Sept. 29, 2011

If drought is often a challenge in your farming operation, you may be interested in new advances in drought tolerant corn and whether to include these hybrids in your crop rotation. This article describes

• the process currently used to create drought tolerant corn;
• why drought tolerance is a trait which is difficult to improve in comparison to herbicide resistance or insect resistance traits; and
• the limitations and benefits of drought tolerant hybrids.

Why Do Crops Need Water?

Crops need water to carry out processes necessary for survival. Water helps cool plants and transports dissolved nutrients throughout the plant and supports photosynthesis and plant growth.

The Nature of Drought Tolerant Traits and How They Affect Plant Processes

Why would you want multiple genes to improve drought tolerance in a corn variety? Drought tolerance is called a quantitative trait. Quantitative traits are heavily influenced by the environment and are controlled by large numbers of genes. Other traits, which are not influenced as much by environmental conditions and are controlled by only a few genes, are called qualitative traits. Examples of these are herbicide resistance and insect resistance. Quantitative traits are more difficult for breeders to improve due to the number of environmental factors influencing how the trait is expressed, coupled with needing to work with a larger number of genes.

Some of the environmental and genetic factors affecting drought tolerance include the timing and duration of water stress, soil type, heat, and humidity. All of these influence plant processes, such as closing of the stomata (openings in the plant leaf surface). The stomata allow carbon dioxide in to make sugars for plant growth and release water, increasing transpiration and protein production.

Genes related to drought stress can affect these and other plant processes differently. For example, one hybrid contains a trait which increases root depth and another which increases silking vigor. In the Corn Belt deeper roots mean the plant will find more available moisture deep in the soil profile. Improving silk vigor is important because prolific silk growth increases the likelihood of good pollination. Silks are 98% water by weight, which explains why drought can be so detrimental to silk development. More traits related to drought tolerance will offer multiple modes of action, such as these, in response to drought stress conditions.

Developing Drought Tolerance in Corn

Drought tolerant corn varieties have been developed using the “native gene” approach. This refers to using traditional plant breeding techniquzes, but with an added twist from using cutting edge molecular tools. In this manner, breeders conduct crossing experiments to determine the relative chromosome location of these native drought tolerant genes, and then use marker-assisted selection to help move along the breeding process more efficiently.The “native gene” approach has allowed breeders to bring in more than one gene affecting drought tolerance. In contrast, a transgenic/biotech approach (which involves genetic engineering) introduces a single new gene into corn from another organism.

How RNA Chaperones are Being Bred into Hybrids

Genes not present in any corn germplasm can be incorporated and then bred into elite crop lines through “native approaches.” These biotech-derived varieties are still in the industry development pipelines and not yet available to producers. Let’s take a look at one drought tolerant trait in the pipeline that uses RNA chaperones.

RNA chaperones are proteins that help produce and protect other proteins during times of stress. Although the specific mechanisms aren’t yet fully understood, in general, RNA chaperones help make sure that RNA molecules and critical plant proteins maintain their proper shape. Therefore, even under drought stress, critical proteins produced by the plant fold into their proper shape. If the shape of a protein is damaged, it cannot properly perform its function. During the reproductive growth stages, damaged proteins can lead to yield loss. RNA chaperones help stabilize yield even during drought conditions.

Precision Phenotyping

You may hear the term “precision phenotyping” as you research corn varieties for your operation. This term refers to a set of tools and designed experiments which help breeders determine which specific genes and/or traits contribute the most toward drought tolerance. Careful control of the environment and precise observations are necessary to accurately assess the drought tolerance levels of corn lines/varieties. In an effort to control the growing environment, fields with uniform soil conditions are selected. These fields also need to meet certain requirements in terms of latitude, altitude, and soil fertility. Researchers select low rainfall areas that would be able to grow good crops if water were present. Drip irrigation is typically used in to control the amount of water the plants receive.

The experiments must also be conducted at the correct time during the corn’s life cycle. Researchers are interested in what is happening during specific plant processes in specific tissues at a critical stage (such as flowering, grain fill, etc). As an example, UV (ultra-violet light) sensors and infrared sensors are often used to create images that allow researchers to “measure” the temperature of the plants. With UV light, the plants that appear purple in the picture tend to be cooler than those that are red or yellow.

Breeders also look for specific DNA markers in plants that exhibit drought tolerance. These markers are then used to plan specific crosses to “stack” multiple drought related traits into drought tolerant hybrids. This approach of carefully controlled environments, DNA work, and detailed field observation is called “precision phenotyping.”

Is Drought Tolerant Corn Right for Your Operation?

Even though drought tolerant corn offers some advantages, care should be taken to determine whether these particular hybrids are suitable for your farming operation. Drought tolerant corn does not offer a yield advantage over other hybrids under irrigated or well-watered conditions. Drought tolerant corn will not grow without water; it simply performs better than other hybrids under limited-water conditions.

It is also important to note drought tolerant corn varieties vary in the way they handle drought. Genetics affect different physiological processes involved in water use so no one variety will work well across all drought ridden environments. Currently, hybrids are best suited for dry land or fields with marginal soil and climate conditions for corn production (areas that typically yield 100-140 bu/ac or lower). This seed will be higher priced due to the new traits and its limited availability. Before purchasing these hybrids, carefully think through the environmental conditions in your particular field(s), look at the genetics offered by the hybrids, and weigh the cost versus the benefits they can provide to your operation.

Amy Lathrop, Distance Education Specialist, Department of Agronomy
Deana Namuth, Extension Plant Genetics Education and Outreach Specialist