2023-08-01 19:04:30
A dreaded parasitic weed destroys crops on a massive scale in Africa and beyond every year. Dutch researchers think they have found a solution: a bacterial fragrance.
The nuisance for African farmers caused by the striga plant is enormous. It’s not for nothing that striga is called ‘witch’s herb’ in English. Yet there is also fascination in the way biologist Jos Raaijmakers talks about this parasite.
“The tiny seeds of striga can lie in the soil for years waiting for a signal from a crop that they can parasitize. These plants inadvertently send out chemical signals, so that the weed seed knows that it must germinate now. The weed then attaches itself to the host’s roots and draws moisture and nutrients from it. Striga keeps that host alive just long enough, until the weeds themselves flower and form new seeds. After that, the parasitized plant dies definitively, due to a lack of moisture and nutrition.”
Biologically this weed may be very interesting, but for the food supply in sub-Saharan Africa it is an unmitigated disaster. Complete crops of grains such as sorghum, millet, rice and maize can be lost when the soil is contaminated with striga seeds. “These crops are the ‘staple food’ for people and livestock in many African countries,” says Raaijmakers. “Straw from these crops is also often used for the construction of huts, so a striga infection affects the lives of millions of people at various levels.”
Microsoft millions
It is estimated by the United Nations Food and Agriculture Organization, FAO, that at least 50 million hectares of farmland in Africa are infected with striga seeds. This causes damage of USD 7 billion annually and affects 300 million people in Africa.
Raaijmakers is the head of the microbial ecology department at the Netherlands Institute of Ecology in Wageningen. In 2015, he was invited to brainstorm at the headquarters of the Bill & Melinda Gates Foundation in Seattle, the philanthropic fund of the Microsoft billionaire’s wife. “Whether I thought that fungi and bacteria could play a role in combating diseases and pests in Africa was their question.” It was a time when research into bacteria and fungi in soils, on plants, but also in the intestines or on the skin of people, was getting more and more interest, so Raaijmakers had some ideas about that.
“After that brainstorming session, we didn’t hear anything for a year, until an unexpected Gates employee literally knocked on my door in Wageningen, asking if I was interested in setting up a research program in Africa to combat the disastrous striga infections with the help of microorganisms. organisms.”
After extensive preliminary research and with funding from the Bill & Melinda Gates Foundation, Raaijmakers and colleagues from the Netherlands Institute of Ecology, the University of Amsterdam, the Westerdijk Institute for fungal cultures and also foreign partners from the US and Ethiopia set up a special research project.
The suicide signal
With “Promise” (a creative acronym for Promoting Microbes for Integrated Striga Eradicationor it promoting microbial use for the integrated eradication of striga), they have spent the past six years researching how bacteria and fungi can lift the dreaded weed leg. “One of the solutions is fertilising,” says Raaijmakers. “The signal that causes the weed seeds to germinate is actually a signal that plants use to attract special fungi to help with the nutrient supply. By fertilizing the soil better, the plant no longer has to call for help and striga cannot eavesdrop on that signal either,” says Raaijmakers.
Yet fertilization is certainly not the whole story, let alone an adequate solution. Because even with sufficient nutrition, the weeds still manage to find the plant regularly. “Other research groups have made substances that emit a kind of ‘interfering signal’. That then works as a kind of ‘suicide signal’ for striga. The seeds will germinate even though there is no host plant nearby to infect. And without host plants, striga cannot survive.”
A scent that inhibits growth
This strategy is promising, but not sufficient in itself to solve the problem. That is why different research groups are now working together on a broader approach. The Promise researchers have now shown that bacteria and fungi in the soil can address the survival and germination of striga seeds in various ways. Raaijmakers: “We have found a fragrance that is naturally produced by soil bacteria. This volatile substance can almost completely inhibit the germination of the weed seeds.”
In a study in Ethiopian greenhouses, Raaijmakers and colleagues used soil from infected areas containing many striga seeds. When they then stimulated the production of this ‘inhibitory odorant’ by soil bacteria a bit, they saw that the infections of the sorghum crop by the noxious striga weed decreased by a factor of ten, compared to untreated soil. “We are still investigating how these volatile substances from bacteria almost completely paralyze germination,” says Raaijmakers. “But the fact that it is effective is shown by these experiments in greenhouses in Ethiopia.”
Money for further research
The first field experiments that the researchers conducted in Ethiopia last year also gave promising results. Until an extreme drought threw a spanner in the works. Most plants perished with or without the experimental treatment. But the first results of the experiments in the greenhouse were promising enough for the Gates foundation to also fund a follow-up study. At the end of May, the foundation signed a check for more than 10 million euros, with which the Promise project can try to fulfill the promise for the next five years.
Raaijmakers: “We focus our research not only on the stimulation of those volatile odorants by bacteria, but also on other bacteria or fungi that can drastically affect the supply of striga seeds in the soil. We also try to strengthen the crops’ own defenses, which strongly inhibits infections. And just as important: we also want to know whether there are any side effects of these new strategies. By changing the bacteria and fungi in the soil, you want to tackle the weed problem, but not cause new problems.”
In the coming years, Raaijmakers and colleagues will conduct field trials in Tanzania and Senegal in addition to Ethiopia. “A condition of funding from the Bill & Melinda Gates Foundation is that small farmers in sub-Saharan Africa have free access to effective and affordable products.”
According to Raaijmakers, the promise of these new integrated methods of weed control goes beyond striga alone. “There are various other diseases and pests that we believe can be tackled with the help of the micro-organisms in the soil.”
Green and social
Corné Pieterse, professor of plant-microbe interactions at Utrecht University, who was not involved in the project, says Promise is indeed a great promise. “A lot of research is being done worldwide into biological alternatives for crop protection. Currently, an enormous amount of chemical agents are used every year. The research by Raaijmakers and colleagues has already provided important insights into how benign bacteria and fungi are able to come to the aid of crops.”
“For example, there are microbes that release nutrients from the soil and make them available to the plant. Other microbes produce antibiotics that can inhibit disease-causing fungi in the soil. These are all properties that have evolved hand in hand with plants in a balanced ecological system. But in today’s intensive agriculture, those mechanisms are no longer sufficiently utilized with the practice of monocultures.”
Pieterse calls the fragrance that has already rolled out of the Promise project a wonderful discovery. “This helps solve a very big problem in the cultivation of crops in developing countries. The solution is also green and social, because it also benefits countries that need it most. A very significant contribution to the world food problem.”
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