When the genes don’t add up

Climate projections indicate that maize breeders will have to start looking for traits that confer tolerance to simultaneous drought and heat stress, according to CIMMYT-CCAFS research. Photo: CIMMYT.

1 + 2 definitely equals 3. No one needs to question elementary math.

But what happens if you try to add words? Does peanut + butter = peanut butter? Not really. Enter the terms separately in a web search engine, and you’ll get a different result than if you enter the two together. And genes? As it turns out, basic addition won’t work with them, either.

Research from the International Maize and Wheat Improvement Center (CIMMYT) has elicited an unusual hiccup in breeding for stress tolerance in maize: drought tolerance + heat tolerance does not = drought and heat tolerance. That is to say, the genes responsible for tolerance to the combined stressors of heat and drought are not the same as the genes for tolerance to either of those stressors alone.

A maize plant that has been bred to tolerate high temperatures or drought, or even both, is distinct from a plant that has been bred to tolerate simultaneous exposure to both stressors – the latter has the “combo” trait, let’s say, while the others have the “solo” trait. Even if a variety features both of the solo traits, that doesn’t necessarily add up to the combo trait. This may seem like a small detail, but it means that when the combo drought/heat trait is not present a cultivar expressing drought tolerance could still experience markedly diminished yields when hit with a simultaneous blow of heat stress.

CIMMYT and CCAFS are now rolling out strategies to ensure that improved varieties are productive even when exposed to multiple, concurrent stressors.

Drought + heat makes for volatile conditions

Malawi, Zimbabwe, and Zambia are the most maize-dependent countries in Southern Africa, relying on the crop for 40 to 50% of both calories and protein consumed. Nevertheless, current yields are barely enough to ensure food security in most years and are often insufficient altogether. That fact, combined with the climate projections for the region, means Southern Africa has every reason to be interested in climate tolerant traits in maize.

Using downscaled climate projections for maize growing regions in Sub-Saharan Africa, CIMMYT researchers determined that Southern Africa in particular is a high priority region for the development and dissemination of stress resistant germplasm. Although temperatures throughout Sub-Saharan Africa are projected to increase by 2050, Southern Africa features the dangerous combination of increasing temperatures in tandem with rainfall scarcity.

Although drought tolerance in maize is relatively well-researched, much less attention has been given to breeding with heat tolerance specifically in mind. This has proved to be a nearsighted tactic, however; a landmark study by Lobell and Burke in 2010 showed that a 2°C increase in temperature would negatively affect maize yields even more than a 20% decrease in precipitation would. With the climate predictions above pointing to increased growing season temperatures in drought-prone regions, maize yields are likely to experience considerable constraints from combined drought and heat stress.

The new task for crop breeders

CIMMYT’s findings are recent enough to mean that many maize varieties in circulation have been bred to express one or both of the “solo” genes, but very few but very few have been bred with the “combo” gene in mind. Many drought tolerant varieties currently in use in Southern Africa could see considerable yield drops if they are exposed to heat stress at the same time, a high likelihood in Zimbabwe and other countries of Southern Africa.

In light of these results the Drought Tolerant Maize for Africa (DTMA) breeding pipeline (the largest public drought breeding program for maize in the world) is now screening maize cultivars to eliminate from the pipeline varieties that will not hold up well to future climate demands. Likewise, seed companies and other national and international breeding programs are starting to look for maize donors that exhibit the combo drought/heat tolerance traits to offset potential losses from climate change.

Breeding pipelines must start screening for the combination drough/heat tolerant trait to prevent the release of vulnerable cultivars. Photo: A. Wangalachi (CIMMYT)

Zimbabwe’s Crop Breeding Institute is teaming up with CIMMYT-CCAFS to screen existing cultivars for the presence of the combo drought/heat tolerance trait. Their aim is to flag drought-tolerant varieties already in circulation that could be susceptible to heat stress – quality control of sorts – and to prevent the release of new varieties that are missing the necessary characteristics. The work was recognized by MAIZE as critical for bolstering national capacity to address drought and heat stress in maize breeding, and resulted in funding for a new breeding program.

CIMMYT project leader Jill Cairns notes the importance of these results for adapting maize production to climate change.

“These technologies will improve climate resilience in parts of Zimbabwe that are (a) heavily dependent on maize and (b) greatly at risk from the impacts of increased drought and elevated temperatures due to climate change.”

No other programs are working on the issue, says Cairns, and the varieties that exhibit combined heat and drought tolerant are relatively unknown so far. As those varieties emerge, she concludes, “the result will be a direct enhancement of the capacity of maize based farming systems to adapt to climate change.” For crop breeders, the implication is a change in the way that stress tolerant varieties are developed and screened – a change in the “math” of cultivar selection. For maize farmers, the result will hopefully be a stable crop that can stand up to whatever conditions the future climate may throw their way.

Read more about CIMMYT research: Cairns JE et al. (2013)  Identification of Drought, Heat, and Combined Drought and Heat Tolerant Donors in Maize

Read more from the CCAFS blog:

Mitigating change in climate relations: Breeders and modelers of Africa unite for climate-smart crops.

Steadying the aim for crop breeders using target population environments.