Grain yield and quality depend on complex physiological processes occurring during plant development. However, protein content is a major determinant of the feed value of cereal grains. The protein content of wheat can be increased by conventional breeding, but not without compromising yield, so genetic engineering approaches have been suggested as having some potential in this context (see also Chap. 11). In an attempt to channel greater amounts of major substrates of protein biosynthesis into the developing grain, the broad bean (Vicia faba) amino acid permease AAP1 gene and the barley sucrose transporter Sut1 gene were ectopically expressed in wheat caryopses. The grain of the resulting transgenic plants had a higher protein content under glasshouse conditions, and a field validation of this result is currently underway (Biosicherheit 2008).
Proteins which can enhance the availability of nutrients or facilitate the downstream processing of cereal grains are of particular interest. Xylans and glucans are major components of the cereal grain cell wall. Neither is readily digested by monogastric animals (such as poultry and pigs), because the monogastric intestine lacks the appropriate hydrolases (Bedford 1995). In addition, the high viscosity of solutions containing xylans and glucans inhibits the digestion and absorption of some of the other nutrients present, and so can result in a poor feed conversion ratio. A common solution to this problem is the supplementation of animal diets by the appropriate enzymes (Bedford 1995; Malathi and Devegowda 2001; Juanpere et al. 2004), but a transgenic approach, where the necessary recombinant hydrolases (glucanases, xylanases, phytases) are accumulated in the endosperm, is also possible (Jensen et al. 1996; Nuutila et al. 1999; Horvath et al. 2000; Patel et al. 2000; Xue et al. 2003). While those glucanases and xylanases can contribute to the digestibility of the grain, the availability of phosphorus, iron and zinc can be improved by the incorporation of a recombinant phytase from Aspergillus niger (Brinch-Pedersen et al. 2000).
Glucanases are also important for the malting and brewing processes. The ectopic expression of glucanases in the endosperm enhances not only the utilization of the glucans themselves, but also the accessibility of other grain constituents. Plant glucanases are irreversibly inactivated at temperatures above 55°C, which limits their relevance in conventional grain processing. Thus, thermo-tolerant enzymes obtained from fungi and bacteria are preferred for this application (Jensen et al. 1996; Nuutila et al. 1999). Kihara et al. (2000) ectopically expressed a thermostable b-amylase gene in barley, and this led to an improvement in kilning and mashing, as well as to increased fermentability later in the brewing process.
Baking quality in wheat depends strongly on the amount and composition of the endosperm storage proteins. Each of the high molecular weight glutenin subunit genes (Glu-1) encodes a pair of subunits, termed x and y. The gene encoding the A genome Glu-1 x subunit (1Ax1) was over-expressed in a wheat cultivar lacking this allele and, while mixing time, loaf volume and water absorbance were all improved in some of the transgenic lines, there were also substantial disturbances observed in the abundance of other storage protein fractions (Altpeter et al. 1996). Barro et al. (1997) undertook a similar analysis involving the D genome Glu-1 x and y subunits (respectively, 1Dx5 and 1Dy10) under the control of their own regulatory sequences. Some of the resulting transgenic lines had improved dough elasticity. Under field conditions, Shewry et al. (2006) were able to show that these transgenes were consistently expressed, without any negative effects on overall plant performance. Similar results have been obtained in both cereal rye (Wieser et al. 2005) and durum wheat (Gadaleta et al. 2008).
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