Modifying thioredoxin expression in cereals leads to improved pre-harvest sprouting resistance and changes in other grain properties

Thioredoxins (Trxs) are widely distributed, small proteins that function in redox regulation in a broad spectrum of cellular reactions. Experimental work with barley, wheat and a legume (Medicago truncatula) has established thioredoxin h (Trx h) as a central regulatory protein in seeds, reducing disulphide (S-S) groups of diverse seed proteins, including storage proteins, enzymes and enzyme inhibitors. Trxs appear to be particularly important in plants, as a large number of genes are present compared to mammalian organisms. In Arabidopsis thaliana there are approximately 20 different genes for classical Trxs, and large Trx gene families have also been found in cereals, such as rice, barley, wheat and sorghum. Extensive evidence indicates that adding Trx, reduced nicotinamide adenine dinucleotide phosphate (NADPH) and NADP-thioredoxin reductase (NTR) to cereal flour or seed preparations reduces disulphide (S-S) linkages of storage proteins. The early in vitro studies have been complemented with transgenic barley seed, overexpressing Trx h in protein bodies of the barley endosperm, which showed accelerated germination and early or enhanced expression of associated enzymes, i. e., alpha-amylase and pullulanase. Overexpression of Trx h levels in wheat was subsequently shown to (1) enhance protein solubility and digestibility, (2) reduce allergenicity of wheat gliadins, and (3) improve dough quality from poor-quality wheat flour. Most recently, we have demonstrated that down-regulation in wheat of Trx h9, a unique thioredoxin, leads to a reduction in the incidence of pre-harvest sprouting, demonstrated in several varieties over multiple generations with field-grown material. Yield and starch content were increased while baking quality in the high-gluten variety remained unchanged. These observations led to the intriguing question of how changes in the endosperm are communicated to the embryo. Studies of Trx h9, a membrane-associated Trx h that can move from cell to cell, provide suggestive evidence for a role of Trx h9 in intercellular communication of redox state.