About 2 billion people are affected by zinc deficiency because of infertile and excessively cultivated agricultural land in Asia, Africa and elsewhere. This problem will probably only get worse as people’s diets become more plant-based in the rest of the world. Now researchers have discovered how to regulate plants’ zinc status and increase zinc accumulation in seeds by 50%. So far, they have only managed to do this in the model plant Arabidopsis thaliana; but if this can be replicated in real crops, the new knowledge may eventually lead to new strategies to improve the nutritional quality of plant-derived food and feed.
The fact that people can be malnourished even if they get plenty to eat may seem paradoxical, but combatting malnutrition means much more than eliminating hunger. People need at least 49 nutrients in their diet. Deficiencies of these nutrients can lead to suboptimal health and eventually to disease and can slow development among children. Deficiencies of such nutrients as vitamin A, iron and zinc cause two thirds of all deaths among children. Researchers have now revealed a mechanism in plants that can be used to tackle these global health problems.
“About 2 billion people have zinc deficiency, mainly those who live on plant-based diets relying on crops grown in zinc-deficient soil, but now we have discovered how to use a molecular switch in the model organism Arabidopsis thaliana that enables it to absorb more zinc from the soil, apparently without adversely affecting its growth and development. We hope to be able to replicate this mechanism in other plants to ensure that people consuming these fortified plants get adequate zinc,” explains first author Grmay Hailu Lilay, Postdoctoral Fellow, Department of Plant and Environmental Science, University of Copenhagen.
50% higher zinc content
Zinc is an essential micronutrient for plants and animals because of its structural and catalytic roles in many proteins, and proteins and the whole organism require an optimal zinc supply to function. Plants thus have advanced mechanisms that sense the amount of zinc in the soil to ensure that they absorb enough, and the researchers investigated these sensors.
“Zinc deficiency affects plant growth. Less zinc means slower growth, but researchers have long tried to understand how plants actually increase and decrease their uptake of zinc. We have identified two proteins, bZIP19 and bZIP23, that act as zinc sensors and determine a plant’s ability to absorb, transport and store zinc through plant tissues,” says Grmay Hailu Lilay.
The two proteins are transcription factors that ensure that the right genes are expressed in the right cell at the right time in the right amount. When the plant has enough zinc, bZIP19 binds the zinc and sends a signal that the plant does not need to absorb additional zinc. In contrast, if the plant lacks zinc, it does not bind to these proteins. In response, the plant begins to absorb more zinc from the soil. The researchers wanted to learn to understand and manage this on-off switch.
“After we showed that bZIP19 and bZIP23 bind to zinc, we tried to remove specific parts of these proteins to determine whether we could disrupt the zinc binding. We found and removed the special zinc binding sites and therefore waited eagerly to see how our plants would react to that change,” explains Grmay Hailu Lilay.
The effect was remarkable. The Arabidopsis thaliana plants with the modified zinc sensors absorbed 50% more zinc from the soil – apparently without adversely affecting their growth and development. By slightly adjusting the sensor, the researchers thus induced the plant to act as if it has permanent zinc deficiency.
“This is a major scientific breakthrough because we have now shown that zinc uptake can be increased in our experimental Arabidopsis thaliana plant. The next step is to replicate the results in actual crops. We are already well on our way to doing this in beans, rice and tomatoes. If we succeed, this may pave the way for creating more nutrient-dense crops,” says Grmay Hailu Lilay.
Recreating a genetically modified organism through breeding
It is too early to conclude whether the zinc-hungry plants will be useful in practice because the researchers must first thoroughly investigate whether the fortified plants with the significantly higher zinc concentrations are also healthy – so they can grow and thrive just as well as other plants.
“These zinc sensors have been preserved for millennia in many plant species, and this usually means that the plants have preserved them for a good reason – possibly to maintain a low zinc content or for a completely different and unknown reason,” explains Grmay Hailu Lilay.
Another recurring challenge is that the researchers genetically modified the zinc sensor, and this is not allowed in food plants in some places. However, the regulation is not quite as restrictive in the parts of the world in which nutrient deficiencies result in millions of children dying annually.
“Another option is to replicate the modification through traditional breeding methods. This is now possible since we know what kind of genes need to be modified, although this takes longer, and ensuring that no other modifications occur is also more difficult. As the rest of the world begins to eat less meat and more plants, this problem will probably only get worse, so we hope to be able to create fortified plants that can solve this challenge globally,” says Grmay Hailu Lilay.