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Semidwarfing genes have greatly increased wheat yields globally, yet the widely used gibberellin (GA)-insensitive genes Rht-B1b and Rht-D1b have disadvantages for seedling emergence. Use of the GA-sensitive semidwarfing gene Rht13 avoids this pleiotropic effect. Here, we show that Rht13 encodes a nucleotide-binding site/leucine-rich repeat ( NB-LRR ) gene. A point mutation in the semidwarf Rht-B13b allele autoactivates the NB-LRR gene and causes a height reduction comparable with Rht-B1b and Rht-D1b in diverse genetic backgrounds. The autoactive Rht-B13b allele leads to transcriptional up-regulation of pathogenesis-related genes including class III peroxidases associated with cell wall remodeling. Rht13 represents a new class of reduced height ( Rht ) gene, unlike other Rht genes, which encode components of the GA signaling or metabolic pathways. This discovery opens avenues to use autoactive NB-LRR genes as semidwarfing genes in a range of crop species, and to apply Rht13 in wheat breeding programs using a perfect genetic marker.

Gibberellin (GA)-insensitive dwarfing genes Rht-B1b and Rht-D1b that are responsible for the 'Green Revolution' have been remarkably successful in wheat improvement globally. However, these alleles result in shorter coleoptiles and reduced vigour, and hence poor establishment and growth in some environments. Rht18, on the other hand, is a GA-sensitive, dominant gene with potential to overcome some of the early growth limitations associated with Rht-B1b and Rht-D1b. We assessed progeny from both a biparental and a backcross population that contained tall, single dwarf, and double dwarf lines, to determine whether Rht18 differs from Rht-D1b and hence verify its value in wheat improvement. Progeny with Rht18 had an almost identical height to lines with Rht-D1b, and both were ~26% shorter than the tall lines, with the double dwarf 13% shorter again. However, coleoptile length of Rht18 was 42% longer than that of Rht-D1b. We detected no differences in time to terminal spikelet and anthesis, and few differences in stem or spike growth. Both dwarfing genes diverted more dry matter to the spike than tall lines from prior to heading. No differences were detected between Rht18 and Rht-D1b that could prevent the adoption of Rht18 in wheat breeding to overcome some of the limitations associated with the 'Green Revolution' genes., (© The Author(s) 2020. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

The induced dwarf mutant Rht12 was previously shown to have agronomic potential to replace the conventional DELLA mutants Rht-B1b/Rht-D1b in wheat. The Rht12 dwarfing gene is not associated with reduced coleoptile length (unlike the DELLA mutants) and it is dominant, characteristics which are shared with the previously characterized dwarfing genes Rht18 and Rht14. Using the Rht18/Rht14 model, a gibberellin (GA) 2-oxidase gene was identified in the Rht12 region on chromosome 5A. A screen for suppressor mutants in the Rht12 background identified tall overgrowth individuals that were shown to contain loss-of-function mutations in GA2oxidaseA13, demonstrating the role of this gene in the Rht12 dwarf phenotype. It was concluded that Rht12, Rht18, and Rht14 share the same height-reducing mechanism through the increased expression of GA 2-oxidase genes. Some of the overgrowth mutants generated in this study were semi-dwarf and taller than the original Rht12 dwarf, providing breeders with new sources of agronomically useful dwarfism., (© The Author(s) 2020. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Fusarium head blight (FHB) and stem rust are among the most devastating diseases of wheat worldwide. Fhb1 is the most widely utilized and the only isolated gene for FHB resistance, while Sr2 is a durable stem rust resistance gene used in rust-prone areas. The two loci are closely linked on the short arm of chromosome 3B and the two genes are in repulsion phase among cultivars. With climate change and the shift in Fusarium populations, it is imperative to develop wheat cultivars resistant to both diseases. The present study was dedicated to developing wheat germplasm combining Fhb1 and Sr2 resistance alleles in the International Maize and Wheat Improvement Center (CIMMYT)'s elite cultivars' backgrounds. Four recombinant inbred lines (RILs) in Hartog background that have the resistant Fhb1 and Sr2 alleles in coupled phase linkage were crossed with seven CIMMYT bread wheat lines, resulting in 208 lines. Molecular markers for both genes were employed in addition to the use of pseudo-black chaff (PBC) as a phenotypic marker for the selection of Sr2 . At various stages of the selection process, progeny lines were assessed for FHB index, Fusarium damaged kernels (FDK), stem rust, and PBC expression as well as other diseases of interest (stripe rust and leaf spotting diseases). The 25 best lines were selected for CIMMYT's wheat breeding program. In addition to expressing resistance to FHB, most of these 25 lines have an acceptable level of resistance to other tested diseases. These lines will be useful for wheat breeding programs worldwide and potentially speed up the resistance breeding efforts against FHB and stem rust.

