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 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.

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 BC7F3 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.