Fine Physical Bin Mapping of the Powdery Mildew Resistance Gene Pm21 Based on Chromosomal Structural Variations in Wheat

Orthologous genes Pm12 and Pm21 from two wild relatives of wheat show evolutionary conservation but divergent powdery mildew resistance

Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is a devastating disease that threatens wheat production worldwide. Pm12, which originated from Aegilops speltoides, a wild relative of wheat, confers strong resistance to powdery mildew and therefore has potential use in wheat breeding. Using susceptible mutants induced by gamma irradiation, we physically mapped and isolated Pm12 and showed it to be orthologous to Pm21 from Dasypyrum villosum, also a wild relative of wheat. The resistance function of Pm12 was validated via ethyl methanesulfonate mutagenesis, virus-induced gene silencing, and stable genetic transformation. Evolutionary analysis indicates that the Pm12/Pm21 loci in wheat species are relatively conserved but dynamic. Here, we demonstrated that the two orthologous genes, Pm12 and Pm21, possess differential resistance against the same set of Bgt isolates. Overexpression of the coiled-coil domains of both PM12 and PM21 induces cell death in Nicotiana benthamiana leaves. However, their full-length forms display different cell death-inducing activities caused by their distinct intramolecular interactions. Cloning of Pm12 will facilitate its application in wheat breeding programs. This study also gives new insight into two orthologous resistance genes, Pm12 and Pm21, which show different race specificities and intramolecular interaction patterns.

Genome-wide association study reveals structural chromosome variations with phenotypic effects in wheat (Triticum aestivum L.)

Structural chromosome variations (SCVs) are large-scale genomic variations that can be detected by fluorescence in situ hybridization (FISH). SCVs have played important roles in the genome evolution of wheat (Triticum aestivum L.), but little is known about their genetic effects. In this study, a total of 543 wheat accessions from the Chinese wheat mini-core collection and the Shanxi Province wheat collection were used for chromosome analysis using oligonucleotide probe multiplex FISH. A total of 139 SCVs including translocations, pericentric inversions, presence/absence variations (PAVs), and copy number variations (CNVs) in heterochromatin were identified at 230 loci. The distribution frequency of SCVs varied between ecological regions and between landraces and modern cultivars. Structural analysis using SCVs as markers clearly divided the landraces and modern cultivars into different groups. There are very clear instances illustrating alien introgression and wide application of foreign germplasms improved the chromosome diversity of Chinese modern wheat cultivars. A genome-wide association study (GWAS) identified 29 SCVs associated with 12 phenotypic traits, and five (RT4AS•4AL-1DS/1DL•1DS-4AL, Mg2A-3, Mr3B-10, Mr7B-13, and Mr4A-7) of them were further validated using a doubled haploid population and advanced sib-lines, implying the potential value of these SCVs. Importantly, the number of favored SCVs that were associated with agronomic trait improvement was significantly higher in modern cultivars compared to landraces, indicating positive selection in wheat breeding. This study demonstrates the significant effects of SCVs during wheat breeding and provides an efficient method of mining favored SCVs in wheat and other crops.

Molecular Cytogenetic Identification of the Wheat–Dasypyrum villosum T3DL·3V#3S Translocation Line with Resistance against Stripe Rust

The annual species Dasypyrum villosum possesses several potentially valuable genes for the improvement of common wheat. Previously, we identified a new stripe rust-resistant line, the Chinese Spring (CS)–D. villosum 3V#3 (3D) substitution line (named CD-3), and mapped its potential rust resistance gene (designated as YrCD-3) on the 3V#3 chromosome originating from D. villosum. The objective of the present study was to further narrow down the YrCD-3 locus to a physical region and develop wheat-3V#3 introgression lines with strong stripe rust resistance. By treating CD-3 seeds with 60Co γ-irradiation, two CS-3V#3 translocation lines, T3V#3S.3DL and T3DS.3V#3L (termed 22-12 and 24-20, respectively), were identified from the M4 generation through a combination of non-denaturing fluorescence in situ hybridization (ND-FISH) and functional molecular markers. Stripe rust resistance tests showed that the line 22-12 exhibited strong stripe rust resistance similarly to CD-3, whereas 24-20 was susceptible to stripe rust similarly to CS, indicating that YrCD-3 is located on the short arm of 3V#3. The line 22-12 can potentially be used for further wheat improvement. Additionally, to trace 3V#3 in the wheat genetic background, we produced 30 3V#3-specific sequence tag (EST) markers, among which, 11 markers could identify 3V#3S. These markers could be valuable in fine-mapping YrCD-3.

