Efficient anchoring of alien chromosome segments introgressed into bread wheat by new Leymus racemosus genome-based markers

Alien Chromatin from Hordeeae Grasses Enhances the Compatibility of Epichloë Endophyte Symbiosis with the Hexaploid Wheat Triticum aestivum

The inoculation of Epichloë endophytes into modern cereals, resulting in systemic infection, depends on the genetics of both the host and the endophyte strain deployed. Until very recently, the only modern cereal to have been infected with Epichloë, in which normal phenotype seed-transmitted associations were achieved, is rye (Secale cereale). Whilst minor in-roads have been achieved in infecting hexaploid wheat (Triticum aestivum), the phenotypes of these associations have all been extremely poor, including host death and stunting. To identify host genetic factors that may impact the compatibility of Epichloë infection in wheat, wheat-alien chromosome addition/substitution lines were inoculated with Epichloë, and the phenotypes of infected plants were assessed. Symbioses were identified whereby infected wheat plants were phenotypically like uninfected controls. These plants completed their full lifecycle, including the vertical transmission of Epichloë into the next generation of grain, and represent the first ever compatible wheat-Epichloë associations to be created.

A Novel Wheat-Rye 2R (2D) Disomic Substitution Line Pyramids Two Types of Resistance to Powdery Mildew

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is a devastating disease of wheat that seriously affects yield and quality worldwide. Because of the extensive growth of wheat cultivars with homogeneous genetic background, exploring novel resistant resources from wheat relatives has become important for increasing the genetic diversity of wheat. Rye (Secale cereale) is a wheat relative possessing abundant resistance genes because of its high variation. Wheat line AL69, resistant to powdery mildew, was developed by crossing, backcrossing, and self-pollination for multiple generations between hexaploid triticale Zhongsi 237 and common wheat cultivar Zimai 17. Through genomic in situ hybridization (GISH) and multicolor fluorescence in situ hybridization (FISH), nondenaturing FISH, multicolor GISH, and selection with specific molecular markers, AL69 was determined to be a wheat-rye 2R (2D) disomic substitution line. Testing with different B. graminis f. sp. tritici isolates and genetic analysis showed that the all-stage resistance (also called seedling resistance) of AL69 was conferred by the cataloged powdery mildew resistance gene Pm4b derived from Zimai 17, and its adult-plant resistance was derived from the alien chromosome 2R of Zhongsi 237, which was found to be different from the previously reported rye-derived Pm genes, including Pm7 on 2RL. In addition, AL69 showed improved spike number per plant, spike length, fertile spikelet number per spike, kernel number per spike, and grain yield per plant compared with its wheat parent Zimai 17. An elite line S251 combining powdery mildew resistance with excellent agronomic performance was selected from the progenies of AL69 and wheat cultivar Jimai 22. Therefore, AL69 has two types of resistance genes to powdery mildew and improved agronomic traits through pyramiding and thus can be used as a promising genetic stock for wheat breeding.

Transfer of the ph1b Deletion Chromosome 5B From Chinese Spring Wheat Into a Winter Wheat Line and Induction of Chromosome Rearrangements in Wheat-Aegilops biuncialis Hybrids

Effective utilization of genetic diversity in wild relatives to improve wheat requires recombination between wheat and alien chromosomes. However, this is suppressed by the Pairing homoeologous gene, Ph1, on the long arm of wheat chromosome 5B. A deletion mutant of the Ph1 locus (ph1b) has been used widely to induce homoeologous recombination in wheat × alien hybrids. However, the original ph1b mutation, developed in Chinese Spring (CS) background has poor agronomic performance. Hence, alien introgression lines are first backcrossed with adapted wheat genotypes and after this step, alien chromosome segments are introduced into breeding lines. In this work, the ph1b mutation was transferred from two CSph1b mutants into winter wheat line Mv9kr1. Homozygous genotypes Mv9kr1 ph1b/ph1b exhibited improved plant and spike morphology compared to Chinese Spring. Flow cytometric chromosome analysis confirmed reduced DNA content of the mutant 5B chromosome in both wheat genotype relative to the wild type chromosome. The ph1b mutation in the Mv9kr1 genotype allowed wheat-alien chromosome pairing in meiosis of Mv9kr1ph1b_K × Aegilops biuncialis F₁ hybrids, predominantly with the M^b-genome chromosomes of Aegilops relative to those of the U^b genome. High frequency of wheat-Aegilops chromosome interactions resulted in rearranged chromosomes identified in the new Mv9kr1ph1b × Ae. Biuncialis amphiploids, making these lines valuable sources for alien introgressions. The new Mv9kr1ph1b mutant genotype is a unique resource to support alien introgression breeding of hexaploid wheat.

