Sweetpotato Square One

Written by Elsa Matthus, CWR Project Correspondent

The domesticated sweetpotato has many wild relatives, surviving on their own in rough conditions in various parts of the Americas. Breeders want to use the strengths of those wild sweetpotatoes in their crop improvement programmes, but they grumble about how much work it is. A group of tough scientists from Peru, Mozambique and the USA has accepted the challenge.

The life saver

Mozambique gained independence in 1975, followed by a civil war (since 1977) – to arrive at Peace Agreement in 1992. When refugees returned home that year, one of the worst droughts of the century struck. With entire crops being lost, the country had to rely on food aid.

This is also one of the countries most affected by climate change, with extreme droughts and floods occurring more frequently in recent years. In 2000, a disastrous cyclone hit and flooded the country. Half a million people were displaced; crop and livestock were lost.

One approach to alleviate the disaster was to distribute sweetpotato planting material to farmers. Sweetpotato plants grow fast, need little input and labor, and produce more energy per land area than any other food crop. Plus, they are not just empty carbohydrates, but packed with micronutrients, with many of the improved varieties grown in Mozambique now containing high levels of the vitamin A precursor. In 2016, the World Food Prize was awarded to a group of scientists from the International Potato Center (CIP): Maria Andrade, Rober Mwanga and Jan Low, as well as Howarth Bouis, Founder of HarvestPlus, for making sweetpotato more nutritious and distributing ‘Orange Fleshed Sweetpotatoes’ to areas in need. Since 2000, production in Mozambique has increased 40 fold.

Mozambique is not the only case of sweetpotato coming to the rescue. The crop has been a life saver in many countries struck by disaster, from India to China to Japan, just to name a few, and has often been nicknamed ‘famine relief crop’. It has been kind to humankind. But, with weather conditions becoming more extreme, even the sweetpotato is starting to struggle.

Who will save the life saver?

“I wanted to work with a crop that was important to my country, and sweetpotato is very much one of those crops”, says Stella Nhanala, PhD student at North Carolina State University, USA. Stella grew up in Mozambique and got exposed to agricultural activities at early age when she used to help her grandmother working in the “machamba” (Mozambican word that means field). In addition to other foods, roasting ‘minlhata’, sweetpotato in the local language, was part of their meals in the field. Now, Stella works on making the cultivated sweetpotato more tolerant to stresses such as drought and heat. Her PhD is funded by the Crop Trust’s Crop Wild Relatives project. Wait – Crop Wild What? Stella worked a little on sweetpotato before starting her PhD, but she had never worked with Crop Wild Relatives (CWRs), the wild cousins of our domesticated food crops.

In the case of domesticated sweetpotato, Ipomoea batatas Lam., there are 15 wild relatives that still grow undisturbed in their center of origin, the wilderness of the Americas. When humans first started domesticating sweetpotato more than 5000 years ago, they kept on selecting for bigger and tastier roots, again and again, to finally get to the sweetpotato we grow and eat today. Its wild relatives did not have to be top of the root size class but instead kept on growing wherever and however they liked, until today. Growing in the wild, without the loving care of a farmer, also means they had to battle stresses on their own – no fungicides or irrigation system in sight. These wild relatives are poorly documented, with many species not being properly represented in genebanks, let alone studied by researchers. This means the diversity of wild sweetpotatoes is completely untapped when it comes to breeding programs.

Nobody said it was easy

Give or take a few sporadic uses of a wild sweetpotato in breeding programmes, “wild relatives of sweetpotato have never been used extensively for sweetpotato improvement”, points out Dr. Hannes Dempewolf, CWR Project manager from the Crop Trust. In theory, the breeding community is keen to exploit the diversity of wild sweetpotato. In practice, however, using wild relatives in breeding programmes is a staggering amount of work. The same is true for many other important crops, such as lentils and carrots, which also need a dose of wildness in them.

