Global food production depends on a few hundred of the roughly 400,000 plant species in nature. Danish researchers will examine the genetic toolbox in-depth to use more plants. According to a researcher, nature has plants with attractive properties lacking in the plants we have domesticated for millennia.
Of the 400,000 possible plant species in nature, humans have chosen to commercially exploit a mere 200 of them, from wheat and lettuce to peas and cabbage.
Many of the domesticated plants, however, lack characteristics that could make them even more attractive.
This may include drought resistance, the ability to extract nitrogen from the atmosphere or being perennial, so that farmers do not have to go through the expensive process of sowing, fertilizing, harvesting and selling the crops every year.
These characteristics are precisely what Danish researchers seek to achieve in future crop plants.
“Drawing on nature has major benefits, because our food plants are based on a small group of fragile plants that we have inbred for millennia, so that today they do not have nearly the same resilience as plants in nature. In the future, if we want plants that can withstand climate change and also ensure that everyone in the world gets enough food, we need to exploit the opportunities nature presents,” explains Michael Broberg Palmgren, Professor, Department of Plant and Environmental Sciences, University of Copenhagen.
The Novo Nordisk Foundation has awarded Michael Broberg Palmgren a Challenge Programme grant over 6 years to enable him to realize his dream of turning semi-domesticated or even wild plants into better crops.
Several reasons to explore wild plants in search for new crops
Sustainability is the core of this major research project.
Sustainability can be understood differently, and agriculture can become sustainable in several ways:
• using less external inputs such as fertilizer and water;
• using less pesticides;
• using fewer machines in cultivating crops; and
• storing more carbon in the soil.
Interestingly, nature already contains plants or plant properties that could make agriculture more sustainable.
• Many wild plants that live in symbiosis with bacteria can extract nitrogen directly from the atmosphere.
• Many wild plants have poisonous leaves, eliminating the need for pesticides.
• Many wild plants are perennial, which could eliminate much of the mechanical work in agriculture.
• Many wild plants, in particular perennials, have deep root systems or large stems that remove carbon from the atmosphere and store it.
“The good thing is that cultivating some enviable properties in our crop plants does not require inventing them from scratch. They already exist in nature. We just have to figure out how to exploit them,” explains Michael Broberg Palmgren.
Peas can extract nitrogen from the atmosphere
Michael Broberg Palmgren is far from the first to explore wild plants in the search for better crops.
For many years, plant breeders and farmers have explored legumes such as peas and beans.
Peas and beans live in symbiosis with bacteria that extract nitrogen from the atmosphere. Then the bacteria supply the legumes with the essential nutrient nitrogen in exchange for sugar.
This characteristic is desirable in crops other than legumes, such as cereal varieties to which large quantities of nitrogen fertilizer are applied.
Nitrogen fertilizer that the plants do not absorb ends up in groundwater.
The problem is that all attempts to transfer this special property from legumes to other plants have failed.
“Improving nature is not as easy as some may think. People have been naive and thought that a gene could just be moved from one plant to another, and that was it. Today we know that hundreds of genes can be involved in managing an advanced characteristic, and the regulation of genes must also function, so this is an incredibly complex system that nature has spent millions of years developing. This is very difficult to do in the laboratory or through breeding, if it is possible at all,” says Michael Broberg Palmgren.
Perennial grasses are more attractive as cereals
Another trait plant breeders and farmers want in crops is perenniality.
Cereal varieties such as wheat, barley, rye and corn are annuals.
Farmers must sow the seeds in the spring and harvest in the autumn. Then the plant dies and the remnants have to be ploughed into the soil, and the farmer starts over the following spring.
Also, the roots of the annual plants do not reach very deeply into the soil. This makes them more vulnerable when drought occurs and water is more than one metre below ground level.
In addition, such plants only have access to the nutrients in the top metre of the soil, and all the fertilizer that seeps further down disappears beyond the reach of the plants.
Perennial grasses do not have these problems.
They do not have to start over every year but can simply produce new grains from existing stalks.