Enlisting wild grass genes to combat nitrification in wheat farming: A nature-based solution

Globally, wheat farming is a major source of nitrogen pollution. Rapid generation of soil nitrates cause nitrogen leakage and damage ecosystems and human health. Here, we show the 3Ns^bS chromosome arm in wild grass (Leymus racemosus) that controls root nitrification inhibitor production can be transferred into elite wheat cultivars, without disrupting the elite agronomic features. Biological nitrification inhibition (BNI)–enabled wheats can improve soil ammonium levels by slowing down its oxidation and generate significant synergistic benefits from assimilating dual nitrogen forms and improving adaptation to low N systems. Deploying BNI-enabled wheat on a significant proportion of current global wheat area (ca. 225 M ha) could be a powerful nature-based solution for reducing N fertilizer use and nitrogen losses while maintaining productivity.

Active nitrifiers and rapid nitrification are major contributing factors to nitrogen losses in global wheat production. Suppressing nitrifier activity is an effective strategy to limit N losses from agriculture. Production and release of nitrification inhibitors from plant roots is termed “biological nitrification inhibition” (BNI). Here, we report the discovery of a chromosome region that controls BNI production in “wheat grass” Leymus racemosus (Lam.) Tzvelev, located on the short arm of the “Lr#3Ns^b” (Lr#n), which can be transferred to wheat as T3BL.3Ns^bS (denoted Lr#n-SA), where 3BS arm of chromosome 3B of wheat was replaced by 3Ns^bS of L. racemosus. We successfully introduced T3BL.3Ns^bS into the wheat cultivar “Chinese Spring” (CS-Lr#n-SA, referred to as “BNI-CS”), which resulted in the doubling of its BNI capacity. T3BL.3Ns^bS from BNI-CS was then transferred to several elite high-yielding hexaploid wheat cultivars, leading to near doubling of BNI production in “BNI-MUNAL” and “BNI-ROELFS.” Laboratory incubation studies with root-zone soil from field-grown BNI-MUNAL confirmed BNI trait expression, evident from suppression of soil nitrifier activity, reduced nitrification potential, and N₂O emissions. Changes in N metabolism included reductions in both leaf nitrate, nitrate reductase activity, and enhanced glutamine synthetase activity, indicating a shift toward ammonium nutrition. Nitrogen uptake from soil organic matter mineralization improved under low N conditions. Biomass production, grain yields, and N uptake were significantly higher in BNI-MUNAL across N treatments. Grain protein levels and breadmaking attributes were not negatively impacted. Wide use of BNI functions in wheat breeding may combat nitrification in high N input–intensive farming but also can improve adaptation to low N input marginal areas.

A robust KASP marker for selection of four pairs of linked leaf rust and stripe rust resistance genes introgressed on chromosome arm 5DS from different wheat genomes

Stripe rust and leaf rust are among the most devastating diseases of wheat, limiting its production globally. Wheat wild relatives harbour genetic diversity for new genes and alleles for all major wheat diseases. However, the use of this genetic variation from wild progenitor and non-progenitor species has been limited in the breeding programs. Reasons include limited recombination of donor and recipient genomes and the lack of tertiary gene pool markers. Here, we describe the development of a SNP based marker from the flow-sorted and sequenced Aegilops umbellulata chromosome 5U which can be used for marker assisted selection of four pair of alien leaf rust and stripe rust resistance genes. Lr57-Yr40_CAPS16 marker was reported earlier to be linked with alien leaf and stripe rust resistance genes introgressed on wheat chromosome 5DS. Due to its dominant nature and laborious to work with, a new SNP-based KASP marker, XTa5DS-2754099_kasp23, was developed from the same CAPS marker contig. XTa5DS-2754099_kasp23 was tested in Aegilops umbellulata, Ae. geniculata, Ae. peregrina and Ae. caudata derived alien introgression lines, which harbour four pairs of linked leaf and stripe rust genes; Lr76-Yr70, Lr57-Yr40, LrP- YrP, LrAc-YrAc, respectively. This KASP marker was found to be effective for the selection of the aforesaid four pairs of leaf rust and stripe rust resistance genes. Further, we tested and validated XTa5DS-2754099_kasp23 on commercial varieties and advanced breeding lines from four countries (India, Egypt, Australia and UK) including hexaploid and durum wheat. Our results provide evidence that KASP marker, XTa5DS-2754099_kasp23 can be used in marker-assisted selection of the four pairs of rust resistance alien genes in wheat breeding programmes.

Development and Cytomolecular Identification of Monosomic Alien Addition and Substitution Lines of Triticale (×Triticosecale Wittmack) With 2Sk Chromosome Conferring Leaf Rust Resistance Derived From Aegilops kotschyi Boiss

