Molecular breeding applies molecular tools and techniques, such as quantitative trait loci (QTL) mapping, marker-assisted selection (MAS) and genomic selection (GS) for crops improvement (Mergoum et al. 2019). The next-generation sequencing (NGS) technologies including DArTseq found prominent role in GS (Xu et al. 2012). Quantitative trait loci analysis based on genetic maps saturated with DArTseq markers may lead to development or optimization of selection tools and to localization of candidate genes potentially associated with agronomically important traits. Understanding of PM resistance factors may contribute to better control of biotic stresses that is important direction in modern breeding, and lead to sustainable food supply (Talas e al. 2016; Karbarz et al. 2020).
Up to date, numerous genetic maps are available for many crop species including cereals: wheat (Somers et al. 2004; Mantovani et al. 2008; Peleg et al. 2008; Avni et al. 2014; Cui et al. 2017; Xu et al. 2020), rye (Korzun et al. 2001; Milczarski et al. 2011; Gawroński et al. 2016), barley (Hearrden 2007; Zhou et al. 2015) and triticale (González et al. 2005; Alheit et al. 2011; Tyrka et al. 2011, 2015, 2018; Karbarz et al. 2020; Dyda et al. 2021; Wąsek et al. 2021). Also, numerous QTL regions associated with resistance to most common fungal diseases were reported (Saintenac et al. 2018; Odilbekov et al. 2019; Lin et al. 2020; Liu et al. 2020). However, number of reports describing QTL regions and associated with them, candidate genes of resistance to Blumeria graminis in triticale is very limited (Karbarz et al. 2020; Dyda et al. 2021). Therefore, analysis of PM resistance in 'Hewo' x 'Magnat' triticale population can result in an unique set of QTL and candidate genes associated with powdery mildew resistance and respective molecular markers useful for molecular breeding.
Powdery mildew caused by fungal pathogen Blumeria graminis (DC.) Speer is the most serious disease limiting cereal production in many regions of the world (Zhang et al. 2016). It affects host-plant’s photosynthesis of nutrient organs (stems, leaves and spikes) which may lead to decrease of grain quality and yield (Gao et al. 2018). In appropriate conditions and in short time, conidia develop a tube which elongates to appresorium, produces the haustoria to penetrate the surface of a host plant (Bruggmann et al. 2005; Wang et al. 2012). Fast spread of this pathogen can significantly affect a whole field (Gao et al. 2018) that is why, breeding and cultivation of resistant triticale varieties of triticale is needed (Mergoum et al. 2019).
Based on observation on the triticale 'Hewo' x 'Magnat' DH population resistance to powdery mildew infection under natural conditions, 12 QTL regions were identified associated with this trait on three wheat (4A, 3B and 4B) and four rye (2R, 4R, 5R and 6R) chromosomes (Table 3, Fig. 1). Three main QTL (on 3B, 4B and 5R), stable across selected environments were associated with the area under disease progress curve (AUDPC) or the average value of powdery mildew infection (avPM) and explained from 18–29% of variation.
On wheat chromosome 4A, locus Qpm.hm.4A.1–2 explained about 11% of phenotypic variation was identified (Table 3). Up to date only one QTL Qpm.gz.4A.1, explaining up to 13.7% of phenotypic variation has been reported as locus associated with PM resistance in triticale (Dyda et al. 2021). Chantret et al. (2001) and Mingeot et al. (2012) described QTL associated with powdery mildew resistance in wheat located on 4A chromosome which may correspond to Pm16 resistance gene or Rp1-like protein coding gene that determines leaf stripe and rust resistance (Reader and Miller 1991; Marone et al. 2012, 2013). Also, a cluster of resistance QTL on 4A chromosome for both powdery mildew and leaf rust resistance was described by Li et al. (2014) that is why this chromosome can be considered as important source of PM resistance in triticale.
Two QTL for AUDPC or avPM traits were found on chromosome 3B (Table 3). Qpm.hm.3B.2–5 was stable in Laski and Choryń locations and explained 8.6% − 18.1% of phenotypic variation. Qpm.hm.3B.2–5 shared the common region between 331.9 cM and 337.5 cM on chromosome 3B and the DArT marker tPt-4541 was closest to the peak (Table 3). QTL on 3B chromosome were also previously described in wheat as loci significantly linked with adult plant resistance (APR) for powdery mildew (Chen et al. 2009; Asad et al. 2013, 2014; Jia et al. 2018). These QTL were in a close distance to previously described Pm13 (Donini et al. 1995) and Pm41 genes (Li et al. 2009) and play a significant role in PM resistance. Furthermore, our previous studies also revealed loci on chromosome 3B significantly linked with APR in triticale evaluated in natural field conditions (Dyda et al. 2021). Physical mapping of those QTL confirmed that Qpm.hm.3B.2–5 is located in the same physical position as in above research, which indicates that loci on 3B chromosome is strongly linked with PM resistance. Two QTL with main effects were found on chromosome 4B including Qpm.hm.4B.1–3 stable across environments (Table 3). The phenotypic variation for this QTL was as in range between 12.1 % and 23.4%, and the DArTseq marker 4366886 was appointed as marker closest to the LOD peak (Table 3). Powdery resistance locus on chromosome 4B flanked by RFLP markers Xpsr593b and Xpsr1112 was described (Keller et al. 1999) as Pm7 resistance gene (Friebe et al. 1994). QTL between SSR markers Xgwm375 and Xgwm251 explaining 5.9% of phenotypic variation was also identified (Liang et al. 2006). Locus QPm.heau-4BL with high and significant effects common for two analyzed wheat RIL mapping populations was also reported (Ren et al. 2017) and compared with other loci described so far (Börner et al. 2002; Asad et al. 2013). But physical mapping confirmed that loci Qpm.hm.4B.1–3 are located in a different physical position than previously described loci so it and can be reported as an unique and a new source of powdery mildew resistance.
