Stress tolerant maize for the developing world – Challenges and prospects

The 2010 Maize Genetics Conference started with a call for maize geneticists to take on one of the greatest challenges of human history – feeding the world. Marianne Bänziger of CIMMYT presented the first plenary talk, titled Stress tolerant maize for the developing world – Challenges and prospects. Find the abstract of her talk at the end of this post.

Of all of the staple grains, maize is the most drought susceptible. Wheat is fairly drought tolerant, and rice is irrigated. Maize is sensitive to variation in rainfall, and since it is typically not irrigated, any year to year variation in rainfall will be seen as year to year variation of yield, with low rainfall years yielding less than high rainfall years. There are some drought tolerant varieties that don’t have such variation with rainfall, but they are consistently low yielding, even in high rainfall years. In order to provide enough food for growing populations, maize must be developed that can maintain reasonably high yields even in drought years.
A second major problem with maize is nitrogen. Maize reacts well to fertilizer application, providing (to a point) higher yields with higher amounts of nitrogen. However, less than 50% of applied fertilizer is used by the plant, leaving much of the nitrogen unused This unused fertilizer can be carried via surface waters to places like the Gulf of Mexico where it can contribute to hypoxic zones. Additionally, synthetic nitrogen fertilizer can be expensive to produce because it requires natural gas. Both synthetic nitrogen and non-synthetic fertilizers take fuel to distribute through fields. Maize that can use applied nitrogen more efficiently without laving so much behind must be developed both in order to provide enough food and to ensure that we are using both renewable and nonrenewable resources efficiently while protecting the environment.
CIMMYT aims to solve problems of drought and nitrogen by breeding under stress conditions. Their fields look more like a field in Africa than a field in Iowa. They simply select for stress tolerant plants that grow successfully under low water and low nitrogen conditions. They’ve found that genetic markers in typical yield selected lines and in stress selected lines are very different. CIMMYT is also looking at breeding under low phosphorus and low potassium.
While CIMMYT is primarily focused on breeding, they believe the key to meeting future food needs lies in matching breeding and transgenics. In particular, CIMMYT has partnered with seed companies to develop transgenics that will enhance productivity. The current traits on the market are protective: Bt protects the crop plants from insect damage which can reduce yield, Roundup Ready protects the crop plants from having to compete with weeds for resources, and virus resistance protects papaya from reduced yield due to virus infection. Productivity traits would directly increase yield instead of protecting it. Castigiloni showed in the 2008 paper Bacterial RNA chaperones confer abiotic stress tolerance in plants and improved grain yield in maize under water-limited conditions that yield could be significantly improved with a transgene.
Finding appropriate transgenes that will improve yield isn’t the end of the story. Each transgene needs to be investigated for genotype x environment interactions to see if the productivity transgenes behave differently under different environmental conditions. In addition, the transgenes may behave differently in different varieties, so each individual variety would need to be tested for yield changes with the productivity transgene. More layers of complication are added when multiple genes of similar and different traits are stacked. The combinations of transgenes may behave differently than each gene alone, and the combinations may have different interactions with each variety and environment.
CIMMYT has partnerships with Monsanto to work on water efficient maize for Africa (WEMA) and with Pioneer to work on improved maize for African soils (IMAS) which is also known as nitrogen use efficiency (NUE). These partnerships have many benefits. They can combine CIMMYT germplasm which is adapted for the farming conditions of low-income farms with the elite germplasm held by the corporations. They can ensure that the poor can aces the seed at no cost or at costs they can afford. They can also depend on the companies to provide funding to develop and deregulate the traits.
Developing and using transgenics is not without barriers, of course. In short, transgenics are expensive. Developing a transgenic trait costs $25 to $100 million dollars or more. Costs include finding a gene that does what you want it to, testing efficacy of the gene in many different varieties and environments, safety testing to ensure that the transgenic plants are substantially equivalent to their non-transgenic sister plants, and so on. For the forseeable future, the cost of transgenic traits will remain high. For the price of one commercial transgenic cultivar, CIMMYT believes they can characterize the entire genetic heritage of the two principal cereal crops, wheat and maize.
During her talk, Marianne announced the new CIMMYT program Seeds of Discovery for the first time. This exciting program will examine ancestral varieties of maize and wheat to enable breeding programs globally to use crop biodiversity in developing new lines. They aim to discover the extent of allelic variation in these varieties. They also hope to better understand how the different varieties are related in core sets. Right now, varieties are organized by geographic origin or phenotype but grouping by genotype will allow for better explanation of genetic similarities and differences. When more is known about the allelic diversity in ancestral varieties, marker assisted breeding can be used to bring those rare useful alleles into breeding programs.
In addition to ancestral varieties, CIMMYT looks at farmers’ varieties. They have partners in 14 countries that are both looking for potential lines for breeding that have traits like drought tolerance and looking into how new traits will work with the varieties farmers are currently using. They are also looking into other traits that are important to farmers in the developing world, including taste and appearance.
Greater than 80% of the required yield grain has to come from breeding. No other method, including fertilizer and transgenic traits, will be able to come close to breeding. Making these increases requires scientists from the developed world and from the developing world to both form partnerships and to work on their own areas of expertise. Marianne called upon the maize genetics community to help characterize the genes and alleles that CIMMYT finds in their Seeds of Discovery program. They plan to provide seed to scientists so they can begin to investigate the traits.
Talk Abstract:
Increasing demands for the main food staples, climate change, and increasing water, nutrient and land costs give a new urgency to developing and making available stress tolerant crops. This urgency is the greatest in the developing world where investments in research, capacity building and infrastructure development still lag far behind the developed world. The presentation gives an overview of CIMMYT’s investment in the development of stress tolerant maize which has recently gained significant leverage through stronger research collaboration with public and private partners, and now extends from native and transgenic trait discovery to large scale application of marker assisted selection approaches tailored to the improvement of highly quantitative traits such as yield under drought and low soil fertility. Many years of CIMMYT research indicate that these traits are highly polygenic, which has implications for the use of transgenics, identification of effects within association mapping studies, and the choice of appropriate marker based breeding strategies. In addition to assessing front line transgenics originating from the private sector for use in particular in Africa, current efforts focus on marker assisted recurrent selection (MARS), which is being implemented in over 40 biparental populations in Africa, Asia, and Latin America. Current MARS populations are selected on an index of 200 to 300 anonymous SNP markers, a density chosen because it is affordable with current genotyping technology. In 2010, pilot projects on the implementation of genomic selection (GS) using much higher marker densities will be initiated on new platforms based on next generation sequencing technologies, and it is expected that by 2011 genotyping costs will have dropped enough to permit their routine application across the CIMMYT maize breeding program and facilitate innovative native gene discovery and allele mining approaches. With that, CIMMYT is among the first public sector breeding programs that integrate cutting edge transgenic and molecular techniques on a large scale for germplasm development and dissemination to the tangible benefit of resource poor farmers.
Castiglioni P, Warner D, Bensen RJ, Anstrom DC, Harrison J, Stoecker M, Abad M, Kumar G, Salvador S, D’Ordine R, Navarro S, Back S, Fernandes M, Targolli J, Dasgupta S, Bonin C, Luethy MH, & Heard JE (2008). Bacterial RNA chaperones confer abiotic stress tolerance in plants and improved grain yield in maize under water-limited conditions. Plant physiology, 147 (2), 446-55 PMID: 18524876