Stripe rust responses in the field
JM44 displayed low DS (10–20%) (Fig. 1a) whereas JM229 was highly susceptible with DS of 90–100%) in all experiments (Fig. 1b). The stripe rust severities of the RILs showed approximate normal distributions, ranging from 0 to 100%, implying that the SR resistance in JM44 was polygenically controlled (Fig. 1c, d). ANOVA revealed lines and environments were the main determinants of phenotypic variation (P < 0.001). No significant variation (P = 0.289) was detected among replications in each environment. Broad-sense heritability of DS was 0.87 (Table 1). Pearson’s correlation coefficients among the phenotype data from five environments ranged from 0.34 to 0.75 (P < 0.001) (Fig. 1d). These results suggested that major SR genes/QTL may be involved in RIL population.
Table 1
Variance components of disease severity (DS) scores for the JM44/JM229 RIL population obtained across five environments
Source of variation
|
DF
|
Mean square
|
F value
|
P-value
|
RILs
|
294
|
2232
|
22.5
|
< 0.0001
|
Environments
|
4
|
157615.5
|
1586.6
|
< 0.0001
|
Lines×Environments
|
1176
|
331.8
|
8.4
|
< 0.0001
|
Replicates
|
1
|
833.6
|
|
0.298
|
Error
|
882
|
99.34
|
|
|
h2 b
|
0.87
|
|
|
|
Genetic linkage map
After removing SNPs with > 10% missing data and distorted segregation, 13,926 (26.27%) among all 53,007 SNPs showing polymorphism in parent lines and RILs remained. By using the “BIN” function in QTL IciMapping v4.2, and removing redundancies and 2,336 SNPs representing each "Bin" were chosen to construct genetic linkage maps covering all chromosomes. The map contained 35 linkage groups spanning 3,227.20 cM. The A, B, and D genomes had 813 (34.80%), 933 (39.94%), and 590 (25.26%) markers covering lengths of 1,072.22, 1,040.00, and 1,114.98 cM with average " Bin " marker intervals of 1.32, 1.11, and 1.89 cM, respectively. Chromosomes 1B, 1D, 2A, 2D, 3B, 3D, 4D, 6A and 6D were represented by single linkage group; the others two or three (Table S2).
QTL analysis
Five QTL for DS with additive effect on chromosome arms 1DS and 2BC in JM229 and 1BL, 6AL and 7DL in JM44 were identified using the ICIM method and a 3.2 LOD value (Fig. 2, Table 2). Two had consistent effects across all five field trials and were named QYr.nwafu-1BL.3 and QYr.nwafu-6AL (hereafter, YrJ44).
Table 2
Quantitative trait loci (QTL) for stripe rust resistance detected in the JM229/JM44 RIL population using disease severity (DS) data from five environments
QTL, environmenta
|
Flanking marker
|
Genetic positionb(cM)
|
Physical intervalc(Mb)
|
LODd
|
PVEe
|
ADDf
|
QYr.nwafu-1BL.3 (Yr29)
|
|
|
|
|
|
20YL
|
AX-94871279-AX-108800083
|
146
|
670.4-670.5
|
17.9
|
13.4
|
-9.2
|
20MY
|
AX-109423718-AX-111104551
|
150
|
675.6-675.7
|
4.2
|
6.2
|
-7.5
|
20WH
|
AX-109423718-AX-111104551
|
150
|
675.6-675.7
|
9.4
|
11.9
|
-3.8
|
21YL
|
AX-94871279-AX-108800083
|
146
|
670.4-670.5
|
13
|
18.5
|
-9.6
|
22YL
|
AX-109898596-AX-109346236
|
149
|
673.9-675.8
|
6.5
|
9.6
|
-8.1
|
Qyr.nwafu-1DS
|
|
|
|
|
|
|
20YL
|
AX-111540850-AX-111594357
|
0
|
8.4–10.6
|
8.1
|
15.9
|
6.3
|
20WH
|
AX-111040861-AX-109330851
|
14
|
18.4–20.3
|
4.8
|
6.7
|
4.6
|
21YL
|
AX-109141822-AX-111853381
|
22
|
21.8–22.9
|
4.4
|
5.2
|
3.8
|
Qyr.nwafu-2BC
|
|
|
|
|
|
|
20YL
|
AX-111559203-AX-109577269
|
22
|
154.6-158.4
|
2.7
|
4.4
|
4.6
|
20WH
|
AX-94480449- AX-112285965
|
28
|
211.9-214.6
|
5.6
|
7.8
|
4.8
|
22YL
|
AX-94480449- AX-112285965
|
28
|
211.9-214.6
|
3.5
|
4.4
|
4.6
|
QYr.nwafu-6AL (YrJ44)
