[show abstract][hide abstract] ABSTRACT: Elevated proportions of amylose in cereals are commonly associated with either the loss of starch branching or starch synthase activity. Goami 2 is a high amylose mutant of the temperate japonica rice variety Ilpumbyeo. Genotyping revealed that Goami 2 and Ilpumbyeo carry the same alleles for starch synthase IIa and granule-bound starch synthase I genes. Analyses of granule-bound proteins revealed that SSI and SSIIa accumulate inside the mature starch granules of Goami 2, which is similar to the amylose extender mutant IR36ae. However, unlike the amylose extender mutants, SBEIIb was still detectable inside the starch granules of Goami 2. Detection of SBEIIb after protein fractionation revealed that most of the SBEIIb in Goami 2 accumulates inside the starch granules while most of it accumulates at the granule surface in Ilpumbyeo. Exhaustive mass spectrometric characterisations of granule-bound proteins failed to detect any peptide sequence mutation or major post-translational modifications in Goami 2. Moreover, the signal peptide was found to be cleaved normally from the precursor protein and there is no apparent N-linked glycosylation. Lastly, no difference was found in the SBEIIb structural gene sequence of Goami 2 compared with Ilpumbyeo. In contrast, a G-to-A mutation was detected in the SBEIIb gene of IR36ae located at the splice site between exon and intron 11, which could potentially introduce a premature stop codon and produce a truncated form of SBEIIb. We suggest that the mutation responsible for producing high amylose in Goami 2 is not due to a defect in SBEIIb gene as was observed in IR36ae, even though it produces a phenotype analogous to the amylose extender mutation. Understanding the molecular genetic basis of this mutation will be important in identifying novel targets for increasing amylose and resistant starch contents in rice and other cereals.
Journal of Agricultural and Food Chemistry 09/2012; · 2.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: The awn, an apical extension from the lemma of the spikelet, plays important roles in seed dispersal, burial, and photosynthesis. Barley typically has long awns, but short-awn variants exist. The short awn 2 (lks2) gene, which produces awns about 50% shorter than normal, is a natural variant that is restricted to Eastern Asia. Positional cloning revealed that Lks2 encodes a SHI-family transcription factor. Allelism tests showed that lks2 is allelic to unbranched style 4 (ubs4) and breviaristatum-d (ari-d), for which the phenotypes are very short awn and sparse stigma hairs. The gene identity was validated by 25 mutant alleles with lesions in the Lks2 gene. Of these, 17 affected either or both conserved regions: the zinc-binding RING-finger motif and the IGGH domain. Lks2 is highly expressed in awns and pistils. Histological observations of longitudinal awn sections showed that the lks2 short-awn phenotype resulted from reduced cell number. Natural variants of lks2 were classified into three types, but all shared a single-nucleotide polymorphism (SNP) that causes a proline-to-leucine change at position 245 in the IGGH domain. All three lks2 natural variants were regarded as weak alleles because their awn and pistil phenotypes are mild compared with those of the 25 mutant alleles. Natural variants of lks2 found in the east of China and the Himalayas had considerably different sequences in the regions flanking the critical SNP, suggesting independent origins. The available results suggest that the lks2 allele might have a selective advantage in the adaptation of barley to high-precipitation areas of Eastern Asia.
Journal of Experimental Botany 07/2012; 63(14):5223-32. · 5.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: Mixed-linkage glucan (MLG) is a cell wall polysaccharide containing a backbone of unbranched (1,3)- and (1,4)-linked β-glucosyl residues. Based on its occurrence in plants and chemical characteristics, MLG has primarily been associated with the regulation of cell wall expansion due to its high and transient accumulation in young, expanding tissues. The Cellulose synthase-like F (CslF) subfamily of glycosyltransferases has previously been implicated in mediating the biosynthesis of this polymer. We confirmed that the rice (Oryza sativa) CslF6 gene mediates the biosynthesis of MLG by overexpressing it in Nicotiana benthamiana. Rice cslf6 knockout mutants show a slight decrease in height and stem diameter but otherwise grew normally during vegetative development. However, cslf6 mutants display a drastic decrease in MLG content (97% reduction in coleoptiles and virtually undetectable in other tissues). Immunodetection with an anti-MLG monoclonal antibody revealed that the coleoptiles and leaves retain trace amounts of MLG only in specific cell types such as sclerenchyma fibers. These results correlate with the absence of endogenous MLG synthase activity in mutant seedlings and 4-week-old sheaths. Mutant cell walls are weaker in mature stems but not seedlings, and more brittle in both stems and seedlings, compared to wild type. Mutants also display lesion mimic phenotypes in leaves, which correlates with enhanced defense-related gene expression and enhanced disease resistance. Taken together, our results underline a weaker role of MLG in cell expansion than previously thought, and highlight a structural role for MLG in nonexpanding, mature stem tissues in rice.
