R. Lance

GA-20 oxidase as a candidate for the semidwarf gene sdw1/denso in barley.

The barley sdw1/denso gene not only controls plant height but also yield and quality. The sdw1/denso gene was mapped to the long arm of chromosome 3H. Comparative genomic analysis revealed that the sdw1/denso gene was located in the syntenic region of the rice semidwarf gene sd1 on chromosome 1. The sd1 gene encodes a gibberellic acid (GA)-20 oxidase enzyme. The gene ortholog of rice sd1 was isolated from barley using polymerase chain reaction. The barley and rice genes showed a similar gene structure consisting of three exons and two introns. Both genes share 88.3% genomic sequence similarity and 89% amino acid sequence identity. A single nucleotide polymorphism was identified in intron 2 between barley varieties Baudin and AC Metcalfe with Baudin known to contain the denso semidwarf gene. The single nucleotide polymorphism (SNP) marker was mapped to chromosome 3H in a doubled haploid population of Baudin × AC Metcalfe with 178 DH lines. Quantitative trait locus analysis revealed that plant height cosegregated with the SNP. The sdw1/denso gene in barley is the most likely ortholog of the sd1 in rice. The result will facilitate understanding of the molecular mechanism controlling semidwarf phenotype and provide a diagnostic marker for selection of semidwarf gene in barley.

Expression level of a gibberellin 20-oxidase gene is associated with multiple agronomic and quality traits in barley

The use of dwarfing genes has resulted in the most significant improvements in yield and adaptation in cereal crops. The allelic dwarfing gene sdw1/denso has been used throughout the world to develop commercial barley varieties. The sdw1 gene has never been used successfully for malting barley, but only for a large number of feed varieties. One of the gibberellin 20-oxidase genes (Hv20ox2) was identified as the candidate gene for sdw1/denso. Semi-quantitative real-time RT-PCR revealed that Hv20ox2 was expressed at different levels in various organs of barley. Transcriptional levels were reduced in leaf blade, sheath, stem and rachis tissue in the barley variety Baudin with the denso gene. Subsequently, the relative expression levels of Hv20ox2 were determined by quantitative real-time RT-PCR in a doubled haploid population and mapped as a quantitative trait. A single expression quantitative trait locus (eQTL) was identified and mapped to its structural gene region on chromosome 3H. The eQTL was co-located with QTLs for yield, height, development score, hectolitre weight and grain plumpness. The expression level of Hv20ox2 was reduced fourfold in the denso mutant, but around 60-fold in the sdw1 mutant, compared to the control variety. The reduced expression level of Hv20ox2 enhanced grain yield by increasing the number of effective tillers, but had negative effects on grain and malting quality. The sdw1 gene can be used only in feed barley due to its severe reduction of Hv20ox2 expression. The gene expression marker for Hv20ox2 can be used to distinguish different alleles of sdw1/denso.

A nonsense mutation in a putative sulphate transporter gene results in low phytic acid in barley

Low phytic acid grains can provide a solution to dietary micronutrient deficiency and environmental pollution. A low phytic acid 1-1 (lpa1-1) barley mutant was identified using forward genetics and the mutant gene was mapped to chromosome 2HL. Comparative genomic analysis revealed that the lpa1-1 gene was located in the syntenic region of the rice Os-lpa-MH86-1 gene on chromosome 4. The gene ortholog of rice Os-lpa-MH86-1 (designated as HvST) was isolated from barley using polymerase chain reaction and mapped to chromosome 2HL in a doubled haploid population of Clipper×Sahara. The results demonstrate the collinearity between the rice Os-lpa-MH86-1 gene and the barley lpa1-1 region. Sequence analysis of HvST revealed a single base pair substitution (C→T transition) in the last exon of the gene in lpa1-1 (M422), which resulted in a nonsense mutation. These results will facilitate our understanding of the molecular mechanisms controlling the low phytic acid phenotype and assist in the development of a diagnostic marker for the selection of the lpa1-1 gene in barley.