Stem rust is an important disease of wheat that can be controlled using resistance genes. The gene SuSr-D1 identified in cultivar 'Canthatch' suppresses stem rust resistance. SuSr-D1 mutants are resistant to several races of stem rust that are virulent on wild-type plants. Here we identify SuSr-D1 by sequencing flow-sorted chromosomes, mutagenesis, and map-based cloning. The gene encodes Med15, a subunit of the Mediator Complex, a conserved protein complex in eukaryotes that regulates expression of protein-coding genes. Nonsense mutations in Med15b.D result in expression of stem rust resistance. Time-course RNAseq analysis show a significant reduction or complete loss of differential gene expression at 24 h post inoculation in med15b.D mutants, suggesting that transcriptional reprogramming at this time point is not required for immunity to stem rust. Suppression is a common phenomenon and this study provides novel insight into suppression of rust resistance in wheat.

The wheat Sr2 locus confers partial resistance to four biotrophic pathogens: wheat stem rust ( Puccinia graminis f. sp. tritici ), leaf rust ( P. triticina ), stripe rust ( P. striiformis f. sp. tritici ), and powdery mildew ( Blumeria graminis f. sp. tritici ). In addition, Sr2 is linked with a brown coloration of ears and stems, termed pseudo-black chaff (PBC). PBC, initially believed to be elicited by stem rust infection, was subsequently recognized to occur in the absence of pathogen infection. The current study demonstrates that the resistance response to stem rust is associated with the death of photosynthetic cells around rust infection sites in the inoculated leaf sheath. Similarly, Sr2 -dependent resistance to powdery mildew was associated with the death of leaf mesophyll cells around mildew infection sites. We demonstrate that PBC occurring in the absence of pathogen inoculation also corresponds with death and the collapse of photosynthetic cells in the affected parts of stems and ears. In addition, Sr2 -dependent necrosis was inducible in leaves by application of petroleum jelly or by heat treatments. Thus, Sr2 was found to be associated with cell death, which could be triggered by either biotic or abiotic stresses. Our results suggest a role for the Sr2 locus in controlling cell death in response to stress.

Background: Good establishment is important for rapid leaf area development in wheat crops. Poor establishment results in fewer, later-emerging plants, reduced leaf area and tiller number. In addition, poorly established crops suffer from increased soil moisture loss through evaporation and greater competition from weeds while fewer spikes are produced which can reduce grain yield. By protecting the emerging first leaf, the coleoptile is critical for achieving good establishment, and its length and interaction with soil physical properties determine the ability of a cultivar to emerge from depth., Results: Here we characterise a locus on chromosome 1AS, that increases coleoptile length in wheat, which we designate as Lcol-A1. We identified Lcol-A1 by bulked-segregant analysis and used a Halberd-derived population to fine map the gene to a 2 cM region, equivalent to 7 Mb on the IWGSC genome reference sequence of Chinese Spring (RefSeqv1.0). By sowing recently released cultivars and near-isogenic lines in the field at both conventional and deep sowing depths, we confirmed that Locl-A1 was associated with increased emergence from depth in the presence and absence of conventional dwarfing genes. Flanking markers IWB58229 and IWA710 were developed to assist breeders to select for long coleoptile wheats., Conclusions: Increased coleoptile length is sought in many global wheat production areas to improve crop emergence. The identification of the gene Lcol-A1, together with tools to allow wheat breeders to track the gene, will enable improvements to be made for this important trait.

