A guide to barley mutants

BACKGROUND

Mutants have had a fundamental impact upon scientific and applied genetics. They have paved the way for the molecular and genomic era, and most of today's crop plants are derived from breeding programs involving mutagenic treatments.

RESULTS

Barley (Hordeum vulgare L.) is one of the most widely grown cereals in the world and has a long history as a crop plant. Barley breeding started more than 100 years ago and large breeding programs have collected and generated a wide range of natural and induced mutants, which often were deposited in genebanks around the world. In recent years, an increased interest in genetic diversity has brought many historic mutants into focus because the collections are regarded as valuable resources for understanding the genetic control of barley biology and barley breeding. The increased interest has been fueled also by recent advances in genomic research, which provided new tools and possibilities to analyze and reveal the genetic diversity of mutant collections.

CONCLUSION

Since detailed knowledge about phenotypic characters of the mutants is the key to success of genetic and genomic studies, we here provide a comprehensive description of mostly morphological barley mutants. The review is closely linked to the International Database for Barley Genes and Barley Genetic Stocks ( bgs.nordgen.org ) where further details and additional images of each mutant described in this review can be found.

Genomic Regions Influencing Seminal Root Traits in Barley

Water availability is a major limiting factor for crop production, making drought adaptation and its many component traits a desirable attribute of plant cultivars. Previous studies in cereal crops indicate that root traits expressed at early plant developmental stages, such as seminal root angle and root number, are associated with water extraction at different depths. Here, we conducted the first study to map seminal root traits in barley ( L.). Using a recently developed high-throughput phenotyping method, a panel of 30 barley genotypes and a doubled-haploid (DH) population (ND24260 × 'Flagship') comprising 330 lines genotyped with diversity array technology (DArT) markers were evaluated for seminal root angle (deviation from vertical) and root number under controlled environmental conditions. A high degree of phenotypic variation was observed in the panel of 30 genotypes: 13.5 to 82.2 and 3.6 to 6.9° for root angle and root number, respectively. A similar range was observed in the DH population: 16.4 to 70.5 and 3.6 to 6.5° for root angle and number, respectively. Seven quantitative trait loci (QTL) for seminal root traits (root angle, two QTL; root number, five QTL) were detected in the DH population. A major QTL influencing both root angle and root number (/) was positioned on chromosome 5HL. Across-species analysis identified 10 common genes underlying root trait QTL in barley, wheat ( L.), and sorghum [ (L.) Moench]. Here, we provide insight into seminal root phenotypes and provide a first look at the genetics controlling these traits in barley.

Induced variations in brassinosteroid genes define barley height and sturdiness, and expand the green revolution genetic toolkit

Reduced plant height and culm robustness are quantitative characteristics important for assuring cereal crop yield and quality under adverse weather conditions. A very limited number of short-culm mutant alleles were introduced into commercial crop cultivars during the Green Revolution. We identified phenotypic traits, including sturdy culm, specific for deficiencies in brassinosteroid biosynthesis and signaling in semidwarf mutants of barley (Hordeum vulgare). This set of characteristic traits was explored to perform a phenotypic screen of near-isogenic short-culm mutant lines from the brachytic, breviaristatum, dense spike, erectoides, semibrachytic, semidwarf, and slender dwarf mutant groups. In silico mapping of brassinosteroid-related genes in the barley genome in combination with sequencing of barley mutant lines assigned more than 20 historic mutants to three brassinosteroid-biosynthesis genes (BRASSINOSTEROID-6-OXIDASE, CONSTITUTIVE PHOTOMORPHOGENIC DWARF, and DIMINUTO) and one brassinosteroid-signaling gene (BRASSINOSTEROID-INSENSITIVE1 [HvBRI1]). Analyses of F2 and M2 populations, allelic crosses, and modeling of nonsynonymous amino acid exchanges in protein crystal structures gave a further understanding of the control of barley plant architecture and sturdiness by brassinosteroid-related genes. Alternatives to the widely used but highly temperature-sensitive uzu1.a allele of HvBRI1 represent potential genetic building blocks for breeding strategies with sturdy and climate-tolerant barley cultivars.