RECENT ADVANCES IN BREEDING BARLEY FOR DROUGHT AND SALINE STRESS TOLERANCE

Barley is the most tolerance cereal crop for drought and salinity and is an ideal model crop for genetic study of drought and salinity tolerance because of its early maturity, diploid and self-pollination. Selection for drought tolerance in convention breeding programs has achieved significant progress to improve yield and yield stability under drought through direct selection or indirect selection for early vigour, coleoptile length or “stay green”. A large number of Quantitative Trait Loci (QTL) were mapped for drought and salinity tolerance related traits, including physiological biochemical traits such as osmotic adjustment capacity, proline content, stomatal conductance, water-soluble carbohydrates, relative water content, leaf turgor, ABA content, transpiration efficiency, water use efficiency and carbon isotope discrimination; and developmental/ morphological traits such as height, leaf emergence, leaf area index, tiller development, flowering time, maturity rate and root characteristics. QTLs for yield and yield components were also identified under drought. Extensive research has been devoted to the characterization of genes induced or up-regulated by drought or salinity. Numerous candidate genes were identified to associate with tolerance to drought or salinity and some of the candidate genes co-located with the QTLs for drought tolerance. Wild barley (Hordeum spontaneum) was demonstrated as a key genetic resource for drought and salinity tolerance. QTLs from the wild barley increased yield by 12–22% under drought. New germplasm and molecular tools make it possible to develop better barley variety faster for drought or salinity tolerance, but challenges still remain due to complexity of drought and salinity tolerance.

Seed dormancy in barley: identifying superior genotypes through incorporating epistatic interactions

A genetic linkage map of barley with 128 molecular markers was constructed using a doubled haploid (DH) mapping population derived from a cross between barley (Hordeum vulgare) cvv. Stirling and Harrington. Quantitative trait loci controlling seed dormancy were characterised in the population. A major quantitative trait locus (QTL) controlling seed dormancy and accounting for over half the phenotypic variation (52.17%) was identified on the distal end of the long arm of chromosome 5H. Minor QTLs were also detected near the centromeric region of 5H and on chromosomes 1H and 3H. These minor QTLs with additive effects accounted for 7.52% of the phenotypic variance measured. Examination of epistatic interactions further detected additional minor QTLs near the centromere of 2H and on the long arm and short arms of 4H. Combinations of parental alleles at the QTL locations in predictive analyses indicated dramatic differences in germination. These results emphasise the potential differences in dormancy that can be achieved through the use of specific gene combinations and highlights the importance of minor genes and the epistatic interactions that occur between them. This study found that the combination of Stirling alleles at the two QTL locations on the 5H chromosome and Harrington alleles at the 1H and 3H QTL locations significantly produced the greatest dormancy. Uncovering gene complexes controlling the trait may enable breeders to produce superior genotypes with the desirable allele combinations necessary for manipulating seed dormancy in barley.

Genes Controlling Low Phytic Acid in Plants: Identifying Targets for Barley Breeding

Phytic acid (myo-inositol 1, 2, 3, 4, 5, 6-hexakisphosphate) is the most abundant form of phosphorus in plant seeds. It is indigestible by both humans and nonruminant livestock and can contribute to human mineral deficiencies. The degradation of phytic acid in animal diets is necessary to overcome both environmental and nutritional issues. The development of plant cultivars with low phytic acid content is therefore an important priority. More than 25 low-phytic acid mutants have been developed in rice, maize, soybean, barley, wheat, and bean, from which 11 genes, belonging to six gene families, have been isolated and sequenced from maize, soybean, rice, and Arabidopsis. Forty-one members of the six gene families were identified in the rice genome sequence. A survey of genes coding for enzymes involved in the synthesis of phytic acid identified candidate genes for the six barley mutants with low phytic acid through comparison with syntenic regions in sequenced genomes.