The elite wheat breeding line ShiCG15-009 shows a high level of resistance to powdery mildew at the seedling and adult stages. In this study, a dominant gene PmCG15-009 was characterized on the long arm of chromosome 2B in ShiCG15-009, further molecular markers analysis showed that PmCG15-009 was flanked by markers XCINAU130 and XCINAU143 with the genetic distances 0.2 and 0.4 cM, respectively, corresponding to a physic interval of 705.14-723.48 Mb on the Chinese Spring reference genome sequence v2.1 [15]. Previous studies reported that a series of formally designated Pm genes on chromosome 2BL were identified, including dominant genes Pm6 [16], Pm33 [17], Pm51 [18], Pm52 [11], Pm63 [19] and Pm64 [20] which indicated the chromosome 2BL is most likely to be an enrichment region for resistance genes.
Pm6 was derived from T. timopheevii 2B/2G introgression and was moderate to highly susceptible to powdery mildew at the one-leaf stage to the two-leaf stage, but gradually increased resistance from the third leaf stage and reached complete resistance at the fourth leaf stage and later [21]. Wan et al. reported that Pm6 was flanked by markers CIT02g-18 and CIT02g-20, corresponding to the physical interval of 706.75-707.63 Mb and the candidate interval of Pm6 had serious recombination suppression due to the introgression of the 2G chromosome segment. In contrast to those genes, PmCG15-009 (705.14-723.48 Mb), derived from common breeding line ShiCG15-009, was highly resistant to powdery mildew from the first leaf stage to the whole stages shows no significant recombination suppression in our mapping population. Additionally, when tested with 57 co-segregated or closely linked markers of Pm6, only ten markers showed polymorphisms in ShiCG15-009, Yannong 21 and their derivative F2:3 families, which revealed a distinct genetic diversity between the candidate intervals of PmCG15-009 and Pm6. In conclusion, PmCG15-009 was significantly different from Pm6.
Pm33 [17], a dominant powdery mildew resistance gene, was introduced from Triticum carthlicum accession PS5 and was mapped on the interval of 782.3-794.37 Mb. Pm52 [22] was derived from the wheat cultivar Liangxing 99 and flanked by SSR markers Xicssl326 and Xicssl795, referring to the physical interval of 589.14-594.63 Mb. In our study, the dominant gene PmCG15-009 was delimited to an interval of 705.14-723.48 Mb on the Chinese Spring reference genome sequence v2.1, which was significantly different from Pm33 and Pm52 based on the physical interval and/or origins.
Pm51 [18], Pm63 [19] and Pm64 [20] were derived from Thinopyrum ponticum, Iranian wheat landrace PI 628024, and wild emmer, respectively. Although the physical interval of PmCG15-009 overlapped that of Pm51 (718.30-747.73 Mb), Pm63 (718.80-731.82 Mb,) and Pm64 (703.85–718.80 Mb), their source was different from each other. More importantly, all the closely linked markers or co-segregated markers of these three genes, including six for Pm51, six for Pm63 and ten for Pm64, were not polymorphic between resistant parent ShiCG15-009 and susceptible parent Yannong 21 and two bulks, which indicated a various genetic diversity between the candidate intervals of PmCG15-009 and these of the tested genes. Taken together, PmCG15-009 was different from those documented genes on chromosome 2BL, which might be a novel gene or allele. To further provide more reliable evidence for their relationship, allelism tests and cloning of these genes are necessary in the future to further provide more reliable evidence for their relationship.
So far, 11 race-specific Pm genes have been cloned successively. Among them, Pm3 [23], Pm8 [24], Pm2 [25], Pm17 [6], Pm60 [26], Pm21 [27, 28], Pm5e [29], Pm41 [30] and Pm1a [31] encoded coiled-coil nucleotide-binding site leucine-rich repeat protein (CC-NBS-LRR) protein. Pm4 [32] and Pm24 [33] encoded a putative serine/threonine kinase and tandem kinase protein (TKP) with putative kinase-pseudokinase domains, respectively. In plants, NLR proteins and protein kinases are the major classes of disease resistance genes. NLR functions as intracellular immune receptor that recognizes pathogen effectors and activates effector-triggered immunity (ETI) and protein kinases are important for transmembrane signaling that regulates plant development and adaptation to diverse environmental conditions [34, 35]. In the candidate interval of PmCG15-009, 194 high confidence genes were annotated based on the IWGSC Chinese Spring reference genome v2.1, and only 14 genes are associated with disease resistance. Furtherly, qRT-PCR analysis showed that the transcript levels of six genes were induced at different degree by the Bgt isolate E09 between the resistant parent ShiCG15-009 and susceptible parent Yannong 21. Notably, the gene TraesCS2B03G1283800, encoding a serine threonine-protein kinase, showed high expression in ShiCG15-009 at 4 hpi following Bgt inoculation. TraesCS2B03G1276100 and TraesCS2B03G1266900 were significantly upregulated in resistant parent ShiCG15-009 but not changed in susceptible Yannong 21. Considering the expression patterns, these genes could be the candidate gene of PmCG15-009 or regulatory genes involved in the resistance process. These data provide a significant direction at dissecting the resistance pathways. Of course, further studies are needed to investigate whether these genes are candidate genes of PmCG15-009.
When a novel gene was discovered, the rational utilization was the next challenge in wheat breeding programs. The elite wheat breeding line ShiCG15-009 showed not only highly resistance to powdery mildew at all the stages but excellent agronomic traits, thus should be a valuable resource for genetic research and wheat resistance improvement. To accelerate the transfer of PmCG15-009 in MAS, we evaluated the availability of 20 markers linked or co-segregated with PmCG15-009 in 46 susceptible commercial cultivars/lines. The results showed that 18 of 20 markers could be used singly or in combination in MAS for tracking PmCG15-009 in the background of those susceptible cultivars. Also, we have made many hybrid combinations between ShiCG15-009 and several susceptible commercial wheat cultivars and obtained the BC1F2 and F3 segregation populations. In future, PmCG15-009 will play an important role in wheat breeding programs.