[show abstract][hide abstract] ABSTRACT: Arabinoxylan arabinofuranohydrolases (AXAHs) are family GH51 enzymes that have been implicated in the removal of arabinofuranosyl residues from the (1,4)-β-xylan backbone of heteroxylans. Five genes encoding barley AXAHs range in size from 4.6 kb to 7.1 kb and each contains 16 introns. The barley HvAXAH genes map to chromosomes 2H, 4H, and 5H. A small cluster of three HvAXAH genes is located on chromosome 4H and there is evidence for gene duplication and the presence of pseudogenes in barley. The cDNAs corresponding to barley and wheat AXAH genes were cloned, and transcript levels of the genes were profiled across a range of tissues at different developmental stages. Two HvAXAH cDNAs that were successfully expressed in Nicotiana benthamiana leaves exhibited similar activities against 4-nitrophenyl α-L-arabinofuranoside, but HvAXAH2 activity was significantly higher against wheat flour arabinoxylan, compared with HvAXAH1. HvAXAH2 also displayed activity against (1,5)-α-L-arabinopentaose and debranched arabinan. Western blotting with an anti-HvAXAH antibody was used to define further the locations of the AXAH enzymes in developing barley grain, where high levels were detected in the outer layers of the grain but little or no protein was detected in the endosperm. The chromosomal locations of the genes do not correspond to any previously identified genomic regions shown to influence heteroxylan structure. The data are therefore consistent with a role for AXAH in depolymerizing arabinoxylans in maternal tissues during grain development, but do not provide compelling evidence for a role in remodelling arabinoxylans during endosperm or coleoptile development in barley as previously proposed.
Journal of Experimental Botany 02/2012; 63(8):3031-45. · 5.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: (1,3;1,4)-β-D-glucans (mixed-linkage glucans) are found in tissues of members of the Poaceae (grasses), and are particularly high in barley (Hordeum vulgare) grains. The present study describes the isolation of three independent (1,3;1,4)-β-D-glucanless (betaglucanless; bgl) mutants of barley which completely lack (1,3;1,4)-β-D-glucan in all the tissues tested. The bgl phenotype cosegregates with the cellulose synthase like HvCslF6 gene on chromosome arm 7HL. Each of the bgl mutants has a single nucleotide substitution in the coding region of the HvCslF6 gene resulting in a change of a highly conserved amino acid residue of the HvCslF6 protein. Microsomal membranes isolated from developing endosperm of the bgl mutants lack detectable (1,3;1,4)-β-D-glucan synthase activity indicating that the HvCslF6 protein is inactive. This was confirmed by transient expression of the HvCslF6 cDNAs in Nicotiana benthamiana leaves. The wild-type HvCslF6 gene directed the synthesis of high levels of (1,3;1,4)-β-D-glucans, whereas the mutant HvCslF6 proteins completely lack the ability to synthesize (1,3;1,4)-β-D-glucans. The fine structure of the (1,3;1,4)-β-D-glucan produced in the tobacco leaf was also very different from that found in cereals having an extremely low DP3/DP4 ratio. These results demonstrate that, among the seven CslF and one CslH genes present in the barley genome, HvCslF6 has a unique role and is the key determinant controlling the biosynthesis of (1,3;1,4)-β-D-glucans. Natural allelic variation in the HvCslF6 gene was found predominantly within introns among 29 barley accessions studied. Genetic manipulation of the HvCslF6 gene could enable control of (1,3;1,4)-β-D-glucans in accordance with the purposes of use.
