Seeking beneficial genetic diversity in the field

Breeding organic wheat to thrive across variety of situations.

By Kevin Matthew Murphy

Glossary Variety: A group of individuals within a species which are distinct in form or function from other similar individuals. Genotype: The genetic identity of an individual. Population: A community of individuals that share a common gene pool. Landraces: A farmer-developed variety of a crop plant that is genetically diverse, adapted to local environmental conditions, and often has its own local name. Pedigree breeding method: A system of breeding in which individual plants are selected from a cross on the basis of their individual desirability.

The unpredictable nature of climate change challenges the long-term viability of many of our established agricultural practices. The concept of food security and yield stability must be addressed with regards to all possible agronomic solutions to future climatic instability. Organic farming, one such solution, offers unique benefits to the agronomic problems associated with climate change.

Statistics for the United Kingdom (Great Britain and Ireland) show that, on average, U.K. organic farming is 26 percent more energy efficient per ton than conventional farming (Azeez et al, 2008), with organic wheat production estimated at 40 percent more energy efficient. A transition to organic farming in the U.K. would reduce fossil fuel energy use by about 20 percent, primarily due to the use of inorganic nitrogen fertilizer by conventional farmers (Azeez et al, 2008). This situation is strikingly similar in the U.S.

But it goes beyond energy efficiency of farming. Agronomic stability over potentially rapid and patternless environmental change should be a research focus for organic farming systems. Climate-change induced environmental shifts include drought, irregular rainfall and warmer seasonal temperatures. Basic organic farming practices designed to conserve soil, increase soil organic matter and improve microbial diversity to provide crop fertility have been shown to add stability to crop production during environmental fluctuations.

These stabilizing practices include green manures in crop rotations, diversified crop sequences, minimal tillage and the use of on-farm manure and composting (Niggli et al, 2008)). With many successful soil conservation and fertility practices in place, how can plant breeding further the capacity of organic farmers to withstand future inconsistent and unpredictable climatic patterns?

There is considerable whole-system diversity among organic farms, even within the same agro-environment. Breeding for adaptation to spatial and temporal variation is one of the key differences between breeding for organic and conventional conditions. Conventional farms are often rendered agronomically similar through the addition of substantial quantities of chemical fertilizers. Disease and insect pressures are more easily mitigated using crop protection chemicals. Rotations in conventional wheat farms are typically similar in a given area, whereas, rotations for organic wheat farms tend to vary significantly. This variation depends on whether the farmer’s primary goals are to: provide supplemental nitrogen and soil fertility using cover crops; suppress weeds through rotations; or prevent disease and insect problems.

Genetic diversity improves odds

Natural selection and crop evolution depend on having sufficient genetic diversity within a given population. A much greater degree of genetic diversity was prevalent in agriculture until the past 100 years when plant breeders and farmers began to exploit the agronomic and practical benefits of genetic uniformity (Wolfe et al, 2008). Genetic diversity in the field, on the other hand, has long been shown to have a buffering effect on year-to-year shifts in weather patterns.

The most common model of breeding self-pollinating cereal crops is to develop genetically uniform varieties that are highly productive under optimal agronomic conditions with high inputs of fertilizers and often crop protection chemicals. Most wheat breeders use some form of the pedigree method which results in a variety that meets the DUS (distinct, uniform and stable) seed-breeding standards. The DUS requirements often necessitate a genetically uniform variety. This genetic uniformity can be restrictive to the ability of the variety to adapt to environmental changes. When agronomic conditions shift, in the case of drought or a new race of pathogen, these varieties typically show a significant reduction in yield.

Breeding for sustainability is a process of fitting varieties to an environment instead of altering the environment (through the addition of fertilizer, water, pesticides, etc.) to fit varieties (Coffman and Smith, 1991). This is the same idea that we should use when breeding for climate change. Though more complex, because the environments are always shifting, the premise is the same. One alternative that should be considered is the use of genetic variation in the field to help address the ever-changing variation in our climate. There exist several different methods wheat breeders employ to harness genetic diversity to develop resilient varieties, including multi-lines, mixtures and evolutionary, or population, breeding.

Multi-lines and variety mixtures

Multi-lines are mixtures of several predominantly uniform breeding lines that usually differ from each other by a single gene. For example, the variety Rely, developed by USDA plant breeder and geneticist Bob Allan, is a mixture of 10 breeding lines all differing in a resistant gene for stripe rust, a common wheat disease. Due to its multi-gene resistance to stripe rust, Rely has been grown on significant acreage in Washington State for over 20 years.

The benefits of multi-lines are that they meet DUS standards while providing some level of genetic adaptability. In the dynamic polyface of climate change however, multi-lines will be limited in their ability to adapt to multiple simultaneous changes in the environment. A multi-line developed for resistance to shifts in stripe-rust races, for example, will not necessarily be suited for large changes in rainfall or temperature.

Mixtures of pure-line varieties often provide sufficient genetic diversity to help provide resistance to shifts in pest and pathogen populations, temperature fluctuations and changes in rainfall patterns and amounts. Mixtures in themselves do not typically evolve in response to natural selection, but rather provide a genetic buffer to the constant changes by allowing the most successful genetics seeded in the field to flourish.

In Washington State in 2008, mixtures in the soft white market class were grown on almost 200,000 acres, or approximately 14 percent of the total soft white acreage, according to the Washington Grain Alliance. Mixtures represent a practical and effective method for farmers to add a layer of stability in their production system.