Alien chromosome introgression has become a valuable tool to broaden the genetic variability of crop plants via chromosome engineering. This study details the procedure to obtain monosomic addition and monosomic substitution lines of the triticale carrying 2S^k chromosome from Aegilops kotchyi Boiss., which harbors Lr54 + Yr37 leaf and stripe rust-resistant gene loci, respectively. Initially, A. kotschyi × Secale cereale artificial amphiploids (2n = 6x = 42 chromosomes, UUSSRR) were crossed with triticale cv. "Sekundo" (2n = 6x = 42, AABBRR) in order to obtain fertile offspring. Cyto-molecular analyses of five subsequent backcrossing generations revealed that 2S^k chromosome was preferentially transmitted. This allowed for the selection of monosomic 2S^k addition (MA2S^k) lines of triticale. Finally, the 2S^k(2R) substitution plants were obtained by crossing MA2S^k with the nullisomic (N2R) plants of triticale. The presence of 2S^k chromosome in subsequent generations of plants was evaluated using SSR markers linked to Lr54 + Yr37 loci. Disease evaluation of the monosomic 2S^k(2R) substitution plants for the reaction to leaf and stripe rust infection were carried out under controlled conditions in a growth chamber. The results showed significant improvement of leaf rust resistance severity of monosomic substitution plants compared with control ("Sekundo"). In contrast, the introgression of the Lr54 + Yr37 loci did not lead to improvement of stripe rust resistance. In summary, the creation of monosomic addition and monosomic substitution lines of triticale is the starting point for the precise and guided transfer of Lr54 + Yr37 loci. The results showed that the developed materials could be exploited for the development of triticale varieties with resistance to leaf rust.

Identification of an Elite Wheat-Rye T1RS·1BL Translocation Line Conferring High Resistance to Powdery Mildew and Stripe Rust

Wheat-rye T1RS·1BL translocations have been widely used worldwide in wheat production for multiple disease resistance and superior yield traits. However, many T1RS·1BL translocations have successively lost their resistance to pathogens due to the coevolution of pathogen virulence with host resistance. Because of the extensive variation in rye (Secale cereale L.) as a naturally cross-pollinating relative of wheat, it still has promise to widen the variation of 1RS and to fully realize its application value in wheat improvement. In the present study, the wheat-rye breeding line R2207 was characterized by comprehensive analyses using genomic in situ hybridization (GISH), multicolor fluorescence in situ hybridization with multiple probes, multicolor GISH, and molecular marker analysis, and then was proven to be a cytogenetically stable wheat-rye T1RS·1BL translocation line. Based on the disease responses to different isolates of powdery mildew and genetic analysis, R2207 appears to possess a novel variation for resistance, which was confirmed to be located on the rye chromosome arm 1RS. Line R2207 also exhibited high levels of resistance to stripe rust at both seedling and adult stages, as well as enhanced agronomic performance, so it has been transferred into a large number of commercial cultivars using an efficient 1RS-specific kompetitive allele specific PCR marker for marker-assisted selection.

Development of Novel Wheat-Rye Chromosome 4R Translocations and Assignment of Their Powdery Mildew Resistance

Rye (Secale cereale L.) is an important gene donor for wheat improvement because of its many valuable traits, especially disease resistance. Development of novel wheat-rye translocations with disease resistance can contribute to transferring resistance into common wheat. In a previous study, a wheat-rye T4BL·4RL and T7AS·4RS translocation line (WR41-1) was developed by distant hybridization, and it was speculated that its resistance to powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), was derived from rye based on pedigree analysis. To make accurate use of chromosome 4R in wheat improvement, a set of new 4R translocations involving different arm translocations (e.g., 4RS monosomic, 4RL monosomic, 4RL disomic, 4RS monosomic plus 4RL monosomic, 4RS monosomic plus 4RL disomic, and 4RS disomic plus 4RL disomic translocations) was developed from crosses with common wheat. Those translocations were characterized by genomic in situ hybridization and expressed sequence tag simple sequence repeat marker analysis. To confirm the source of powdery mildew resistance, the translocation plants were tested against Bgt isolate E09. The results indicated that all translocations with 4RL were resistant at all tested growth stages, whereas those with only 4RS translocation or no alien translocation were susceptible. This further indicated that the powdery mildew resistance of WR41-1 was derived from the alien chromosome arm 4RL. To effectively use 4RL resistance in wheat improvement, two competitive allele-specific PCR markers specific for chromosome arm 4RL were developed to detect the alien chromosome in the wheat genome. These new translocation lines with diagnostic markers can efficiently serve as important bridges for wheat improvement.

Novel molecular marker-assisted strategy for production of wheat-Leymus mollis chromosome addition lines

Developing wheat–alien chromosome introgression lines to improve bread wheat’s resistance to stresses, such as drought, salinity stress and diseases, requires reliable markers to identify and characterize the alien chromatins. Leymus mollis is a wild relative of bread wheat resistant to salinity and economically important diseases of wheat, but its genome sequence and cytological markers are not available. We devised a molecular marker-assisted strategy for L. mollis chromosome identification and applied it to produce 10 wheat–L. mollis chromosome addition lines. Using 47 L. racemosus genome polymorphic PCR markers and DArTseq genotyping, we distinguished the L. mollis chromosomes and differentiated disomic and monosomic lines by progeny test. DArTseq genotyping generated 14,530 L. mollis SNP markers and the chromosome-specific SNP markers were used to determine the homoeologous groups of L. mollis chromosomes in the addition lines. To validate the marker-based results, genomic in situ hybridization was applied to confirm the presence and cytological status of L. mollis chromosomes in the lines. This study demonstrates that adequate molecular markers allow the production and characterization of wheat–alien addition lines without in situ hybridization, which saves considerable time and effort.