1. Introduction
Powdery mildew caused by
Blumeria graminis f. sp.
tritici and stripe rust caused by
Puccinia striiformis f. sp.
tritici are devastating diseases of wheat growing in cold and wet regions of the world [
1,
2]. Although numerous genes conferring resistance to powdery mildew and stripe rust have been reported in wheat and its wild relatives, major resistance genes are rapidly overcome due to rapid virulence changes in the pathogen populations [
3]. Therefore, it is essential for wheat breeding to continuously explore new genetic resources and transfer new genes from wild relatives that have potential resistance to powdery mildew and stripe rust.
Thinopyrum intermedium (Host) Barkworth and Dewey (genomes, JJ
sSt, 2n = 6x = 42), is a hexaploid species that confers potentially favorable traits that can be exploited for wheat genetic improvement [
4].
Th. intermedium has wide range of adaptation to soils and climate and it displays a vast genetic diversity in the species [
5]. The diversified
Th. intermedium germplasms exhibited superior resistance to multiple wheat diseases including powdery mildew and stripe rust pathogens.
Th. intermedium can be easily hybridized with common wheat, and a number of wheat-
Th. intermedium derivatives were developed by chromosome engineering and breeding practices [
6]. Previously developed wheat-
Th. intermedium partial amphiploids including Zhong 1 to Zhong 5 [
7,
8], Otrastsjuskaya [
9], TE-3 [
10], TAI8335 [
11] and TE253 [
12] were reported to possess outstanding resistance to powdery mildew and/or stripe rust. However, it has not been reported so far that any wheat-
Th. intermedium translocation lines carry both powdery mildew and stripe rust resistance genes.
A wheat-
Th. intermedium partial amphiploid TAI7047 is effective against all existing Chinese powdery mildew and stripe rust races [
11]. With the aim to transfer novel resistance traits to common wheat, TAI7047 was crossed with cultivar Mianyang11 (MY11) and the hybrids were backcrossed by using MY11 as recurrent parent. Among the obtained wheat-
Th. intermedium derived progeniess, a new wheat-
Th. intermedium introgression line CH13-21 showing high level of resistance to both powdery mildew and stripe rust was selected. In the present study, we determined the chromosomal constitution of the CH13-21 using multiple molecular cytogenetic approaches including the genomic and fluorescence
in situ hybridization in combination with functional marker analysis.
3. Discussion
The production of wheat-
Thinopyrum compensating Robertsoanian translocations with the targeted alien chromosomes by the centric breakage-fusion mechanism, is the first important step for transferring genes from
Thinopyrum species to wheat for breeding purpose. Friebe
et al. [
16] firstly characterized radiation-induced wheat-
Th. ponticum chromosome translocation lines and located the stem rust resistance gene
Sr26 on the translocation chromosome T6AS.6AL-6Ae#1L. Whelan and Hart [
17] identified two independent Robertsonian translocations, involving the fusion between the short arm of a group 6 chromosome of
Th. Ponticum and the long arm of chromosome 6D by using chromosome painting and PCR analysis. A gene for resistance to wheat streak mosaic
Cmc2 was located on the compensating translocated chromosomes T6DL·6Ae#2S [
17] and T6AL·6Ae#2S [
18]. Hu
et al. [
19] reported that a wheat-
Th. ponticum 6J
s(6B) substitution line X005 carried novel stripe rust resistance gene(s) from chromosome 6J
s. Recently, Tang
et al. [
20] reported that a powdery mildew resistance line 08-723 possessed a fusion of homozygous translocation between St-chromosomes of
Th. intermedium and chromosome 6A of wheat. Therefore, the
Thinopyrum homologous group 6 chromosomes contained novel disease resistance genes and easily compensated the wheat homologous chromosomes. In the present study, the CH13-21 contained
Th. intermedium chromatin 6Ai#1L carrying both powdery mildew and stripe rust genes by spontaneous chromosome translocation which originated from the crosses between wheat and wheat-
Th. intermedium partial amphiploid. It is interesting to further identify new resistance genes in homologous group 6 chromosomes of
Thinopyrum genomes.
Recently described wheat-
Th. intermedium cryptic translocations carried two powdery mildew resistance genes,
Pm40 on 7B [
21] and
Pm43 on 2D [
22], as well as two stripe rust resistance gene,
Yr50 on 4B [
23], and
YrL693 on 1B [
24]. They were presumed to be transferred from
Th. intermedium to common wheat on cryptic translocations. There is no report on powdery mildew or stripe rust resistance genes that have been transferred from a group 6 of
Th. intermedium chromosome to wheat. The CH13-21 displayed novel resistance to multiply powdery mildew and stripe rust races at both seedling and adult plant stages. It will be interesting to reveal the novel genes or gene complex conferring the Pm and/or Yr resistance on chromosome 6Ai#1L in CH13-21. Moreover, pyramiding multiple and effective resistance genes is a potential method for durable resistance of genetic control [
25]. The alien resistance genes complex such as the
S. cereale 1RS-derived
Pm8/Yr9/Lr26 and the
Ae. ventricosa 2N
v derived
Yr17/Lr34 confirmed to have multi-diseases resistance, which are effective to long-term resistance for several decades of global wheat breeding [
2,
16]. It is worthwhile to further investigate the wheat-
Th. intermedium translocation line CH13-21 for possibly long-lasting resistance to wheat against stripe rust and powdery mildew.
Characterization of alien chromatins in wheat background is essential for effectively utilized new germplasm in breeding programs. The C-banding, GISH and the sequential FISH have been used widely for rapid identification of alien chromatins in wheat [
14,
26,
27]. In the present study, the GISH and FISH were used to visualize the chromosome constitution of the wheat-
Th. intermedium translocation line CH13-21 (
Figure 2). However, The
in situ hybridization was not able to precisely determine the homologous group of alien chromatin on wheat background. The PCR-based landmark unique gene (PLUG) primers are developed based on the orthologous gene conservation between rice and wheat, and presumably amplify fragments corresponding to the A, B and D genomes of wheat [
15]. Our previous and other studies also confirmed that the PLUG markers have been used as effective tools for assigning the chromosomal homology of alien chromatins including
Dasypyrum,
Secale and
Thinopyrum species [
19,
28,
29,
30]. Moreover, the PLUG primers were effective to detect the physical location of alien chromatin in wheat background [
15]. In the present study, the PLUG markers were used to identify
Th. intermedium alien fragments and revealed the alien chromosome arm belonging to homology group 6 in the wheat-
Th. intermedium derivative line CH13-21. The molecular evidence confirmed that the translocation line CH13-21 conferred potential resistance genes which originated from its wild donor
Th. intermedium. The PLUG markers specific to
Th. intermedium chromosome 6Ai#1L, can be used as marker assisted selection for this Robertsonain translocation in the future wheat breeding. The CH13-21 can be used for further chromosome engineering by shortening the
Th. intermedium segment in line CH13-21, using
ph1b induced homoeologous recombination for wheat resistance breeding.