QTL mapping analysis
A total of 23 QTLs associated with PH and SC were detected under the two environmental conditions on chromosomes 1A, 1B, 2B, 2D, 3A, 3B, 4B, 5A, 5B, 6B, 6D, 7A, 7B, and 7D, respectively. Among these QTLs, qPH2B.1, qPH2D, qPH4B, qSC1B, qSC2B.1, qSC2D.1, qSC7D, could be stably detected in the two environments (Fig. 3; Table 1).
Table 1
QTLs associated with plant height (PH), heading date (HD), and spikelet compactness (SC) in the Zhongpuchang and Changping environments identified with IciMapping 4.1.
Environment | Trait Name | QTL | Chromosome | Left Marker | Right Marker | LOD | PVE (%) | Add |
Zhongpuchang | PH | qPH2B.1 | 2B | A44806 | A44812 | 11.4684 | 7.994 | 2.5902 |
qPH2B.2 | 2B | A48303 | A48463 | 8.2893 | 5.5607 | 2.1144 |
qPH2D | 2D | A61464 | A61529 | 22.651 | 19.2436 | 4.0568 |
qPH3A | 3A | A69302 | A69519 | 5.8247 | 3.943 | -1.7645 |
qPH4B | 4B | A112983 | A113076 | 30.1221 | 29.7012 | -4.901 |
qPH7B.1 | 7B | A193948 | A194156 | 10.2915 | 6.9504 | 2.414 |
qPH7B.2 | 7B | A194789 | A194814 | 3.9033 | 2.449 | -1.4358 |
SC | qSC1B | 1B | A18513 | A19081 | 5.0487 | 3.7798 | 0.0687 |
qSC2B.1 | 2B | A44792 | A44806 | 24.5623 | 24.9755 | -0.1801 |
qSC2B.2 | 2B | A45030 | A45063 | 11.673 | 9.482 | 0.1119 |
qSC2D.1 | 2D | A61140 | A61372 | 13.6018 | 13.006 | -0.1288 |
qSC2D.2 | 2D | A64193 | A64287 | 5.4426 | 3.9538 | -0.0703 |
qSC7B | 7B | A193641 | A193948 | 3.9214 | 2.7592 | -0.058 |
qSC7D | 7D | A202015 | A202077 | 2.7953 | 1.9425 | 0.0493 |
Changping | PH | qPH2B.1 | 2B | A44792 | A44806 | 4.0065 | 2.1155 | 1.6056 |
qPH2B.3 | 2B | A56299 | A56338 | 5.2987 | 2.8879 | 1.9184 |
qPH2D | 2D | A61464 | A61529 | 23.5792 | 17.8166 | 4.7225 |
qPH3B | 3B | A94610 | A94625 | 4.2256 | 2.2634 | -1.6462 |
qPH4B | 4B | A112816 | A112833 | 40.1532 | 41.7687 | -7.0218 |
qPH5A | 5A | A133195 | A133216 | 7.5183 | 4.2383 | 2.6582 |
qPH5B | 5B | A134432 | A141070 | 5.1174 | 3.5318 | -2.0132 |
qPH7A | 7A | A179460 | A179521 | 6.4762 | 3.6198 | 2.132 |
SC | qSC1A | 1A | A7344 | A7362 | 4.4992 | 4.6783 | -0.0555 |
qSC1B | 1B | A18513 | A19081 | 6.2316 | 6.9554 | 0.0686 |
qSC2B.1 | 2B | A44757 | A44782 | 12.7453 | 14.9828 | -0.1018 |
qSC2D.1 | 2D | A61578 | A61731 | 19.362 | 27.6476 | -0.1415 |
qSC6B | 6B | A161877 | A161905 | 3.8673 | 3.9563 | 0.0521 |
qSC6D | 6D | A174738 | A174762 | 3.3927 | 3.504 | 0.0514 |
qSC7B | 7B | A192944 | A193088 | 3.8252 | 3.8496 | -0.0502 |
qSC7D | 7D | A202015 | A202077 | 5.3974 | 5.6152 | 0.0617 |
Three stable QTLs associated with PH were identified on chromosomes 2B, 2D, and 4B in the two environments. The major QTL qPH4B showed higher LOD scores (30.1 and 40.2) that explained 29.7% and 41.8% of the phenotypic variation, respectively. In addition, qPH2D were identified with LOD scores 22.7 and 23.6 under the two environments and accounted for 19.2% and 17.8% of the phenotypic variation, respectively; while qPH2B.1 had LOD scores 11.5 and 4.0 explained 7.9% and 2.1% of the phenotypic variation, respectively, being detected on the two environments. Finally, the qPH4B allele from ND5181 decreased PH, while the Rht8-2 allele in qPH2D and qPH2B.1 reduced PH (Table 1).