Semidwarfing genes have improved crop yield by reducing height, improving lodging resistance, and allowing plants to allocate more assimilates to grain growth. In wheat ( Triticum aestivum ), the Rht18 semidwarfing gene was identified and deployed in durum wheat before it was transferred into bread wheat, where it was shown to have agronomic potential. Rht18 , a dominant and gibberellin (GA) responsive mutant, is genetically and functionally distinct from the widely used GA-insensitive semidwarfing genes Rht-B1b and Rht-D1b In this study, the Rht18 gene was identified by mutagenizing the semidwarf durum cultivar Icaro ( Rht18 ) and generating mutants with a range of tall phenotypes. Isolating and sequencing chromosome 6A of these "overgrowth" mutants showed that they contained independent mutations in the coding region of GA2oxA9 GA2oxA9 is predicted to encode a GA 2-oxidase that metabolizes GA biosynthetic intermediates into inactive products, effectively reducing the amount of bioactive GA (GA 1 ). Functional analysis of the GA2oxA9 protein demonstrated that GA2oxA9 converts the intermediate GA 12 to the inactive metabolite GA 110 Furthermore, Rht18 showed higher expression of GA2oxA9 and lower GA content compared with its tall parent. These data indicate that the increased expression of GA2oxA9 in Rht18 results in a reduction of both bioactive GA content and plant height. This study describes a height-reducing mechanism that can generate new genetic diversity for semidwarfism in wheat by combining increased expression with mutations of specific amino acid residues in GA2oxA9 ., (© 2018 American Society of Plant Biologists. All Rights Reserved.)

Key Message: Many deletions of the wheat Della ( Rht - B1 ) gene and its flanking regions were isolated in a simple phenotypic screen, and characterised by modified analysis of SNP hybridisation data and cytogenetics. In a dwarf wheat suppressor screen, many tall 'revertants' were isolated following mutagenesis of a severely dwarfed (Rht-B1c) hexaploid wheat. About 150 lines were identified as putative deletions of Rht-B1c, based on the PCR analysis. Southern blot hybridisation established that most of them lacked the Rht-B1 gene, but retained the homoeologues Rht-A1 and Rht-D1. PCR assays were developed for orthologues of two genes that flank Rht-1/Della in the genomes of the model species Brachypodium and rice. Deletion of the B-genome-specific homoeologues of these two genes was confirmed in the Rht-B1 deletion lines, indicating loss of more than a single gene. SNP chip hybridisation analysis established the extents of deletion in these lines. Based on the synteny with Brachypodium chromosomes 1 and 4 g, and rice chromosomes 3g and 11g, notional deletion maps were established. The deletions ranged from interstitial deletions of 4BS through to loss of all 4BS markers. There were also instances, where all 4BS and 4BL markers were lost, and these lines had poor fertility and narrow stems and leaves. Cytogenetic studies on selected lines confirmed the loss of portions of 4BS in lines that lacked most or all 4BS markers. They also confirmed that lines lacking both 4BS and 4BL markers were nullisomics for 4B. These nested deletion lines share a common genetic background and will have applications in assigning markers to regions of 4BS as well as to 4BL. The potential for this type of analysis in other regions of the wheat genome is discussed.