|
|
|
|
|
|
20YL
|
AX-109373479-AX-109563479
|
141
|
606.6-611.9
|
13.5
|
19.4
|
-11.6
|
20MY
|
AX-109373479-AX-109563479
|
148
|
606.6-611.9
|
5.2
|
6.5
|
-10.1
|
20WH
|
AX-109373479-AX-109563479
|
149
|
606.6-611.9
|
18.1
|
25.2
|
-5.6
|
21YL
|
AX-109373479-AX-109563479
|
150
|
606.6-611.9
|
13.1
|
18.9
|
-8.5
|
22YL
|
AX-109373479-AX-109563479
|
150
|
606.6-611.9
|
15.5
|
22
|
-10.8
|
Qyr.nwafu-7DL
|
|
|
|
|
|
|
21YL
|
AX-110967909- AX-108909298
|
128
|
449.1-453.6
|
4.9
|
3.9
|
-4.4
|
aYL, MY and WH represent Yangling, Mianyang and Wuhan. 20, 21 and 22 represent growing seasons 2019–2020, 2020–2021 and 2021–2022. |
bPeak position in centimorgans (cM) from the first linked marker in the relevant linkage group |
cPhysical location in mega bases (Mb) from linked markers in the wheat genome |
dLogarithm of odds score |
ePercentages of phenotypic variation explained by individual QTL |
fAdditive effect of the allele. Negative signs indicate that the favorable allele is from JM44. |
QYr.nwafu-1BL.3 (Yr29) was represented by a 4.6-cM interval flanked by AX-94871279 and AX-111104551 and explained 6.2–18.5% of the variation in DS (Fig. 3b, Table 2). According to positions of expressed sequence tags in the 1B deletion bin map, QYr.nwafu-1BL.3 was located in deletion bin 1BL3-0.85-1.00 (Fig. 3a). By aligning the flanking sequences of linked SNPs with IWGSC RefSeq v1.1, the QYr.nwafu-1BL.3 region corresponded genomic interval 670.4-675.7 Mb in chromosome 1B (Fig. 3c).
YrJ44 was limited in a 3.1-cM interval spanned by AX-109373479 and AX-109563479 and placed in deletion bin 6AL8-0.90-1.00 (Fig. 4a, b, Table 2). The physical position of its confidence interval was 606.6-611.8 Mb on chromosome 6AL (Fig. 4c). YrJ44 explained 18.9–25.2% of the phenotypic variation during 2020, 2021 and 2022 seasons at Yangling and the 2020 season at Wuhan, but only 6.5% during the 2020 season at Mianyang (Table 2).
The other three QTL were named Qyr.nwafu-1DS, Qyr.nwafu-2BC and Qyr.nwafu-7DL, respectively (Table 2). These loci had minor effects on stripe rust response (explained phenotypic variation < 10%) and were detected in only one or two environments.
Slowing rusting and additive effects of YrJ44
To investigate the effects of YrJ44, the RILs were classified into two groups (with and without YrJ44) based on the presence/absence of flanking markers AX-109373479 and AX-109563479 (Fig. 5a). The mean DS for RILs carrying YrJ44 was 36.5% compared with 53.8% RILs without the QTL (Fig. 5a, Table S4). To assess the additive effective between YrJ44 and Yr29, The RILs were further divided into four groups based with the additional use of KASP marker ucw.k23 tightly linked with Yr29/QYr.ucw-1BL (Fig. 5b, Table 3). The group lacking Yr29 and YrJ44 had the highest mean DS (60.5%) in all environments and the group with both genes had the lowest DS (29.5%); the other groups were intermediate in mean response (Fig. 5b Table 3). The combination of YrJ44 and Yr29 reduce DS by 22.5–40.6% relative to the none group across environments (Table 3). DS of the Yr29 and YrJ44 groups were reduced by 4.5–20% and 9.7–24.7%, respectively, compared to the group lacking both genes. These results indicated that combining YrJ44 and Yr29 should be an effective breeding strategy for improving resistance to stripe rust. Moreover, lower DS values for the YrJ44 group in environments 20WH, 20YL and 21YL indicated that YrJ44 was a more effective than Yr29 reducing stripe rust levels.