And, we identified four stable QTLs associated with SC on chromosomes 1B, 2B, 2D and 7D in the two environments. qSC2B.1 was detected in both environments with LOD scores 24.6 and 12.7, explaining 25.0% and 15.0% of the phenotypic variation, respectively. qSC2D.1 with LOD scores 13.6 and 19.4 was also identified in two environments and explained 13.0% and 27.6% of the phenotypic variation, respectively. qSC1B, with LOD scores 5.0 and 6.2, explained 3.8% and 7.0% of the phenotypic variation, respectively and was detected in the two environments. Finally, qSC7D with LOD scores 2.8 and 5.4 explained 1.9% and 5.6% of the phenotypic variation, respectively. Among these QTLs in ND5181, the qSC1B and qSC7D alleles increased SC and the qSC2B.1 and qSC2D.1 alleles decreased SC (Table 1).
Rht8-B1 is the candidate gene of qPH2B.1
To validate the mapped region of qPH2B.1, we successfully developed 9 KASP makers around this region based on 660K SNP array analysis between the ND5181 and Rht8-2 varieties, and delimited it to a physical interval of 3.5 Mb between markers 2B-4 and 2B-5. This QTL showed a LOD score of 3.4 and explained 11.8% of the observed phenotypic variation (Fig. 4). Given that this region included Rht8-B1 (TraesCS2B02G073600), an homoeologous gene to Rht8, we sequenced the region and uncover two genetic variants (39418567–39418568, GC to TT) in the coding region of the gene that resulted in an amino acid change from G (ND5181) to V (Rht8-2) at the 175th position. We then analyzed the effects of Rht8-B1 on PH and SC in the RIL population, and found that TT alleles reduced PH by 6.2% and 3.6%, shortened spike length by 6.6% and 4.7%, and increased SC by 5.8% and 6.3% in the Zhongpuchang and Changping field experiments, respectively (Fig. 4).
Rht8-B1 exhibited a lower impact on PH reduction than Rht8-D1
We compared the effects of Rht8-B1 and previously reported Rht8-D1 on PH reduction by knocking out both genes in a Fielder background and evaluating the PH of the T2 lines. The results showed that the PH of edited plants was significantly lower than control plants. Specifically, edited Rht8-B1 and Rht8-D1 plants showed a PH reduction of 5.6% and 17.5%, respectively, suggesting a lower impact of Rht8-B1 than Rht8-D1 (Fig. 5A, B).
Expression comparison of Rht8-B1 and Rht8-D 1
In order to examine the difference of expression pattern between Rht8-B1 and Rht8-D1, we analyzed their transcript level using Hexaploid Wheat Expression Dataset [35], and found that both genes were highly expressed in stem at the jointing stage, while the expression of Rht8-D1 was significantly higher than that of Rht8-B1 (Fig. 6A). Our previous study suggested that the frameshift mutation in Rht8-D1 caused dwarfism phenotype in Rht8-2 and its expression was significantly decreased in the mutant compared to that of WT [10]. To investigate the effects of mutation of Rht8-D1 on the B subgenome of Rht8, we analyzed the expression of Rht8-B1 in the first internode below spike of the mutant Rht8-2 and WT. The results showed that the expression of Rht8-B1 was remarkably increased in Rht8-2 (Fig. 6B), indicating that the mutation of Rht8-D1 affected the transcript expression of Rht8-B1.
Distribution of Rht8-B1 in wheat varieties worldwide
We used a total of 305 worldwide accessions from the Wheat Union Database (http://wheat.cau.edu.cn/WheatUnion/), including 193 accessions from China and 112 accessions from other countries, for the analysis of Rht8-B1 allelic variation. Among these, 68 Chinese accessions (35.2%) contained the Rht8-B1b TT alleles, compared to only 6 accessions (5.4%) in other countries (Fig. 7). In China, we analyzed 118 modern varieties and 75 landrace accessions, with 20.3% and 58.7% with Rht8-B1b alleles, respectively (Table S3). These results suggest Rht8-B1b alleles were not widely used historically for wheat breeding.