Further, because of their longer life, their roots reach up to 4 metres into the soil. This gives them access to more nutrients and water during drought, and they can intercept nutrients that would otherwise end up in the groundwater and are less vulnerable when no rain falls for a long time.
Plant breeders give up
Interestingly, half of all grass species are perennial.
Humans have more or less accidentally selected only annual plants as the basis for the domesticated cereal varieties.
However, plant breeders have always dreamed of making wheat a perennial since the 1930s, but similar to nitrogen fixation by legumes, turning an annual plant into a perennial has proved to be impossible so far.
“The problem is that the genetic package to turn an annual plant into a perennial may simply be too large and complex to enable plant breeders or even modern genetic technology to simply transfer the package. In some cases, a perennial plant has been crossed with an annual and produced perennial offspring, but this has never succeeded with wheat, for example,” explains Michael Broberg Palmgren.
No reason to try to improve existing crops
The new thinking in the project led by Michael Broberg Palmgren is that we should not think of improving existing plants by trying to give them new properties they have never had – or at least not had for many millennia.
Instead, we should explore the 400,000 plants in nature and, with a nudge in the right direction, can transform them into new crops with attractive properties.
“That is our concept. Instead of sticking to these 200 plants and trying to improve them in ways that will not work, we need to explore the other 400,000 plant species for the properties we want. Then we need to breed them from scratch. We have to start all over again,” says Michael Broberg Palmgren.
Mutation in one gene makes tomatoes larger
Proof-of-concept studies with tomatoes have already demonstrated an example of what Michael Broberg Palmgren seeks to do with many crops.
Wild tomatoes are merely small berries of 1 gram on a creeper.
The amount of CLAVATA3 peptide in the plant is one factor that regulates the growth of these tomatoes. CLAVATA3 inhibits cell division, so if the tomato plant has a large amount of CLAVATA3, the berries do not grow as much. However, if the plant does not have much CLAVATA3, the tomatoes grow large.
All commercially available tomato varieties, from cherry tomatoes to beef tomatoes, have mutations in the CLAVATA3 gene, which reduce peptide production. These tomatoes grow larger than their wild counterparts.
In three proof-of-concept studies here, here and here, researchers used CRISPR gene-editing technology to edit the gene sequence that regulates the CLAVATA3 gene. The wild tomato plants immediately developed large tomatoes rather than small berries.
“The insight we have obtained from plant physiology and plant genetics shows that very few mutations are often sufficient to convert a wild plant into a domesticated plant. The experiment with the tomato plant showed how easy it is. Breeders did not create a miracle in the plant’s genetics but just destroyed something that actually weakened the plant slightly, and then it has to use more resources for making unnecessarily large berries, which we can enjoy eating,” explains Michael Broberg Palmgren.
Rice had to mutate to avoid shedding its grains into the water
The example of the tomato plant is not unique.
The same applies to all cereals. After very few mutations, they have acquired properties that have made them interesting for humans to cultivate.
One example is the rice plant.
Rice grows in water, and nature developed wild rice to very easily shed the rice grains so that they can fall into the water, germinate and become new rice plants.
Sliding a hand over a wild rice plant makes all the rice grains fall off. This is not very practical for rice farmers.
The same problem applies to wild wheat and other cereals. Nature has designed them to quickly shed the grains, and the axes are fragile and split more easily.
However, domestication of both rice and the other cereals over millennia has produced mutations in their genomes that make the grains attach more firmly and avoid breaking the axes.
“Again, defects in wild plants have made them useful to cultivate because they can be harvested and produce a greater yield. Perhaps only 10 to 15 genes need to mutate to transform a wild cereal into one that is largely domesticated,” says Michael Broberg Palmgren.
Introducing mutations into intermediate wheatgrass and making it suitable as a crop
And that brings us to the research that Michael Broberg Palmgren and his colleagues have undertaken.
Today, advances in genetic engineering have reached a point at which genome sequencing reveals the innermost secrets of plants.