Table 3
Comparison of stripe rust severities and the relative area under the disease progress curve (rAUDPC) among JM44/JM229 RIL grouped based on QTL status defined by flanking molecular markers
Group
|
No. of RILs
|
Disease Severity
|
rAUDPC 22YL
|
20WH
|
20MY
|
20YL
|
21YL
|
22YL
|
Mean
|
None
|
48
|
45.0a
|
62.0a
|
63.2a
|
75.4a
|
56.7a
|
60.5a
|
0.8a
|
Yr29
|
72
|
40.5a
|
51.3b
|
57.5a
|
60.8b
|
36.7b
|
49.4b
|
0.5b
|
YrJ44
|
75
|
24.0b
|
52.3b
|
49.8b
|
55.1c
|
32.0b
|
42.7c
|
0.5b
|
YrJ44 + Yr29
|
65
|
17.1c
|
39.5c
|
37.3c
|
37.5d
|
16.1c
|
29.5d
|
0.2c
|
Mean disease severities and rAUDPC followed by different letters (a, b, c, and d) are significantly different (p < 0.01) based on tests of least significant difference. YL, MY and WH, Yangling, Mianyang and Wuhan. 20, 21 and 22, growing seasons 2019–2020, 2020–2021 and 2021–2022. |
In addition, the dynamic development progress of Pst on several selected RILs were recorded in the 2022YL environment at 42, 45, 48, 52, 58 days after inoculation and the relative area under the disease progress curve (rAUDPC) were calculated, respectively (Fig. 5c, Table 3&S4). The none group with the highest rAUDPC value showed significant difference with the other three groups. The Yr29 and YrJ44 group with a mean rAUDPC of 0.5 showed no significant difference. The YrJ44 + Yr29 group showed the lowest rAUDPC and can reduce 0.3–0.5 rAUDPC value compared with other groups. These results indicated that both Yr29 and YrJ44 retard the growth of Pst races and more effective slow rusting can be obtained by combination of them.
To further validate the slowing-rust resistance of YrJ44, RIL018 (neither gene), RIL003 (Yr29), RIL037 (YrJ44), RIL064 (Yr29 + YrJ44), and the parents were inoculated with a mixture of CYR32, CYR33 and CYR34 in greenhouse. JM229 showed a highly susceptible response and JM44 showed a low DS. The responses of RIL064 and RIL018 were similar to JM44 and JM229, respectively; RIL003 and RIL037 exhibited intermediate levels of response compared with RIL037 having a lower DS (Fig. 5d).
YrJ44 AQP markers for molecular-assisted selection
Flanking AQP markers AX-109373479 and AX-109563479 were chosen for molecular-assisted selection (MAS) of YrJ44. The validation study showed that both AX-109373479 and AX-109563479 were polymorphic in the worldwide panel of 1,019 wheat accessions (Fig. 6a, b; Table S1). According to presence/absence of the JM44-alelle for AX-109373479 and AX-109563479 panel members were haplotyped as 651 (63.9%) CC-CC (JM44), 77 (7.5%) CC-TT, 226 (22.2%) TT-CC and 65 (6.4%) TT-TT (JM229) (Fig. 6c, Table S5). The high frequency of the CC-CC haplotype implied that YrJ44 was widely distributed in wheat. The significant difference on the significantly different mean DS scores between panel members with CC-CC and TT-TT haplotypes indicated that AX-109373479 and AX-109563479 were associated with resistance to stripe rust (Fig. 6c) and that these AQP markers could be used in marker assisted selection of YrJ44 in wheat breeding programs.