Rising CO2 concentration altered wheat grain proteome and flour rheological characteristics
Introduction
The current atmospheric carbon dioxide concentration ([CO2]) has reached the level of 400 μmol CO2 mol−1 and is predicted to be ±550 μmol CO2 mol−1 by the middle of the 21st century according to the Intergovernmental Panel on Climate Change (IPCC) under “mid-range” emission scenario A1B (Carter, Jones, & Lu, 2007). This will have a direct impact on the growth and development and yield formation of crops, particularly for C3 plants including wheat and rice. It is predicted that grain yield will increase by 15–17% under an atmospheric CO2 concentration (a[CO2]) of about 550 μmol CO2 mol−1 (Leakey et al., 2009). However, the positive influence of [CO2] on plant growth and grain yield is counteracted by inferior grain quality (Fernando et al., 2012b, Fernando et al., 2012a, Högy et al., 2009a).
Mostly, wheat is consumed after processing, therefore, end product quality is important (Shewry, 2009). Wheat end product quality is dependent on the functional properties of flour, which is mainly determined by grain protein concentration and composition (Shewry, 2009). Protein concentration in wheat grains varies from 8% to 20% and the major protein fractions can be classified into three main groups, namely structural, metabolic and storage proteins based on their functional characteristics (Shewry, Tatham, Forde, Kreis, & Miflin, 1986). Storage proteins are considered as gluten proteins, which form viscoelastic networks during dough mixing and are highly correlated with the rheological properties of wheat flour (Shewry, 2009).
According to the primary structure, gluten protein can be classified into three main groups, each group consisting of two or three protein types: high molecular weight (HMW) prolamins (consisting of x- and y- type HMW glutenin subunits), S-poor prolamins (comprises ω-gliadins) and S-rich prolamins (includes α-, β-, and γ- type gliadins and low molecular weight (LMW) glutenin subunits) (Shewry et al., 1986). Gliadins are mainly monomeric proteins that make up 35–45% of the total wheat protein (molecular weight range from 28 to 55 kD) and are soluble in aqueous alcohol. Glutenins are insoluble and are larger polymeric, aggregated proteins linked by disulphide bonds (Shewry & Halford, 2002) mainly associated with dough strength and extensibility (Wieser, Antes, & Seilmeier, 1998). Synthesis of gluten proteins (different fractions) is strongly influenced by genotype, growth conditions and fertilizer application (Wieser, Manderscheid, Erbs, & Weigel, 2008). It has been previously reported that elevated [CO2] (e[CO2]) also alters the wheat grain proteome under well watered, high yielding and temperate conditions (Högy, Zorb, Langenkamper, Betsche, & Fangmeier, 2009). However, there are no such data on the impact of rising CO2 on wheat grain protein quality under low rainfall Mediterranean climate conditions which cover a significant proportion of the global wheat growing areas (Braun, Rajaram, & Ginkel, 1996).
Limited water supply in rain-fed agriculture, common in many wheat growing regions, is predominant in the Mediterranean and semi-arid Australian wheat-belt. It was reported that larger reduction of grain protein concentration under e[CO2] in semi-arid conditions than under higher rainfall conditions (Fernando et al., 2012a, Fernando et al., 2012b).
Furthermore, Mediterranean and semi-arid growing conditions commonly coincide with high temperatures and drought episodes during the crop growing season (Nicolas, Gleadow, & Dalling, 1984), and such events are predicted to increase in the future (Mpelasoka, Hennessy, Jones, & Bates, 2008). Such dry and hot spells, if occurring during grain filling, can have a significant impact on grain protein composition (Panozzo & Eagles, 2000). Yet it has not been evaluated how rising [CO2] will affect wheat flour protein concentrations, protein quality and the associated changes in rheological characteristics in low rainfall areas under a Mediterranean climate. The current study was conducted in the AGFACE (Australian Grains Free Air Carbon Dioxide Enrichment) facility established in Horsham (Victoria, Australia) within the major wheat production area (Mollah, Norton, & Huzzey, 2009). This area receives only 250–300 mm of rainfall during the growing season, making it one of the driest grain FACE experimental locations in the world. Furthermore, in Mediterranean climate conditions, high temperatures (22–30 °C) often dominate during grain filling. In these experiments, the following hypotheses were tested: (1) e[CO2] will modify the wheat grain proteome under Mediterranean rain-fed conditions and (2) changes in wheat grain proteome lead to changes in the rheological properties of wheat flour.
Section snippets
Experimental conditions and CO2 exposure
The experiment was conducted under field conditions in the AGFACE (Australian Grains Free Air CO2 Enrichment) facility in Horsham, Victoria, Australia (36°45′07″S, 142°06′52″E; 128 m above sea level) during the 2009 growing season. Four 16 m diameter a[CO2] plots (“rings”, ±389 μmol CO2 mol−1) and four e[CO2] rings (±550 μmol CO2 mol−1) were used for this experiment. Wheat (Triticum aestivum L. cv. H45) was grown on subplots (4 m long × 1.8 m width) in each ring. Carbon dioxide concentration in the FACE
Wheat grain protein and proteome
Grain protein concentration on a dry weight basis, was reduced by 9% when plants were exposed to e[CO2] under FACE condition. However, the grain protein yield (grain protein concentration × grain yield/m2) was not different between the [CO2] treatments (Table 1). The comparative visual and software guided analysis of the representative 2-DE proteome profiles of the wheat grain results showed that 1188 protein spots could be reproducibly detected in the range of pH 5–8 at 8–18% gel gradient (Fig. 1
Discussion
These results indicate that rising [CO2] concentrations lower grain protein concentration by 9% for spring wheat when plants were grown in a Mediterranean climate (Table 1). Similar results have been reported in other environments such as for irrigated winter wheat in the US (Kimball, Morris, Pinter, Wall, Hunsaker, Adamsen, et al., 2001), Germany (Högy et al., 2009a, Högy et al., 2009b) and rain-fed semi-arid cropping systems in Australia (Fernando et al., 2012a, Fernando et al., 2012b).
Conclusions
This study shows that e[CO2] alters grain protein quality, most prominently reflected in decreases in the HMW-GS protein concentration. The reduction of HMW-GS concentration were associated with lower flour rheological properties and thus lower bread quality at e[CO2].
Acknowledgements
The Australian Grains Free Air CO2 Enrichment (AGFACE) facility is jointly run by the Victorian State Department of Primary Industries (DPI) and the University of Melbourne (UM). The authors gratefully acknowledge financial support by the Australian Commonwealth Department of Agriculture, Fisheries and Forestry (DAFF), The Grains Research and Development Corporation (GRDC), and the International Plant Nutrition Institute. Nimesha Fernando is supported by a Melbourne International Research
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