Last year the genome of a wild prairie grass was mapped: intermediate wheatgrass (Thinopyrum intermedium).
Intermediate wheatgrass is interesting because it is similar to wheat in many ways.
However, there are some minor differences.
Intermediate wheatgrass is perennial: its roots extend several metres further into the soil than wheat, and it does not need to be replanted every year.
Nevertheless, its long grains are very thin and not very attractive to farmers.
The idea that the researchers from the University of Copenhagen are exploring is therefore based on using the knowledge about the mutations that initially started to make wild wheat attractive to cultivate to introduce the same mutations into intermediate wheatgrass.
“Wheat and intermediate wheatgrass are genetically very similar, so the genes that cause the attractive qualities of wheat are almost identical in intermediate wheatgrass. The plan, therefore, is to develop intermediate wheatgrass with the same minor changes in the genes as those that arose in wheat when this grass was bred,” explains Michael Broberg Palmgren.
Wheat is like a mule bred with a zebra
Researchers have tried to breed more attractive properties into intermediate wheatgrass since the 1970s – but without much success.
The researchers can see why this has not yet succeeded based on the knowledge they have today.
Wheat originates from three very closely related species that merged at some prehistorical point. The same applies to intermediate wheatgrass.
It is a bit like breeding a horse with a donkey and producing a mule that you then pair with a zebra. This ends up as a mixed bag.
Wheat still has the genes from all three ancestors and if, for example, the gene that enables the grains to be easily released is to be destroyed, it must mutate not merely in one place but in six places: three species times two genes from each species – one from the “father” and one from “the mother”.
“Wheat is allohexaploid, so using traditional breeding to introduce the same properties in intermediate wheatgrass that wheat has requires succeeding in introducing a destructive mutation in all six genes at the same time to affect the plant. This requires a miracle, and that is why plant breeders have not yet succeeded in making intermediate wheatgrass the new form of wheat,” says Michael Broberg Palmgren.
Lupin may replace soya imports from South America
In their research, Michael Broberg Palmgren and colleagues focus on six plants, which they will purposefully manipulate to obtain commercially attractive properties.
The research, led by Michael Broberg Palmgren, will introduce the same mutations in wheat into intermediate wheatgrass through both traditional breeding experiments and CRISPR gene-editing technology.
The CRISPR research is being carried out with the expert assistance of Caixia Gao, Professor at the Chinese Academy of Sciences and visiting researcher at the University of Copenhagen.
“I think we can get very close to creating a real type of wheat, which will then be perennial. We have to show that this can be done by mutating the right genes, and then the plant breeders can pick up from there and try to introduce them through traditional breeding experiments,” explains Michael Broberg Palmgren.
Two other wild plants that researchers want to make commercially more attractive are quinoa (led by Rosa Lopez-Marquez, Associate Professor, University of Copenhagen) and alfalfa (led by Stephan Wenkel, Associate Professor, University of Copenhagen).
They also seek to make lupin (Lupinus) edible (led by Fernando Geu-Flores and Hussam Hassan Nour Eldin, Associate Professors, University of Copenhagen).
Lupin has special potential because it is a legume, with many of the attractive properties mentioned earlier. Most importantly, it does not need nitrogen fertilizer because it can extract nitrogen from the atmosphere.
Lupin seeds have very high protein content, which makes them very useful as animal feed. Today large quantities of soyabeans are imported from South America, but farmers in Denmark would probably prefer to use local lupin if lupin leaves were not poisonous.
“We want to remove the poison from the lupin seeds but retain it in the leaves. If we remove the poison from the leaves, pesticides must be applied to prevent snails and other animals from eating them, and then the whole idea is ruined. We will therefore only try to remove the poison from the seeds. Incidentally, lupin is really easy to cultivate in Denmark,” says Michael Broberg Palmgren.
Wild barley can grow on a stone
The fifth plant the researchers will work with is barley (by Henrik Brinch-Pedersen, Professor, University of Aarhus).
The barley that farmers grow in fields today is not particularly robust and must be looked after and cared for in order to get a meaningful yield.
