I want to clarify that there are no transgenic or “GM” wheat varieties in the marketplace. The wheat varieties that you refer to are “genetically modified” through traditional plant breeding, and this question brings some important issues to the surface: the manner in which new wheat varieties are created and traits that serve as realistic targets. Singled out here is plant height, and whether or not changes in plant height may impact other characteristics of wheat.
First, let’s step back and briefly examine the fundamental process by which new wheat varieties are created by scientists employed either by land-grant universities or private companies. No matter where it occurs, today’s wheat varieties are the product of the painstaking process of crossing parents and selecting offspring, often called conventional breeding. To suggest anything else would be misrepresentative of the current science, and art, of wheat breeding, which truly began in the United States. in the 1920s. More specifically, parents with complementary traits are hybridized via natural fertilization to produce offspring with new genetic combinations (not new genes). New combinations are chosen that may lead to slightly higher yield potential, better resistance to diseases or insects, or perhaps better characteristics that enable such a wide variety of foods to be consumed from one plant species.
Quite often, parents are chosen from wheat’s ancient lineage to reintroduce genes indigenous to ancestral or related species of modern-day wheat. This method still employs crossing, not genetic engineering. However, if we think of the ancient relatives of modern-day wheat as genetic reservoirs, then at some point we will have exhausted those reservoirs as a means to combat ever-emerging diseases and insects, or to address other challenges our farmers may face. With that perspective, university and private-company scientists are committed to adopting the technologies we so desperately need to unleash or capture new genetic reservoirs critical to wheat’s stable production and healthy consumption. That is in our future.
But let’s stay focused on the past, as one characteristic or trait that has received worldwide attention in wheat breeding-research has been plant height. Wheat evolved and was domesticated in many sizes, and that diversity is readily reflected in ancient and modern wheat accessions maintained at the USDA National Small Grains Collection, varying from less than 12 inches to more than 60 inches. As a reference point, a healthy wheat plant growing in the U.S. Great Plains will average about 30 to 35 inches. In my own nurseries, I will look for progeny that are waist height or slightly shorter. Quite frankly, Oklahoma wheat farmers tell me, “Your varieties are too tall!” With greater height comes a higher center of gravity, which does not bode well when the wind comes sweeping down the plains. Farmers wish to have a wheat crop that is tall enough to swiftly harvest, but not so tall that the crop cannot remain standing through harvest. Wheat naturally has the ability to produce extremely short or “dwarf” types, but those would not be advantageous either to a mechanical harvest or to a bountiful yield.
Fortunately for wheat breeders, the wide genetic diversity for plant height, often associated inversely with straw strength, allows the development of varieties best fit for a certain environment. For example, in more arid environments, farmers prefer varieties with greater potential for height, whereas in those environments with greater rainfall, reduced height is preferred. This is true even within a given state, depending on rainfall distribution patterns.
In wheat, multiple genes confer varying degrees of height expression. In the scientific literature you will find certain height genes named, or numbered, with the prefix “Rht” for "Reduced height." The presence of certain Rht genes, or combinations thereof, sets the genetic range of height expression for a given variety. “Semidwarf” wheat varieties represent the predominant wheat type cultivated around the world. The height level mentioned above (30 to 35 inches) is common for semidwarf varieties grown in the Great Plains, though not all varieties grown in the Great Plains possess a known Rht gene.
The Rht genes which reside in many U.S. varieties today come with a lifesaving story, courtesy of the old 19th-century Japanese wheat variety, daruma, from which height reduction was transferred by conventional breeding in the 1950s by wheat breeders Orville Vogel and Norman Borlaug. Their research spawned a new era of wheat varieties that would eventually ignite the Green Revolution and save this planet from malnutrition, or worse yet, starvation (for more, see The Viking in the Wheat Field, by Susan Dworkin, Walker and Co., New York). Semidwarf varieties of both wheat and rice helped turn entire countries once starving for food into leading exporters of food. Semidwarf varieties can also contribute toward a sustainable agriculture. A wheat variety that expresses some degree of height reduction directs precious resources, such as water and fertilizer, primarily to that part of the plant consumed by humans, and not to the vegetative parts. I emphasized here the notion that height reduction in wheat is not “all-out.” Published research has shown that average plant height in the Great Plains has decreased from about 39 inches in 1960 (before semidwarf varieties emerged) to about 31 inches now.
Let’s now consider the relationship between plant height and nutritional content of the grain, specifically gluten, which as a storage protein provides a young wheat seedling with energy and as a grain protein provides us with essential amino acids for our own growth and development. Very simply, most of the wheat storage protein genes can be found on different chromosomes than the Rht genes. Thus the two traits are not inherited together, and, further, as already mentioned, many modern varieties of wheat grown today in the United States do not possess a Rht gene.
On a final note, starch and protein contents in wheat occur in inverse proportions, and thus, as one fraction increases on a percentage basis, the other decreases. Depending on the end use of wheat flour, some wheat varieties are higher in protein (and thus gluten), while others are lower in protein. The prevailing types of gluten molecules may vary depending on the end use. For flour used in confectionary products, a more mellow gluten and lower gluten content (more starch) are preferred, whereas for certain types of bread and pasta, a stronger gluten and higher gluten content are preferred. The desired amount and type of gluten in a wheat crop is determined not only by the variety but also by the environmental conditions (weather, soil, etc.) under which it is grown. With such inherent variability and fluctuation in the amount, or kind, of gluten that enters our daily diet, and with no scientific evidence that gluten content has systematically shifted with wheat breeding (Kasarda, D., J. Agric. Food Chem., 2013, 61:1155-1159), it is no surprise that any apparent increase in the incidence of celiac disease, or obesity rate, is unattributable to wheat breeding or to varieties with a certain level of height reduction. For a more complete and scientific examination of nutritional myths connected to wheat, see the article by nutrition specialist Dr. Julie Miller Jones in Cereal Foods World (57:177-189; 2012).