Journal of Experimental Botany 09/2011; 63(1):381-92. · 5.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this study of barley starch synthesis, the interaction between mutations at the sex6 locus and the amo1 locus has been characterized. Four barley genotypes, the wild type, sex6, amo1, and the amo1sex6 double mutant, were generated by backcrossing the sex6 mutation present in Himalaya292 into the amo1 'high amylose Glacier'. The wild type, amo1, and sex6 genotypes gave starch phenotypes consistent with previous studies. However, the amo1sex6 double mutant yielded an unexpected phenotype, a significant increase in starch content relative to the sex6 phenotype. Amylose content (as a percentage of starch) was not increased above the level observed for the sex6 mutation alone; however, on a per seed basis, grain from lines containing the amo1 mutation (amo1 mutants and amo1sex6 double mutants) synthesize significantly more amylose than the wild-type lines and sex6 mutants. The level of granule-bound starch synthase I (GBSSI) protein in starch granules is increased in lines containing the amo1 mutation (amo1 and amo1sex6). In the amo1 genotype, starch synthase I (SSI), SSIIa, starch branching enzyme IIa (SBEIIa), and SBEIIb also markedly increased in the starch granules. Genetic mapping studies indicate that the ssIIIa gene is tightly linked to the amo1 locus, and the SSIIIa protein from the amo1 mutant has a leucine to arginine residue substitution in a conserved domain. Zymogram analysis indicates that the amo1 phenotype is not a consequence of total loss of enzymatic activity although it remains possible that the amo1 phenotype is underpinned by a more subtle change. It is therefore proposed that amo1 may be a negative regulator of other genes of starch synthesis.
Journal of Experimental Botany 08/2011; 62(14):5217-31. · 5.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: The inactivation of starch branching IIb (SBEIIb) in rice is traditionally associated with elevated apparent amylose content, increased peak gelatinization temperature, and a decreased proportion of short amylopectin branches. To elucidate further the structural and functional role of this enzyme, the phenotypic effects of down-regulating SBEIIb expression in rice endosperm were characterized by artificial microRNA (amiRNA) and hairpin RNA (hp-RNA) gene silencing. The results showed that RNA silencing of SBEIIb expression in rice grains did not affect the expression of other major isoforms of starch branching enzymes or starch synthases. Structural analyses of debranched starch showed that the doubling of apparent amylose content was not due to an increase in the relative proportion of amylose chains but instead was due to significantly elevated levels of long amylopectin and intermediate chains. Rices altered by the amiRNA technique produced a more extreme starch phenotype than those modified using the hp-RNA technique, with a greater increase in the proportion of long amylopectin and intermediate chains. The more pronounced starch structural modifications produced in the amiRNA lines led to more severe alterations in starch granule morphology and crystallinity as well as digestibility of freshly cooked grains. The potential role of attenuating SBEIIb expression in generating starch with elevated levels of resistant starch and lower glycaemic index is discussed.
Journal of Experimental Botany 07/2011; 62(14):4927-41. · 5.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: Cell walls in commercially important cereals and grasses are characterized by the presence of (1,3;1,4)-β-d-glucans. These polysaccharides are beneficial constituents of human diets, where they can reduce the risk of hypercholesterolemia, type II diabetes, obesity and colorectal cancer. The biosynthesis of cell wall (1,3;1,4)-β-d-glucans in the Poaceae is mediated, in part at least, by the cellulose synthase-like CslF family of genes. Over-expression of the barley CslF6 gene under the control of an endosperm-specific oat globulin promoter results in increases of more than 80% in (1,3;1,4)-β-d-glucan content in grain of transgenic barley. Analyses of (1,3;1,4)-β-d-glucan fine structure indicate that individual CslF enzymes might direct the synthesis of (1,3;1,4)-β-d-glucans with different structures. When expression of the CslF6 transgene is driven by the Pro35S promoter, the transgenic lines have up to sixfold higher levels of (1,3;1,4)-β-d-glucan in leaves, but similar levels as controls in the grain. Some transgenic lines of Pro35S:CslF4 also show increased levels of (1,3;1,4)-β-d-glucans in grain, but not in leaves. Thus, the effects of CslF genes on (1,3;1,4)-β-d-glucan levels are dependent not only on the promoter used, but also on the specific member of the CslF gene family that is inserted into the transgenic barley lines. Altering (1,3;1,4)-β-d-glucan levels in grain and vegetative tissues will have potential applications in human health, where (1,3;1,4)-β-d-glucans contribute to dietary fibre, and in tailoring the composition of biomass cell walls for the production of bioethanol from cereal crop residues and grasses.
[show abstract][hide abstract] 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.