Abstract
Brachypodium distachyon is a small, fast growing grass species in the Pooideae subfamily that has become established as a model for other temperate cereals of agricultural significance, such as barley (Hordeum vulgare) and wheat (Triticum aestivum). The unusually high content in whole grains of β-D-(1,3;1,4)-glucan or mixed linkage glucan (MLG), considered a valuable dietary fibre due to its increased solubility in water compared with cellulose, makes B. distachyon an attractive model for these polysaccharides. The carbohydrate composition of grain in B. distachyon is interesting not only in understanding the synthesis of MLG, but more broadly in the mechanism(s) of carbon partitioning in cereal grains. Several mutants in the major MLG synthase, cellulose synthase like (CSL) F6, were identified in a screen of a TILLING population that show a loss of function in vitro. Surprisingly, loss of cslf6 synthase capacity appears to have a severe impact on survival, growth, and development in B. distachyon in contrast to equivalent mutants in barley and rice. One mutant, A656T, which showed milder growth impacts in heterozygotes shows a 21% (w/w) reduction in average grain MLG and more than doubling of starch compared with wildtype. The endosperm architecture of grains with the A656T mutation is altered, with a reduction in wall thickness and increased deposition of starch in larger granules than typical of wildtype B. distachyon. Together these changes demonstrate an alteration in the carbon storage of cslf6 mutant grains in response to reduced MLG synthase capacity and a possible cross-regulation with starch synthesis which should be a focus in future work in composition of these grains. The consequences of these findings for the use of B. distachyon as a model species for understanding MLG synthesis, and more broadly the implications for improving the nutritional value of cereal grains through alteration of soluble dietary fibre content are discussed.
Highlights
Brachypodium distachyon, a fast growing grass of small stature with a small diploid (271.9 Mbp) sequenced genome (The International Brachypodium Initiative, 2010), has emerged as a model species for other temperate grasses of agricultural importance
In order to identify a reduced mixed linkage glucan (MLG) variant of B. distachyon, a Targeted Induced Local Lesions IN Genomes (TILLING) mutant population described by Dalmais et al (2013) was screened for genetic lesions in CslF6 causing amino acid changes predicted to lead to loss of function
The MLG and other soluble non-starch polysaccharides are both considered components contributing to the increased soluble dietary fibre in the grain which have been shown to improve indices of general bowel health (Bird et al, 2008; McOrist et al, 2011) and result in lower plasma insulin levels (King et al, 2008) in humans
Summary
Brachypodium distachyon, a fast growing grass of small stature with a small diploid (271.9 Mbp) sequenced genome (The International Brachypodium Initiative, 2010), has emerged as a model species for other temperate grasses of agricultural importance. Whilst the cell walls of aerial tissues of seedlings are very similar between B. distachyon, wheat, and barley (Christensen et al, 2010), the endosperm walls of B. distachyon grain are thicker with greater deposition of polysaccharides (Guillon et al, 2011; Opanowicz et al, 2011; Trafford et al, 2013) This is largely due to the amount of the polysaccharide β-D-(1,3;1,4)-glucan or mixed linkage glucan (MLG), at much higher amounts per grain weight, upwards of four times that of close relatives including barley and wheat (Guillon et al, 2011; Opanowicz et al, 2011; Hands and Drea, 2012; Trafford et al, 2013). The converse is true for B. distachyon where most carbohydrate is in MLG, up to 45% (w/w), and the starch content is only 6% of grain weight (Guillon et al, 2011), suggestive of a role for MLG as an alternative carbon sink of readily hydrolysed sugars available to germinating grain (Guillon et al, 2011; Burton and Fincher, 2012; Trafford et al, 2013)
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