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Tapping into tepary

PhD student Jodi Souter conducted trials in Puerto Rico to assess the drought tolerance of tepary beans, common beans and tepary/common bean crosses. Photo courtesy of Jodi Souter.

With all the climate vagaries we’re going through globally, people are very interested in tepary as a crop and as a source of genes for [crossing] into the more popular common bean crop. So says Kirstin Bett, a professor of plant breeding and genetics at the University of Saskatchewan’s (U of S) department of plant sciences. Bett leads the Crop Development Centre’s breeding program for dry beans, which are a type of common bean. The program’s current activities include work to transfer cold- and drought-tolerant genes from tepary bean into dry beans.

“Our goal in the bean breeding program is to try to increase the bean acres in Saskatchewan, and one of the big problems with growing beans here is that they’re a high-risk crop [because of their low tolerance to stresses like cold and drought]. Tepary bean is super stress tolerant,” Bett says. “So, even though they are a high-return crop, a lot of producers don’t want to grow them. But if we can reduce the risk, we might be able to encourage more people to try this crop.”

For Saskatchewan bean production, drought stress can be a yield-limiting factor, but frost is an even greater issue. She explains, “Common bean hates cold temperatures. It won’t grow while the temperature is below about 10 C, and if the temperature hits -2 C, it’s dead.”

Improved cold tolerance would lower the risk of crop loss from an early frost. It might also mean the crop could be planted earlier, allowing Saskatchewan producers to grow longer maturity varieties, which generally have higher yields.

Tepary bean is well-known for its ability to tolerate drought and heat. “Tepary was a traditional crop in the Southwest U.S. and northern Mexico,” Bett notes. “They are trying to revive it as a crop in that region for the local indigenous tribes. Tepary bean has also been taken to Africa for regions that are too marginal for common bean production, as an alternate protein crop for local consumption.”

Tepary bean (Phaseolus acutifolius) is a cousin to common bean (Phaseolus vulgaris). Both species were domesticated in the Americas as food crops thousands of years ago. Along with the ability to tolerate weather stresses, tepary also has resistance to certain diseases and insect pests; for instance, it is a source of genes for tolerance to common bacterial blight for common bean breeders. Tepary seeds are small and vary in colour.

Before Bett arrived at the U of S over 10 years ago, breeders there had been working with a different bean species to try to bring cold tolerance into common bean, but that species was extremely difficult to cross with common bean. So one of Bett’s post-doctoral students, Valarmathi Gurusamy, started exploring the possibility of getting cold tolerance through various crosses with other bean species. Then a graduate student checked tepary for its frost tolerance.

“Tepary bean turned out to be just as frost-tolerant as the other species, but you can actually cross [common bean] with tepary bean – it’s not easy but you can. So we switched to tepary beans, and we’ve been working with them ever since,” Bett explains.

“We’ve developed a population of common bean with some little bits of tepary bean genome, through traditional crossing; they are not transgenic. We have had those lines out in the field for a number of years now, evaluating them and culling out the terrible ones. For instance, one important problem is that the parent we used is daylength sensitive. Tepary comes from a region with short days, and if you grow it in a region with long summer days, then it won’t flower until the days are short, which of course coincides in Saskatchewan with frost in September. So we had to get rid of everything that didn’t flower early. Also, tepary beans tend to grow prostrate and we need upright plants, so we got rid of all the prostrate ones.”

Their work to select the most frost-tolerant lines has been harder than you might think. Earlier research by Bett’s research group had shown that screening for frost tolerance in the Phytotron, the University’s Controlled Environment Facility, was not very practical. She says, “The cold response interacts with the light quality. What we thought were cold-tolerant plants in the Phytotron were not always superior when we grew them outside. You can’t really mimic outdoor light indoors. And space is limited in the Phytotron.”

So, they have to do their frost-tolerance screening outside. To do that, they plant the different lines in August in Saskatchewan. That way, the weather is warm enough for the plants to germinate and grow, plus a frost is pretty much guaranteed to occur at some point in the fall.

“For cold tolerance, we’re talking about -3 to -4 C, so it’s not a huge advantage over common bean, but some days that is the difference between having a crop and not having a crop,” Bett explains. “Of course, trying to evaluate cold tolerance in the field is a nightmare. We keep our fingers crossed for a mild frost. But as you can imagine, when you want a -2 or -3 C frost, what you get is either no frost or -8 C, which kills all the plants.”

Consequently, one aspect of the breeding program’s current work with its common bean/tepary bean crosses involves looking for an alternative way to select for cold tolerance. The idea is to select for drought tolerance and then to see if those drought-tolerant lines also have some measure of cold tolerance.

In keeping with Murphy’s Law, the researchers were screening for drought tolerance just when Saskatchewan was having unusually wet weather. “So we teamed up with Tim Porch from the USDA-ARS [Agricultural Research Service] in Puerto Rico, where in theory you can do drought trials in the winter. Jodi Souter, the PhD student working on the project, spent time there over three winters. It rained but she did get enough drought data to show that the teparies are much more drought-tolerant; in fact they prefer drought to being well-watered. And some of our interspecies hybrids are performing more like tepary than common bean; they are not as drought-tolerant as tepary, but they are pretty good,” Bett says.

Souter is now analyzing the study data to determine if selecting for drought tolerance also selects for cold tolerance.

In another component of the tepary work, Gurusamy has returned as a research associate in Bett’s group, and is integrating stress-tolerant common/tepary lines into the dry bean breeding program. “We’ve made selections and decided which ones are probably promising lines, but they are not going to turn into varieties because the seed is the wrong colour or the wrong size or the wrong shape. So we need to cross them with elite dry bean material from here,” Bett explains.

As a side project, Bett’s group is also dabbling in breeding tepary beans as a crop. “They are more stress tolerant, so one could ask why aren’t you just growing teparies instead? The argument is that there is not a huge market for teparies yet, and one of the problems with them is the seed is small, slightly smaller than a black bean,” Bett says.

“However, stress tolerance is a very complicated trait, whereas traits like seed size and colour are much less complicated genetically. So we wondered if we could improve tepary bean instead of improving common bean – maybe we could cross common bean into tepary bean and try to increase the size of tepary beans and then grow them as a crop. Tepary bean would never rival common bean, but it could be a small niche market bean that could cater to farmers’ markets or the restaurant trade.”

The bean breeding program’s current work to transfer cold and drought tolerance from tepary to common bean is funded by the Saskatchewan Pulse Growers, the Alberta Pulse Growers, Saskatchewan’s Agriculture Development Fund and the Western Grains Research Foundation.

January 21, 2016  By Carolyn King