Review: Amylopectin synthesis and hydrolysis – understanding isoamylase and limit dextrinase and their impact on starch structure on barley (Hordeum vulgare) quality

Background Starch contributes to barley grain and malt quality which in turn contributes to beer quality and flavour; through fermentable sugar profiles, rates of fermentation and Mallard reactions. Both amylopectin and amylose are enzymatically degraded to release maltose, maltotriose and higher order sugars. Scope and approach Amylopectin is highly branched [α-(1\ua0→\ua06) glycoside bond branch points] with numerous short branches while amylose is a long chained polymer with a few side branches. During grain development, the final level of branching is controlled by two enzymes namely; isoamylase and limit dextrinase (LD). Mutations in either of these genes can also result in changes to structure, content, and granule formation and size. During the malting free LD will to cleave the α-(1\ua0→\ua06) bonds but during mashing processes, bound LD is release, resulting in chains of various length available for other starch degrading enzymes to hydrolyse. Findings and conclusions While there is a good understanding of most of the individual aspects in amylopectin formation, structure and degradation; the story remains incomplete, as most of this understanding has been gained from experiments with only a limited number of barley varieties, limitations in the technology for structural measurement, and since no data is available to link structure to fermentable sugar profiles

Near infrared spectral assessment of stay-green barley genotypes under heat stress

This research aimed to correlate near infrared (NIR) spectral data to physiological and biochemical responses associated with stay-green (SG) traits expression in barley (Hordeum vulgare L.) plants experiencing heat stress. One hundred lines were randomly sub-sampledfrom a doubled haploid ND24260 ~ Flagship population consisting of 334 lines. A glasshouse trial with partial sample duplicationwas grown under terminal heat stress to induce SG expression during grain-fill. The ggreennessh of the first leaves under the flag leaf (FL.1) was assessed using NIR spectra. A handheld NIR spectrometer was used to understand and describe some of the physiological and biochemical mechanisms and responses related to SG expression and vice versa which cannot be observed using visual assessments. The use of NIR spectroscopy made it possible both to differentiate between cosmetic (changes in pigments with senescence of spike but no functional chlorophyll affects) and functional (effects on chlorophyll catabolism) SG expression and also to differentiate between the two groups of functional SG. The delayed onset and reduced rate of leaf senescence was linked to plant moisture (water) and plant maturity, which is dependent on the level of SG expression. Variance in the dominant water peak at 1450 nm in the NIR spectrum can be used to differentiate between cosmetic and functional SG expression. The spectral data from the leaves showed significant correlation, with protein (R2 = 0.62) and starch (R2 = 0.70) composition of the grain. The use of NIR spectroscopy allows for the rapid, non-destructive analysis of leaves; enabling multiple traits to be assessed with a single measurement. Understanding the relationship between spectra and SG expression may help develop NIR spectroscopy as a rapid, high-throughput methodology for phenotyping breeding populations, with a view to improve drought resistant barley cultivars

Drought-proofing barley (Hordeum vulgare): The effects of stay green on starch and amylose structure

The identification of the plant physiological trait called stay green (SG) was first identified in sorghum (Sorghum bicolor L. Moench), followed by other cereals, including barley. The effects of this drought tolerance trait on starch biosynthesis, structure, and properties have not been extensively investigated. Using size-exclusion chromatography, the impact of SG expression on starch molecular structure in barley (Hordeum vulgare L.) under heat- and water-stress conditions was examined. Differences were found in total starch and amylose contents within and between the treatments. The chain-length distribution of the amylose in a heat-stressed doubled haploid, ND24260 × Flagship population expressing SG showed significant differences (P < 0.05), whereas no such differences were observed in the water-stressed samples. However, significant differences (P < 0.05) in protein content were observed corresponding to SG expression, with higher levels of SG expression having higher protein content. These differences in composition and starch structure could influence functional properties. Understanding physiological responses in plants to abiotic stress and its impact on grain quality and starch biosynthesis may allow for the future manipulation of plants to improve drought tolerance, while maintaining desirable grain quality and yield potential

Discovery of QTL for stay-green and heat-stress in barley (Hordeum vulgare) grown under simulated abiotic stress conditions

This study maps genomic regions associated with terminal heat- and drought-stress tolerance in barley (Hordeum vulgare L.). One hundred lines were randomly sampled from a ND24260\ua0×\ua0Flagship doubled haploid population and evaluated for stay-green (SG) expression. SG expression including that of parental controls and commercial check varieties was evaluated in two controlled environments; one simulating terminal heat-stress, the other terminal water-stress. During grain-fill the greenness of the spikes (S), flag leaf (FL) and the first leaf under the flag leaf (FL-1) were phenotyped; visually (using a 0–9 scale) and via single-photon avalanche diode measurements. From the visual assessments, the green leaf area of the plant was determined, by using the difference in green area of the S and FL. Composite interval mapping detected 10 quantitative trait loci (QTL) for SG, positioned on chromosomes 3H, 4H, 5H, 6H and 7H; six of which were associated with terminal heat-stress and four with terminal water-stress. None were co-located with previously reported barley stress-response QTL and thus represent novel barley QTL. Although novel, some SG QTL mapped near chromosomal regions previously reported; such as the two heat-stress QTL mapped to bPb-5529 on 5H, adjacent to QTL reported for root length and root-shoot ratio. Detection of SG QTL in barley grown under simulated heat- and water-stressed conditions offers the potential of high through put screening for these traits. If confirmed in field trials, these genomic regions will be candidates for barley breeding programs targeting improved abiotic stress tolerance via marker-assisted selection

Barley genotype expressing "stay-green"-like characteristics maintains starch quality of the grain during water stress condition