Stem rust, caused by Puccinia graminis (Pgt), is a damaging disease of wheat that can be controlled by utilizing effective stem rust resistance genes. 'Thatcher' wheat carries complex resistance to stem rust that is enhanced in the presence of the resistance gene Lr34. The purpose of this study was to examine APR in 'Thatcher' and look for genetic interactions with Lr34. A RIL population was tested for stem rust resistance in field nurseries in Canada, USA, and Kenya. BSA was used to find SNP markers associated with reduced stem rust severity. A major QTL was identified on chromosome 3BL near the centromere in all environments. Seedling testing showed that Sr12 mapped to the same region as the QTL for APR. The SNP markers were physically mapped and the region carrying the resistance was searched for sequences with homology to members of the NB-LRR resistance gene family. SNP marker from one NB-LRR-like sequence, NB-LRR3 co-segregated with Sr12. Two additional populations, including one that lacked Lr34, were tested in field nurseries. NB-LRR3 mapped near the maximum LOD for reduction in stem rust severity in both populations. Lines from a population that segregated for Sr12 and Lr34 were tested for seedling Pgt biomass and infection type, as well as APR to field stem rust which showed an interaction between the genes. We concluded that Sr12, or a gene closely linked to Sr12, was responsible for 'Thatcher'-derived APR in several environments and this resistance was enhanced in the presence of Lr34.

As there are numerous pathogen species that cause disease and limit yields of crops, such as wheat (Triticum aestivum), single genes that provide resistance to multiple pathogens are valuable in crop improvement. The mechanistic basis of multi-pathogen resistance is largely unknown. Here we use comparative genomics, mutagenesis and transformation to isolate the wheat Lr67 gene, which confers partial resistance to all three wheat rust pathogen species and powdery mildew. The Lr67 resistance gene encodes a predicted hexose transporter (LR67res) that differs from the susceptible form of the same protein (LR67sus) by two amino acids that are conserved in orthologous hexose transporters. Sugar uptake assays show that LR67sus, and related proteins encoded by homeoalleles, function as high-affinity glucose transporters. LR67res exerts a dominant-negative effect through heterodimerization with these functional transporters to reduce glucose uptake. Alterations in hexose transport in infected leaves may explain its ability to reduce the growth of multiple biotrophic pathogen species.

Quantitative trait loci (QTLs) for shoot biomass were identified in wheat grown on a soil high in total phosphorus (P) but low in plant-available P. The two populations screened included recombinant inbred lines (RILs) from Chuan-Mai 18/Vigour 18 and doubled-haploid lines from Kukri/Janz. Glasshouse-grown plants were harvested at the five-leaf stage. Seven QTLs for shoot biomass were identified in the RILs, with the largest on chromosome 7A accounting for 7.4% of the phenotypic variance. RILs from the upper tail had larger embryos than RILs from the lower tail. Tail lines were then grown in non-limiting P and the results indicated that early vigour and the capacity to access P contributed to the initial distribution. The influence of early vigour on P nutrition was examined further with advanced vigour lines (AVLs). The AVLs accumulated more shoot biomass, maintained lower shoot P concentrations, and showed greater P-acquisition efficiency than Vigour 18. Nine QTLs for shoot biomass were identified in the Kukri/Janz population. Two on chromosomes 4B and 4D accounted for 24.8% of the variance. Candidates underlying these QTLs are the Rht genes. We confirmed the influence of these genes using near-isogenic lines with different Rht alleles. The dwarf and semi-dwarf alleles affected shoot and root biomass at high and low P but not the efficiency of P acquisition. We conclude that early vigour contributed to the distributions in both populations. Early vigour can increase plant growth at suboptimal P and some sources can also improve the efficiency of P acquisition., (© The Author 2015. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

We identify the wheat stem rust resistance gene Sr50 (using physical mapping, mutation and complementation) as homologous to barley Mla, encoding a coiled-coil nucleotide-binding leucine-rich repeat (CC-NB-LRR) protein. We show that Sr50 confers a unique resistance specificity different from Sr31 and other genes on rye chromosome 1RS, and is effective against the broadly virulent Ug99 race lineage. Extensive haplotype diversity at the rye Sr50 locus holds promise for mining effective resistance genes.