The genetics and transcriptional profiles of the cellulose synthase-like HvCslF gene family in barley

Abstract

Cellulose synthase-like CslF genes have been implicated in the biosynthesis of (1,3;1,4)-beta-d-glucans, which are major cell wall constituents in grasses and cereals. Seven CslF genes from barley (Hordeum vulgare) can be divided into two classes on the basis of intron-exon arrangements. Four of the HvCslF genes have been mapped to a single locus on barley chromosome 2H, in a region corresponding to a major quantitative trait locus for grain (1,3;1,4)-beta-d-glucan content. The other HvCslF genes map to chromosomes 1H, 5H, and 7H, and in two cases the genes are close to other quantitative trait loci for grain (1,3;1,4)-beta-d-glucan content. Spatial and temporal patterns of transcription of the seven genes have been defined through quantitative polymerase chain reaction. In developing barley coleoptiles HvCslF6 mRNA is most abundant. Transcript levels are maximal in 4- to 5-d coleoptiles, at a time when (1,3;1,4)-beta-d-glucan content of coleoptile cell walls also reaches maximal levels. In the starchy endosperm of developing grain, HvCslF6 and HvCslF9 transcripts predominate. Two peaks of transcription are apparent. One occurs just after endosperm cellularization, 4 to 8 d after pollination, while the second occurs much later in grain development, more than 20 d after pollination. Marked varietal differences in transcription of the HvCslF genes are observed during endosperm development. Given the commercial importance of cereal (1,3;1,4)-beta-d-glucans in human nutrition, in stock feed, and in malting and brewing, the observation that only two genes, HvCslF6 and HvCslF9, are transcribed at high levels in developing grain is of potential relevance for the future manipulation of grain (1,3;1,4)-beta-d-glucan levels.

Figure 1.

An unrooted phylogenetic tree of CslF genes from barley (HvCslF) and rice (OsCslF). Amino acid sequence identities were used to identify potential orthologs between the gene families of barley and rice. The rice gene numbers are as follows: OsCslF1, Os07g36700; OsCslF2, Os07g36690; OsCslF3, Os07g36750; OsCslF4, Os07g36740; OsCslF6, Os08g06380; OsCslF7, Os10g20260; OsCslF8, Os07g36630; OsCslF9, Os07g36610. The OsCslF5 gene (Os07g36680) is not included because it is a pseudogene and could not be easily aligned with the other CslF genes.

Figure 2.

Structures of the barley (yellow) and rice (green) CslF genes. The positions of introns are indicated by the triangles and the length of the introns in base pairs is indicated within each triangle. The red bars show the positions of the D,D,D,QVRRW motifs, which are believed to be involved in sugar nucleotide binding and catalytic activity of the enzymes. The lengths of the encoded proteins range from 810 to 947 amino acid residues. The blue bars in the HvCslF3 gene are indicative of the positions of sequences encoding trans-membrane helices, which are located (but not shown here) in similar positions in all the barley and rice genes. The gray bar in the HvCslF6 gene indicates a position encoding a loop of 54 amino acid residues in the HvCslF6 protein that is only 15 to 20 residues in the other barley HvCslF proteins.

Figure 3.

Genetic maps of barley chromosomes 1H, 2H, 5H, and 7H showing the positions of barley HvCslF genes as mapped in a ‘Clipper’ × ‘Sahara’ population. Additional loci are shown to provide a genetic context for the map positions of the HvCslF genes. Approximate positions of the genes HvGlb1 and HvGlb2 are shown, based on mapping in other populations. Approximate positions of previously reported QTLs for grain (1,3;1,4)-β-d-glucan content are shown (A–G), based on alignment of the ‘Clipper’ × ‘Sahara’ map with genetic maps from other populations. Graphs A and B show statistically significant test statistics (log of the odds) values reported by Han et al. (1995) based on mapping in a ‘Steptoe’ × ‘Morex’ population. The letters C, D, and E show the approximate locations of markers at which Molina-Cano et al. (2007) detected QTLs in a ‘Beka’ × ‘Logan’ population, F shows the approximate location of the peak QTL test statistic value reported by Igartua et al. (2002) based on mapping in a ‘Derkado’ × ‘B83-12/21/5’ population, and G shows the approximate location of a marker at which Kim et al. (2004) detected marker-trait association in a ‘Yonezawa Mochi’ × ‘Neulssalbori’ population.

Figure 4.

A, Normalized levels of HvCslF6 gene transcripts (arbitrary units) in a range of tissues. These transcripts were detected in all tissues examined and very high levels of HvCslF6 mRNA were detected in some tissues. Error bars on all Q-PCR plots indicate sds for each mRNA. B, Levels of HvCslF gene transcripts in the different tissues. The values for HvCslF6 mRNA have been omitted to more clearly show the relative abundance of other mRNAs from the gene family. C, Levels of the relatively low abundance HvCslF7 mRNA transcripts in the series of tissues. D, Normalized transcript levels of the seven barley HvCslF genes in a range of tissues, expressed as a ratio of maximal levels observed for the specific gene. Values of 1.0 therefore indicate the tissue in which a particular gene is transcribed at its maximal rate.

Figure 5.

Normalized expression levels for the barley HvCslF genes in developing coleoptiles at various times (days) after the initiation of germination.

Figure 6.

A, Normalized transcript levels of the barley HvCslF9 genes in developing endosperm at various times postpollination. The two data sets are for the variety ‘Sloop’ (SLP) and ‘Himalaya’ (HIM). B, Levels of the HvCslF6 gene transcripts in developing endosperm at various times postpollination. The two data sets are for the variety ‘Sloop’ (SLP) and ‘Himalaya’ (HIM). C, Levels of HvCslF mRNA for genes that are transcribed at lower levels in the developing endosperm. The two data sets are for the variety ‘Sloop’ (SLP) and ‘Himalaya’ (HIM). D, Levels of HvGlb1 mRNA transcripts in developing endosperm at various times postpollination. The HvGlb1 gene encodes barley (1,3;1,4)-β-d-glucan endohydrolase isoenzyme EI. The two data sets are for the variety ‘Sloop’ (SLP) and ‘Himalaya’ (HIM).