One benefit of mixtures is that they can be constantly tinkered with and reformulated to best take advantage of environmental conditions, marketing opportunities and farming systems. Mixtures could be formulated using varieties that best suppress weeds, or varieties with a high nutritional value. And in light of climate change, mixtures can be developed to buffer extreme fluctuations in seasonal temperature, rainfall patterns, pathogen shifts and insect outbreaks.

Evolutionary breeding creates resiliency

In the 1920s and 30s, Harry Harlan, a barley breeder from the University of California (UC) began making composite cross populations between many diverse barley varieties from around the world. A composite cross population is developed by making crosses among all the different varieties chosen to each other (Harlan and Martini, 1929). For example, composite cross II is a population developed by Harlan in 1929 using 30 varieties crossed in all possible combinations.

Dr. Coit Sunesen

In an evolutionary breeding approach using these composite cross populations, Dr. Coit Sunesen and Dr. Robert Allard, cereal breeders from UC Davis, planted these populations each year. They grew them under standard agronomic conditions, and harvested them each fall. This pattern was repeated for over 50 years (Suneson 1956; Allard 1996). No human selection was conducted on these populations. Instead, they were allowed to respond to multiple environmental stresses and the resulting harvest often reflected the populations’ response to pressure from natural selection. These pressures include multiple diseases, prolonged droughts and extreme temperatures. Results from numerous studies on these populations have shown steady increases in grain yield and disease resistance over time and yield stability over diverse environmental conditions.

In contrast to mixtures, populations have much greater variation as typically every plant represents a distinct genotype. This abundant genetic variation in the field has the potential to adapt through natural selection to different and changing environmental conditions and thereby provide more stable performance across variable environments. Composite crosses that adapt to climatic conditions and farming systems have the potential to become “modern landraces” (Wolfe et al, 2008).

While fitness traits, particularly grain yield, will typically improve over time in response to natural selection, quality traits including milling and baking have the potential to be negatively affected through natural selection. Therefore, care must be taken when choosing the parents for population breeding. Populations usually reflect the mid-parent value for most quality traits (Murphy et al, 2008). Therefore, the higher quality parents used in a cross, the better quality the resulting population is likely to be, unless negative genetic correlations prove otherwise.

Bringing the farmer back into the breeding equation

Lexi Roach, a granddaughter of Kahlotus, Washington, farmer Jim Moore, first became interested in participatory wheat breeding in the fourth grade when WSU winter wheat breeder Stephen Jones gave a talk to her school science class. A few years later she came to the greenhouse at Washington State University and made crosses between two wheat varieties that did well on her farm. Roach and Moore, with help from Dr. Jones, wanted to develop evolutionary breeding populations on their farm.

Stephen Jones at WSU test plots.

Each year the three would plant the populations on their farms, let the wheat grow, observe the wheat and harvest the grain in the summer. Each year the process was repeated, in much the same manner as evolutionary composite cross populations were treated—with one key exception. The composite cross populations were left to the vagaries of natural selection alone. Roach’s populations had to pass an additional test – they had to meet her approval. Each summer Roach and her grandfather Jim would walk her wheat rows together, pulling out any plants with stripe-rust symptoms, signs of other diseases, or other negative growth traits.

Now in its sixth year, Roach has evolutionary participatory breeding populations that are not only adapted to the unique hot and dry climate, the deep sandy loess soil, and their low input system of farming, but also able to adapt to changes in climate in the future. The 2008 harvest was a good one for Roach and Moore; their population out-yielded all other varieties on their farm by approximately 5 bushels per acre.

This is the kind of integrated approach—farmer selection working with natural selection on a genetically diverse population—that will allow maximum adaptation to static characters (latitude, soil structure) and maximum resilience in the face of dynamic changes in the environment.

For a background interview with Dr. Stephen Jones on participatory wheat breeding with non-genetically modified techniques, click here.

Kevin Murphy, Ph.D., is currently doing post-doctoral work managing Dr. Stephen Jones’ organic wheat breeding program at Washington State University

References

1. Azeez, G & Hewlett, K (2008) The comparative energy efficiency of organic farming. 16th IFOAM Organic World Congress 2 562-565.

2. Niggli, U, Hepperly, P, Fliessbach, A & Mader, P (2008) Does organic farming have greater potential to adapt to climate change? 16th IFOAM Organic World Congress 2 586-589.

3. Wolfe, M, Baresel, J, Desclaux, D, Goldringer, I, Hoad, S, Kovacs, G, Loschenberger, F, Miedaner, T, Ostergard, H & Lammerts van Bueren, E (2008) Developments in breeding cereals for organic agriculture. Euphytica in press. doi: 10.1007/s10681-008-9690-9.

4. Coffman, WR & Smith, ME (1991) in Plant breeding and sustainable agriculture, considerations for objectives and methods, ed. DA Sleper, TBPB-C (CSSA Special Publication no. 18, pp. 1-9.

5. (2008) Washington Wheat Top Five Varieties & Acreage, by Class and Reporting District, 2008. http://wwwwashingtongrainalliancecom/images/E0177801/WheatVarietySurveypdf.

6. Harlan, HV & Martini, ML (1929) A composite hybid mixture. Journal of the American Society of Agronomy 21 407-409.

7. Suneson, CA (1956) An evolutionary plant breeding method. Agronomy Journal 48 188-191.

8. Allard, RW (1996) Genetic basis of the evolution of adaptedness in plants. Euphytica 92 1-11.

9. Murphy, K, Lammer, D, Lyon, S, Carter, B & Jones, SS (2005) Breeding for organic and low-input farming systems: An evolutionary–participatory breeding method for inbred cereal grains. Renewable Agriculture and Food Systems 20 48-55. doi:10.1079/RAF200486