The identification of "stay-green" in sorghum and its positive correlation with yield increases has encouraged attempts to incorporate "stay-green"-like traits into the genomes of other commercially important cereal crops. However, knowledge on the effects of "stay-green" expression on grain quality under extreme physiological stress is limited. This study examines impacts of "stay-green"-like expression on starch biosynthesis in barley (Hordeum vulgare L.) grain under mild, severe, and acute water stress conditions induced at anthesis. The proportions of long amylopectin branches and amylose branches in the grain of Flagship (a cultivar without "stay-green"-like characteristics) were higher at low water stress, suggesting that water stress affects starch biosynthesis in grain, probably due to early termination of grain fill. The changes in long branches can affect starch properties, such as the rates of enzymatic degradation, and hence its nutritional value. By contrast, grain from the "stay-green"-like cultivar (ND24260) did not show variation in starch molecular structure under the different water stress levels. The results indicate that the cultivar with "stay-green"-like traits has a greater potential to maintain starch biosynthesis and quality in grain during drought conditions, making the "stay-green"-like traits potentially useful in ensuring food security

Barley genotype expressing "stay-green"-like characteristics maintains starch quality of the grain during water stress condition

The identification of "stay-green" in sorghum and its positive correlation with yield increases has encouraged attempts to incorporate "stay-green"-like traits into the genomes of other commercially important cereal crops. However, knowledge on the effects of "stay-green" expression on grain quality under extreme physiological stress is limited. This study examines impacts of "stay-green"-like expression on starch biosynthesis in barley (Hordeum vulgare L.) grain under mild, severe, and acute water stress conditions induced at anthesis. The proportions of long amylopectin branches and amylose branches in the grain of Flagship (a cultivar without "stay-green"-like characteristics) were higher at low water stress, suggesting that water stress affects starch biosynthesis in grain, probably due to early termination of grain fill. The changes in long branches can affect starch properties, such as the rates of enzymatic degradation, and hence its nutritional value. By contrast, grain from the "stay-green"-like cultivar (ND24260) did not show variation in starch molecular structure under the different water stress levels. The results indicate that the cultivar with "stay-green"-like traits has a greater potential to maintain starch biosynthesis and quality in grain during drought conditions, making the "stay-green"-like traits potentially useful in ensuring food security

QTL associated with barley (Hordeum vulgare) feed quality traits measured through in situ digestion

Barley (Hordeum vulgare) is a major feed source for the intensive livestock industry. Competitiveness against other cereal grains depends largely on the price per unit of expressed feed quality. The traits which contribute to feed quality in barley are largely quantitative in nature but little is known about their genetic control. A study to identify the quantitative trait loci (QTLs) associated with feed quality was performed using a F6-derived recombinant inbred barley population. Samples from each line were incubated in the rumen of fistulated cattle, recovered, washed and dried for determination of in situ dry matter digestibility. Additionally, both pre- and post-digestion samples were analysed to quantify the content of key quality components relating to acid detergent fibre, total starch and protein. The data was used to identify trait-associated QTLs. Genetic analysis identified significant QTLs on chromosomes 2H, 5H and 7H. Genetic markers linked to these QTL should provide an effective tool for the selection and improvement of feed barley in the future

Correlation between NIRS generated and chemically measured feed quality data in barley (Hordeum vulgare), and potential use in QTL analysis identification

A study was performed to investigate the value of near infrared reflectance spectroscopy (NIRS) as an alternate method to analytical techniques for identifying QTL associated with feed quality traits. Milled samples from an F6—derived recombinant inbred Tallon/Scarlett population were incubated inthe rumen of fistulated cattle, recovered, washed and dried to determine the in-situ dry matter digestibility(DMD). Both pre- and post-digestion samples were analysed using NIRS to quantify key quality components relating to acid detergent fibre, starch and protein. This phenotypic data was used to identify trait associated QTL and compare them to previously identified QTL. Though a number of genetic correlations were identified between the phenotypic data sets, the only correlation of most interest was between DMD and starch digested (r = -0.382). The significance of this genetic correlation was that the NIRS data set identified a putative QTL on chromosomes 7H (LOD = 3.3) associated with starch digested. A QTL for DMD occurred in the same region of chromosome 7H, with flanking markers fAG/CAT63 and bPb-0758. The significant correlation and identification of this putative QTL, highlights the potential of technologies like NIRS in QTL analysis

The effects of variable nitrogen application on barley starch structure under drought stress

Under well-watered conditions, agronomic yield increases have been observed to correlate with nitrogen supply. Thus there is a need for proper fertilizer regimes to increase both metabolic and regulatory processes during kernel development in cereal crops. However, the impact of varying levels of nitrogen application on starch biosynthesis, structure and properties in grain under drought stress is not well known. This study examines the impact of different nitrogen application rates, in conjunction with drought stress, on starch biosynthesis in barley (Hordeum vulgare L.) grain. The proportions of short amylopectin branches and long amylose branches in the grain of Fitzroy and Grout were higher under drought stress with high nitrogen. This suggests that starch biosynthesis was affected, probably owing to early termination of grain fill. These changes in the long branches can affect starch properties, such as the rates of enzymatic degradation, and hence fermentability and nutritional value. In contrast, the chain length distribution (CLD) of the debranched starch from the grain grown under favourable conditions (Hermitage) did not show the same level of qualitative variations among the nitrogen treatments. The similar CLDs between these grain samples suggest that starch biosynthesis was not negatively impacted by the different nitrogen applications. However, with the grain under drought stress conditions, the results indicate that starch biosynthesis and quality could be impacted by nitrogen application. This has the potential to give rise to beneficial structural changes that are useful for some value-added products