Identification of QTL and genes associated with powdery mildew resistance in rye is poorly described so far, comparing to wheat. So far, only eight Pm resistance genes were characterized and reported on rye chromosomes (Tyrka and Chelkowski 2004; Huang and Röder 2004). But due to close relationship with common wheat, rye has been extensively used as a valuable source of genes, adaptation to environment, yield improvement and mostly, for various wheat diseases (Schlegel 2016; An et al. 2019). Moreover, for triticale R homologous group, number of QTL regions and candidate genes associated with PM resistance is limited (Karbarz et al. 2020; Dyda et al. 2021). In present study, total of seven QTL regions were identified on chromosomes 2R, 4R, 5R and 6R (Table 3, Fig. 1).
Locus Qpm.hm.2R.1 was found for avPM evaluated in 2014 in Choryń location and explained 15.9% of phenotypic variation (Table 3). It has been reported that Pm7 gene has been derived from 2RL of Rosen rye (Rahmatov et al. 2016) but no QTL regions on rye chromosome 2R associated with powdery mildew resistance has been described so far.
Locus Qpm.hm.4R.1 was also found for avPM measured in 2014 in Choryń explaining 13.8% of phenotypic variance. Lind (1982) and Heun and Friebe (1990) reported that chromosome 4R did not condition resistance against powdery mildew isolates tested on wheat-rye addition lines. But afterwards, An et al. (2013) confirmed stable wheat-rye T4BL·4RL and T7AS·4RS translocation line WR41 which exhibited a high level of resistance to PM. Also, Fu et al. (2014) described that lines of 'Mianyang 11'×'Kustro' rye carry powdery mildew resistance at the adult stage and located a resistant gene on chromosome 4RL of Kustro rye. Only few QTL regions have been reported so far on 4R chromosome. Karbarz et al. (2020) identified locus QPm-4R in triticale resistance based on AUDPC method. Also, our previous analysis revealed total of six loci for both, AUDCP and avPM methods explaining up to 15.2% of phenotypic variation (Dyda et al. 2021). These QTL can be considered as a significant PM resistance regions as their physical position correspond to physical position of Qpm.hm.4R.1 described in this paper together with identified gene coded protein associated with race non-specificity and incomplete resistance (Table 4).
Two QTL were identified on 5R chromosome, one minor QTL for avPM in 2014 for Choryń location and one major Qpm.hm.5R.1–2 for Laski and Modzurów locations in 2015 explaining up to 29.3% of phenotypic variation (Table 3). Additionally, Qpm.hm.5R.1–2 shared a common region on 5R chromosome between 27.3 cM and 31.4 cM (Table 3). Up to date few reports show presence of QTL regions on 5R chromosome associated with biomass yield (Busemeyer et al. 2013) and plant height (carrying a dwarfing gene Ddw1; Alheitet al. 2011). In contrast, only one describe QTL, Qpm.gz.5R.1 linked to PM resistance in triticale (Dyda et al. 2021). It has been also confirmed that 5RL might be associated with that resistance as includes Pm4 resistance gene (Schlegel et al. 1998). Therefore, loci on chromosome 5R presented in this study is located in a different physical position comparing to Qpm.gz.5R.1 so it can be reported as a new source of powdery mildew resistance, strongly related with that trait.
Three QTL on 6R were significant in selected environments only and two of them contributed minor effects. Main locus, Qpm.hm.6R.1–2 was associated with avPM measured in Laski location and explained 27.6% of phenotypic variation (Table 3). Only one report describes identification of QTL linked with powdery mildew resistance on 6R chromosome (Dyda et al. 2021). Locus Qpm.gz.6R.2 identified for AUDCP method explaining 11.1% of phenotypic variation was found together with gene encoded cyclin-dependent kinase A-2-like protein linked to those regions (Dyda et al. 2021) however we suggest Qpm.hm.6R.1–2 as a new source of resistance, located in a different physical position than Qpm.gz.6R.2. For confirmation of importance of 6R chromosome, the Pm20 resistance gene has been identified and derived from 6RL of Prolific rye (Zhuang 2003; An et al. 2015).
Information of significant and unique QTL regions associated with powdery mildew resistance together with candidate genes coded proteins taking part in triticale defense against fungal pathogen can be an important tool used in modern breeding programs. Physical comparison of loci and sequences of flanking markers available in the literature showed that four QTL regions identified in present study are unique. Loci Qpm.hm.4B.1–3 were situated outside the QPm.heau-4BL region reported by Ren et al. (2017). Similarly, loci Qpm.hm.5R.1–2 and Qpm.hm.6R.1–2 are located in different physical regions comparing to Qpm.gz.5R.1 and Qpm.gz.6R.2, respectively described in our previous studies (Dyda et al. 2021). Additionally, locus Qpm.hm.2R.1 classified as the main effect was reported for the first time. All of these newly reported sources of PM resistance, after careful validation in available triticale varieties and lines can be used in marker-assisted selection (MAS) and assist molecular breeding programs.