Sweetpotatoes can’t help being difficult to work with, it’s their genetics. The domesticated sweetpotato originated a few thousand years ago, most likely when some of its wild ancestors mingled. It is not quite clear who mingled with whom and in which order – the fact is that our domesticated sweetpotato is hexaploid. This means it has six sets of chromosomes. Many of its wild relatives however have anything between two and four sets of chromosomes. What might sound like gruesome sci-fi, with genetic material doubling and quadrupling, is quite a common thing to happen – just look at banana, wheat and sugar beet. Having a different genetic built is natural but it complicates the work of breeders massively, as wild and domesticated sweetpotato won’t cross easily.

“Using sweetpotato’s wild relatives for breeding purposes requires a start from the proverbial ‘square one’”, says Dempewolf. So far, nobody really wanted to go back to square one, no matter the benefits in the long run.

Let the sweetpotato games begin

The Crop Trust got two champions on board to systematically characterize the wild relatives of sweetpotato for the first time. At North Carolina State University it is Stella, working in the lab of Professor Craig Yencho, a renowned sweetpotato breeder. In Peru, at the International Potato Center (CIP), it is Dr. Bettina Heider and her team. Not quite coincidentally, CIPs’ genebank hosts the world’s biggest collection of sweetpotato, with close to 8000 wild and domesticated types.

Heider’s role is to build a bridge between that collection and the breeders. She takes plant material from the genebank collection, grows it, notes down every important plant trait, and checks which of the diverse plant lines are more stress tolerant than others. When she finds, let’s say, a wild sweetpotato whose roots don’t shrivel up and die in the presence of sweetpotato viruses, Heider does a little (and probably metaphorical) happy dance –and years of hard work in the greenhouse and laboratories start, to make sure that it can be included in a conventional breeding program.

How high is high enough?

When Heider started to explore the world of wild sweetpotatoes, she had very little initial information to work with. A priority was to clarify the fuzzy family tree of sweetpotato relatedness and define which of the wild ones can easily cross with each other.

“The easier it is to cross the different wild sweetpotatoes, the more it shows that the individual populations have not been separated long enough to develop into two completely distinct species”, explains Heider.

If two wild sweetpotatoes lines do not cross easily, it indicates that they became separated a long time ago. Their crossing barrier has become too high to jump over.

So far, Heider and her team have selected nine of the wild sweetpotatoes, carrying out over 11,000 crosses in the process – uff. Very skilled workers are needed to do the crosses – flowering times between the different plants need to be synchronized, and sweetpotatoes in general flower way too early in the morning. Up to now, about one quarter of the crosses have led to seeds being set, hence suggesting that these wild sweetpotatoes cross fairly easily and have not yet separated into completely distinct forms.

A good start is half the battle

Once the family relations are sorted, Heider and Stella will need to figure out how stress tolerant each one is. And try to cross the wild ones with the domesticated sweetpotato, to transfer any tolerance they find.

“We’d be very lucky to get a cross between wild and cultivated sweetpotato straight away”, says Heider. “Many intermediate steps will be required, to introduce traits of interest to the domesticated sweetpotato, ultimately resulting in new varieties available to the breeding community.”

However, Heider and Stella have already come a long way, and both agree that the effort is worth it.

Stella laughs: “You know, there are endless uses for sweetpotato. You can cook it in all kinds of different ways. It’s very nutritious. You can use it to make alcohol. Or even as an ornamental plant. Especially the wild ones have beautiful flowers. It’s – I am trying to find the right word – a very dynamic crop.”

Heider adds the sweetpotato’s most outstanding feature: “And it grows everywhere, even on poor soils. I like to call it almost a weed. This is of course good news for people farming on marginal lands”, as are most farmers in developing countries. And it will be even better news, once our cultivated sweetpotato has acquired the survival tricks of its wild relatives.

This project is part of ‘Adapting Agriculture to Climate Change: Collecting, Protecting and Preparing Crop Wild Relatives’, funded by the Norwegian Government, and coordinated by the Crop Trust and the Millennium Seed Bank, Kew.


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