However, wild barley (Hordeum spontaneum) is cultivated in, for example, Ethiopia.
Wild barley can practically grow on a rock, and it is resilient to pests. Unfortunately, the yield from wild barley is not very impressive, but it could certainly be if the researchers introduce the right mutations into it.
“Most of our crop plants have not merely experienced a few mutations that make them attractive to cultivate. They have also been exposed to harmful mutations so that they are not as resilient as their wild counterparts. You can easily call all crop plants disabled war veterans, because their genome contains mutations that have weakened them. The plants we want to find have only the mutations that are attractive to us and not those that make the plants weak,” explains Michael Broberg Palmgren.
Making potatoes more resistant to diseases
The last plant the researchers will try to improve is the potato (by Kim Hebelstrup, Associate Professor, University of Aarhus).
Cultivated potatoes are very susceptible to various diseases, best demonstrated during the potato blight in Ireland from 1845 to 1849, when the water mould Phytophthora infestans destroyed the potato harvest for several years.
Twelve percent of Ireland’s population died from starvation, and about 1 million people emigrated.
Wild potatoes, in contrast, are very resilient but merely produce a few small, bulky potatoes that are not worth writing home about.
The researchers will introduce mutations into wild potatoes from South America to create more resilient potato plants with large and tasty potatoes.
“For all six plants, we will search for the genes that are mutated in bred plants that have produced benefits. Then we will search for the same gene in the wild plants and introduce the same mutations. The concept is that we do not actually have to create anything. We just have to destroy,” says Michael Broberg Palmgren.
Opening the door to making 400,000 plants edible
Michael Broberg Palmgren will not promise successful outcome, but he believes that succeeding with just one of the six plants would be hugely beneficial, because then they will have shown how 400,000 plants could potentially be made edible.
This would be an instructive proof of concept.
How will these plants taste? After all, they are not being selected based on their culinary potential.
But Michael Broberg Palmgren explains that this is a minor problem.
Very few domesticated plants taste like nature created them to taste.
Cucumber peel used to be very bitter, and grapefruit were also much more bitter just 30 years ago. Plant breeders have played a role and slowly cultivated the flavours that consumers have been demanding.
The same thing can be done with the plants that Michael Broberg Palmgren aims to develop.
“And remember that many plants are bred based on the desire for greater yield and not necessarily better taste. For example, wild tomatoes are much more aromatic than the domesticated ones, but we have just made them bigger and bigger at the expense of the aroma. But we do not have to lose the aroma if we are more precise in the mutations we introduce into the plants,” explains Michael Broberg Palmgren.
Traditional breeding is like performing an appendectomy with a saw
At a time when genetic engineering is viewed unfavourably, many people want to avoid the thought of creating new crops with CRISPR.
Here, however, Michael Broberg Palmgren asks us to keep our eyes on the ball.
Traditional breeding introduces many random mutations in the plants, of which only a few give rise to desirable features in the plants. So the plants we stuff into our mouths every day resulted from many mutations that no one has managed or mastered.
“Traditional plant breeding is like performing an appendectomy with a saw. Many other things are destroyed at the same time,” says Michael Broberg Palmgren.
Using a precision tool such as CRISPR to introduce a mutation, in contrast, would be the equivalent in plant breeding to using a scalpel during an appendectomy. Only the necessary incisions are made.
“Unfortunately, plant breeding using genetic engineering has come to a complete halt, so we still have not progressed beyond the plants that have been developed to be more resistant to glyphosate and the like. But with all the knowledge we have today about plant breeding and plant genetics, we can create a much more sustainable agriculture that will protect nature and its resources to a greater extent,” concludes Michael Broberg Palmgren.
“Accelerating the domestication of new crops: feasibility and approaches” has been published in Trends in Plant Science. In 2019, the Novo Nordisk Foundation awarded Michael Broberg Palmgren a Challenge Programme grant for the project NovoCrops: Accelerated Domestication of Resilient Climate Change–Friendly Plant Species.