Analysis of the barley bract suppression gene Trd1

A typical barley (Hordeum vulgare) floret consists of reproductive organs three stamens and a pistil, and non-reproductive organs-lodicules and two floral bracts, abaxial called 'lemma' and adaxial 'palea'. The floret is subtended by two additional bracts called outer or empty glumes. Together these organs form the basic structural unit of the grass inflorescence, a spikelet. There are commonly three spikelets at each rachis (floral stem of the barley spike) node, one central and two lateral spikelets. Rare naturally occurring or induced phenotypic variants that contain a third bract subtending the central spikelets have been described in barley. The gene responsible for this phenotype was called the THIRD OUTER GLUME1 (Trd1). The Trd1 mutants fail to suppress bract growth and as a result produce leaf-like structures that subtend each rachis node in the basal portion of the spike. Also, floral development at the collar is not always suppressed. In rice and maize, recessive mutations in NECK LEAF1 (Nl1) and TASSEL SHEATH1 (Tsh1) genes, respectively, have been shown to be responsible for orthologous phenotypes. Fine mapping of the trd1 phenotype in an F(3) recombinant population enabled us to position Trd1 on the long arm of chromosome 1H to a 10 cM region. We anchored this to a conserved syntenic region on rice chromosome Os05 and selected a set of candidate genes for validation by resequencing PCR amplicons from a series of independent mutant alleles. This analysis revealed that a GATA transcription factor, recently proposed to be Trd1, contained mutations in 10 out of 14 independent trd1 mutant alleles that would generate non-functional TRD1 proteins. Together with genetic linkage data, we confirm the identity of Trd1 as the GATA transcription factor ortholog of rice Nl1 and maize Tsh1 genes.

Origin of leaf rust adult plant resistance gene Rph20 in barley

Rph20 is the only reported, simply inherited gene conferring moderate to high levels of adult plant resistance (APR) to leaf rust ( Puccinia hordei Otth) in barley ( Hordeum vulgare L.). Key parental genotypes were examined to determine the origin of Rph20 in two-rowed barley. The Dutch cultivar ‘Vada’ (released in the 1950s) and parents, ‘Hordeum laevigatum’ and ‘Gull’ (‘Gold’), along with the related cultivar ‘Emir’ (a derivative of ‘Delta’), were assessed for APR to P. hordei in a disease screening nursery. The marker bPb-0837-PCR, co-located with Rph20 on the short arm of chromosome 5H (5HS), was used to screen genotypes for the resistance allele, Rph20.ai. Results from phenotypic assessment and DNA analysis confirmed that Rph20 originated from the landrace ‘H. laevigatum’ (i.e., Hordeum vulgare subsp. vulgare). Tracing back this gene through the pedigrees of two-rowed barley cultivars, indicated that Rph20 has contributed APR to P. hordei for more than 60 years. Although there have been no reports of an Rph20-virulent pathotype, the search for alternative sources of APR should continue to avoid widespread reliance upon a single resistance factor.

High resolution mapping of Dense spike-ar (dsp.ar) to the genetic centromere of barley chromosome 7H

Spike density in barley is under the control of several major genes, as documented previously by genetic analysis of a number of morphological mutants. One such class of mutants affects the rachis internode length leading to dense or compact spikes and the underlying genes were designated dense spike (dsp). We previously delimited two introgressed genomic segments on chromosome 3H (21 SNP loci, 35.5 cM) and 7H (17 SNP loci, 20.34 cM) in BW265, a BC(7)F(3) nearly isogenic line (NIL) of cv. Bowman as potentially containing the dense spike mutant locus dsp.ar, by genotyping 1,536 single nucleotide polymorphism (SNP) markers in both BW265 and its recurrent parent. Here, the gene was allocated by high-resolution bi-parental mapping to a 0.37 cM interval between markers SC57808 (Hv_SPL14)-CAPSK06413 residing on the short and long arm at the genetic centromere of chromosome 7H, respectively. This region putatively contains more than 800 genes as deduced by comparison with the collinear regions of barley, rice, sorghum and Brachypodium, Classical map-based isolation of the gene dsp.ar thus will be complicated due to the infavorable relationship of genetic to physical distances at the target locus.

High resolution mapping of Dense spike-ar (dsp.ar) to the genetic centromere of barley chromosome 7H

Spike density in barley is under the control of several major genes, as documented previously by genetic analysis of a number of morphological mutants. One such class of mutants affects the rachis internode length leading to dense or compact spikes and the underlying genes were designated dense spike (dsp). We previously delimited two introgressed genomic segments on chromosome 3H (21 SNP loci, 35.5 cM) and 7H (17 SNP loci, 20.34 cM) in BW265, a BC(7)F(3) nearly isogenic line (NIL) of cv. Bowman as potentially containing the dense spike mutant locus dsp.ar, by genotyping 1,536 single nucleotide polymorphism (SNP) markers in both BW265 and its recurrent parent. Here, the gene was allocated by high-resolution bi-parental mapping to a 0.37 cM interval between markers SC57808 (Hv_SPL14)-CAPSK06413 residing on the short and long arm at the genetic centromere of chromosome 7H, respectively. This region putatively contains more than 800 genes as deduced by comparison with the collinear regions of barley, rice, sorghum and Brachypodium, Classical map-based isolation of the gene dsp.ar thus will be complicated due to the infavorable relationship of genetic to physical distances at the target locus.

