, except for PlArg and Pl15, are the Pl genes recently identified, providing a diverse gene pool for sunflower breeding programs (Zhang et al 2017; Ma et al. 2017; Pecrix et al. 2018b; Qi et al. 2015, 2016, 2019; Gilley et al. 2020). Although 803–1 has been used as a differential host for more than 20 years, it is still effectively resistant to most of P. halstedii races identified to date and only susceptible to races 770 and 774 (Gascuel et al. 2015; Gilley et al. 2020). To maximize impedance for the pathogen to become virulent, it is necessary to combine Pl genes with different but complementary resistance spectra, providing durable resistance to DM. The most recently discovered DM genes of Pl17-Pl20 have been fine mapped, which are located on sunflower chromosomes 4 (Pl17 and Pl19), 2 (Pl18), and 8 (Pl20) (Ma et al. 2019, 2020). The diagnostic SNP markers closely linked to these genes will facilitate marker-assisted selection by pyramiding them with Pl36 to achieve durable and broad-spectrum resistance against P. halstedii.

Supplementary Information

Below is the link to the electronic supplementary material.

Supplementary file1 (XLSX 12 KB) (11.9KB, xlsx)
Supplementary file2 (XLSX 16 KB) (16.4KB, xlsx)
Supplementary file3 (XLSX 12 KB) (11.5KB, xlsx)

Acknowledgements

We thank Angelia Hogness for the technical assistance.

Author contribution

LLQ conceived and designed the experiments. LLQ and XWC acquired the funding. LLQ performed the experiments. LLQ and XWC analyzed the data, and LLQ wrote the paper. XWC reviewed and edited the paper.

Funding

This project was supported by the US National Sunflower Association Project No. 12-D02 and the USDA-ARS CRIS Project No. 3060–2100-043-00D.

Data availability

Not applicable.

Declarations

Ethics approval and consent to participate

The experiments were performed in compliance with the current laws of the USA.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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