Predominant wheat-alien chromosome translocations in newly developed wheat of China

Founder wheat lines have played key role in Chinese wheat improvement. Wheat-Dasypyrum villosum translocation T6VS·6AL has been widely used in wheat breeding in recent years due to its high level of powdery mildew resistance and other beneficial genes. Reference oligo-nucleotide multiplex probe (ONMP)-FISH karyotypes of six T6VS·6AL donor lines were developed and used for characterizing 32 derivative cultivars and lines. T6VS·6AL was present in 27 cultivar/lines with 20 from southern China. Next, ONMP-FISH was used to study chromosome constitution of randomly collected wheat cultivars and advanced breeding lines from southern and northern regions of China: 123 lines from the regional test plots of southern China and 110 from northern China. In southern China, T6VS·6AL (35.8%) was the most predominant variation, while T1RS·1BL (27.3%) was the most predominant in northern China. The pericentric inversion perInv 6B derived from its founder wheat Funo and Abbondaza was the second most predominant chromosome variant in both regions. Other chromosome variants were present in very low frequencies. Additionally, 167 polymorphic chromosome types were identified. Based on these variations, 271 cultivars and lines were clustered into three groups, including southern, northern, and mixed groups that contained wheat from both regions. Different dominant chromosome variations were seen, indicating chromosome differentiation in the three groups of wheat. The clearly identified wheat lines with T6VS·6AL in different backgrounds and oligonucleotide probe set will facilitate their utilization in wheat breeding and in identifying other beneficial traits that may be linked to this translocation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-021-01206-3.

Genetic Diversity and Evolutionary Analyses Reveal the Powdery Mildew Resistance Gene Pm21 Undergoing Diversifying Selection

Wheat powdery mildew caused by Blumeria graminis f. sp. tritici (Bgt) is a devastating disease that threatens wheat production and yield worldwide. The powdery mildew resistance gene Pm21, originating from wheat wild relative Dasypyrum villosum, encodes a coiled-coil, nucleotide-binding site, leucine-rich repeat (CC-NBS-LRR) protein and confers broad-spectrum resistance to wheat powdery mildew. In the present study, we isolated 73 Pm21 alleles from different powdery mildew-resistant D. villosum accessions, among which, 38 alleles were non-redundant. Sequence analysis identified seven minor insertion-deletion (InDel) polymorphisms and 400 single nucleotide polymorphisms (SNPs) among the 38 non-redundant Pm21 alleles. The nucleotide diversity of the LRR domain was significantly higher than those of the CC and NB-ARC domains. Further evolutionary analysis indicated that the solvent-exposed LRR residues of Pm21 alleles had undergone diversifying selection (dN/dS = 3.19734). In addition, eight LRR motifs and four amino acid sites in the LRR domain were also experienced positive selection, indicating that these motifs and sites play critical roles in resistance specificity. The phylogenetic tree showed that 38 Pm21 alleles were divided into seven classes. Classes A (including original Pm21), B and C were the major classes, including 26 alleles (68.4%). We also identified three non-functional Pm21 alleles from four susceptible homozygous D. villosum lines (DvSus-1 to DvSus-4) and two susceptible wheat-D. villosum chromosome addition lines (DA6V#1 and DA6V#3). The genetic variations of non-functional Pm21 alleles involved point mutation, deletion and insertion, respectively. The results also showed that the non-functional Pm21 alleles in the two chromosome addition lines both came from the susceptible donors of D. villosum. This study gives a new insight into the evolutionary characteristics of Pm21 alleles and discusses how to sustainably utilize Pm21 in wheat production. This study also reveals the sequence variants and origins of non-functional Pm21 alleles in D. villosum populations.