Mapping Rph20: a gene conferring adult plant resistance to Puccinia hordei in barley

A doubled haploid (DH) barley (Hordeum vulgare L.) population of 334 lines (ND24260 × Flagship) genotyped with DArT markers was used to map genes for adult plant resistance (APR) to leaf rust (Puccinia hordei Otth) under field conditions in Australia and Uruguay. The Australian barley cultivar Flagship carries an APR gene (qRphFlag) derived from the cultivar Vada. Association analysis and composite interval mapping identified two genes conferring APR in this DH population. qRphFlag was mapped to the short arm of chromosome 5H (5HS), accounting for 64-85% of the phenotypic variation across four field environments and 56% under controlled environmental conditions (CEC). A second quantitative trait locus (QTL) from ND24260 (qRphND) with smaller effect was mapped to chromosome 6HL. In the absence of qRphFlag, qRphND conferred only a low level of resistance. DH lines displaying the highest level of APR carried both genes. Sequence information for the critical DArT marker bPb-0837 (positioned at 21.2 cM on chromosome 5HS) was used to develop bPb-0837-PCR, a simple PCR-based marker for qRphFlag. The 245 bp fragment for bPb-0837-PCR was detected in a range of barley cultivars known to possess APR, which was consistent with previous tests of allelism, demonstrating that the qRphFlag resistant allele is common in leaf rust resistant cultivars derived from Vada and Emir. qRphFlag has been designated Rph20, the first gene conferring APR to P. hordei to be characterised in barley. The PCR marker will likely be effective in marker-assisted selection for Rph20.

Genetic dissection of barley morphology and development

Since the early 20th century, barley (Hordeum vulgare) has been a model for investigating the effects of physical and chemical mutagens and for exploring the potential of mutation breeding in crop improvement. As a consequence, extensive and well-characterized collections of morphological and developmental mutants have been assembled that represent a valuable resource for exploring a wide range of complex and fundamental biological processes. We constructed a collection of 881 backcrossed lines containing mutant alleles that induce a majority of the morphological and developmental variation described in this species. After genotyping these lines with up to 3,072 single nucleotide polymorphisms, comparison to their recurrent parent defined the genetic location of 426 mutant alleles to chromosomal segments, each representing on average <3% of the barley genetic map. We show how the gene content in these segments can be predicted through conservation of synteny with model cereal genomes, providing a route to rapid gene identification.

Use of a subset of doubled-haploid lines for RAPD interval mapping in barley

Molecular markers have been used in barley to locate genes and quantitative trait loci. Only a few RAPD markers have been located on barley marker maps. The objectives of this study were (i) to place RAPD markers in specific intervals on the barley linkage map developed from the cross Steptoe (S) × Morex (M), (ii) to examine the distribution of RAPD markers, and (iii) to compare markers amplified by Taq DNA polymerase with those amplified by the Stoffel fragment of Taq DNA polymerase. Screening of DNA from S and M with 362 decamer primers identified 85 that amplified 127 reliable RAPDs. A subset of 15 doubled-haploid (DH) lines from the 150 DH line mapping population was used to place these RAPD markers in intervals on the SM map. This subset can be used for rapid placement of any new markers on the SM linkage map. Most of the RAPD markers were dominant but four codominant RAPDs were identified. The RAPDs were not evenly distributed, with many clustered around the centromeric region of each chromosome. Two of these clusters were located in intervals larger than 15 cM. Testing of 38 to 42 additional DH lines provided more precise placement of eight of the markers in these clusters. Reliable RAPDs were detected with 44% of the primers tested with the Stoffel fragment, but with only 17% of the primers tested with Taq DNA polymerase. These RAPDs provide additional markers for use in barley improvement.Key words: Hordeum vulgare, interval mapping, molecular markers.

Quantitative trait locus effects and environmental interaction in a sample of North American barley germ plasm

Quantitative trait locus (QTL) and QTL x environment (E) interaction effects for agronomic and malting quality traits were measured using a 123-point linkage map and multi-environment phenotype data from an F1-derived doubled haploid population of barley (Hordeum vulgare). The QTL × E interactions were due to differences in magnitude of QTL effects. Highly significant QTL effects were found for all traits at multiple sites in the genome. Yield QTL peaks and support intervals often coincided with plant height and lodging QTL peaks and support intervals. QTL were detected in the vicinity of a previously mapped Mendelian maturity locus and known function probes forα- andβ-amylase genes. The average map density (9.6 cM) should be adequate for molecular marker-assisted selection, particularly since there were few cases of alternative favorable alleles for different traits mapping to the same or adjacent intervals.