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A wild bean’s genes may help a key crop thrive on a hotter Earth

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The climate change point-of-no-return may still be 1 degree C away. But that is of little solace to the people whose lives have already been upended by a warmer climate. They include growers and consumers of one of the most important protein sources in low-income countries: the common bean, a staple in diets from the highlands of Central America to the vast expanses of sub-Saharan Africa.

Through warming and unpredictable weather, bean fields are being abandoned or declining in productivity. In doing so, they are joining the growing list of climate casualties ranging from malnutrition and food insecurity to migrations and social unrest.

Already a staple crop for hundreds of millions of people in low-income countries, beans are now pointed to as an iron-rich replacement for meat in rich-world diets. If current dietary trends continue – and new ones catch on – the world is going to have to figure out how to produce a lot more beans on a warmer, hungrier planet.

Fortunately, scientists are working on a globe-spanning project to make the common bean more resistant to heat. The secret lies in its wild ancestor, the tepary bean, which has been grown for at least 2,500 years in the hot, arid regions of the Southwestern United States and Mexico. Researchers are working to exploit the genes responsible for climate robustness in tepary bean – known as Phaseolus acutifolius – and breed them into the common bean, or Phaseolus vulgaris.

“Low-fertility soils have been a problem for a long time but we didn’t address heat before,” said Celestina Xerinda, a bean breeder at the Mozambique Institute of Agricultural Research (IIAM), referring to her institution’s research priorities. “Under this project, we expect to develop a variety that is tolerant to heat stress because we are starting to get very high temperatures and the current genotypes (used by farmers in Mozambique) are not tolerant to heat.”

In Mozambique, a Southeast African nation with a population of about 30 million people, beans are the second most important source of protein for most people after meat, said Xerinda. Heat is exacerbating a “complex series of problems” facing the agricultural sector, she said, during a recent meeting of colleagues at the International Center for Tropical Agriculture (CIAT) (now part of the Alliance of Bioversity International and CIAT), as part of the Crop Trust-led Crop Wild Relative Project.

“Remarkable” performance

Adelmaro Clará Melara lives 14,000 km away from Xerinda but he has watched the same story unfold. As a plant breeder and researcher in El Salvador, he works with farmers whose fields are succumbing to the heat.

“There are many areas of the country where people have stopped planting beans due to drought,” said Clará, adding that disease, humidity, and extreme weather events are increasingly causing headaches for farmers in the small Central American nation. And just like Xerinda, he is testing common beans crossed with P. acutifolius in El Salvador to try to reverse the trend.

Beans are in trouble when overnight temperatures do not drop below 20 C. Daytime highs above 35 degrees also affect plant development. Even the best management practices – including irrigation and planting beans alongside corn plants to increase shaded, cooler areas for beans – cannot always compensate.

“When we talk about climate change, obviously the instability of rainfall will be a big issue. But you can manage, to some extent, your water deficit with better soil management, and in some lucky cases, through irrigation,” said Steve Beebe, a senior scientist at CIAT and leader of the center’s Bean Program. “But when you talk about high temperatures, which we’re going to have year after year, there is no alternative to genetic tolerance to high temperatures – short of moving cultivation to other environments.”

But that could lead to increased deforestation, a driver of climate change. Clearing cooler land for beans is not a desirable solution.

Recent research shows that P. acutifolius can be more productive in almost any type of warm environment. Beebe believes agricultural land that is traditionally too wet or too hot for bean cultivation could be a new frontier for bean farmers.

“There’s still a lot we don’t understand about P. acutifolius,” said Beebe, adding that there is promising research on tepary hybrids studied in hot environments with environmental variables spanning high humidity, low humidity, drought, and different soil types.

“The tepary bean comes out on top almost across the board,” he said. “We always knew it was good. I think we’re starting to appreciate just how good it is.”

Keeping ahead of change

Xerinda, Clará, Beebe, and other bean breeders gathered recently at CIAT headquarters in Colombia to discuss this research and to standardize protocols to help compare their advances across continents and ecosystems. The meeting was part of the Crop Trust-led Crop Wild Relatives Project (CWR), which builds on years of research at CIAT into breeding hardier bean varieties.

The initiative is intended to benefit smallholder farmers in Colombia, Honduras, and Mozambique, who started evaluating new materials in 2019.

“The hope is that we can rapidly evaluate these materials … and accelerate the process of getting these (heat-resistant bean varieties) to farmers if we find materials that have the advantages we are looking for,” said Juan Carlos Rosas, the research director at the Zamorano Panamerican Agricultural University, a partner in the project.

Quick results will be appreciated in neighboring Costa Rica, where bean-growing regions remain just below the temperature threshold, said Néstor Felipe Chaves. In 2015 with a mild El Niño event, Chaves visited bean fields in the country that showed the tell-tale signs of too warm nights – irregular flowering and low-production – which he recognized from his research on drought and high temperatures at CIAT.

“The production area right now is at the limit,” said Chaves, who did his Ph.D. research at CIAT and now works at the University of Costa Rica. “There aren’t many other problems but if the temperatures rise one or two degrees, we’re going to have serious problems.”