Tinkering with the plant’s engine
Wageningen Climate Solutions
BY Marion de Boo - September 2019
If we want to find a solution in plants for the issues of food security, sustainable energy sources and fewer CO2 emissions, we need to look under the hood. Wageningen researchers are trying to improve photosynthesis so as to cultivate more efficient crops.
After years of drought and scorching heat, suddenly there is a deluge in the desert. Plants shoot up apparently out of nowhere, swiftly flowering and dispersing their seed. “Many desert plants are able to develop so quickly thanks to their incredibly efficient photosynthesis,” says plant scientist and biochemist Dr René Klein Lankhorst of Wageningen University & Research (WUR). “We can learn a lot from them in agriculture.”
Sunlight is important in letting plants produce nutrients and energy. Photo: Shutterstock
Photosynthesis is the green engine in agriculture and the basis for the food chain. Chloroplasts in green plants capture sunlight and use its energy to convert water and carbon dioxide (CO2) in the atmosphere into sugars, which the plant uses to grow and flower. Oxygen is released as a waste product.
BETTER PHOTOSYNTHESIS MEANS HIGHER CROP YIELDS
If photosynthesis goes twice as fast, plant yields will double
But this process is not very efficient. Agricultural crops use only 0.5 to 1 per cent of the sunlight that falls on them. Louise Fresco, President of WUR’s Executive Board, even calls improving photosynthesis the holy grail of agriculture. “If photosynthesis were to go twice as fast, the yield from green plants would double,” explains Klein Lankhorst. And the larger the plant, the more CO2 it absorbs. That evokes pictures of fields full of fast-growing crops slurping up CO2.
“More efficient crops are good for the climate,” says Klein Lankhorst. “As you know, there is huge pressure on the global land area for food production. By 2050, we will have to feed 10 billion people. Incomes are rising so more and more land is also being taken up for the production of animal feed for the burgeoning livestock population.”
In photosynthesis, plants use sunlight to produce their own nutrients and energy. If we want to conserve the planet and its growing population sustainably, we will need plants to produce far more food, energy and usable biomass than they do now.
FARMLAND WILL BE USED MORE EFFICIENTLY
Klein Lankhorst continues: “At the same time, we want to phase out fossil fuels — which are bad for the climate — and replace them with green fuels where necessary. Industry also needs to become part of the circular economy and to switch to sustainable, biobased inputs. So an estimated 30 per cent more land would be needed worldwide for producing green fuels and biobased raw materials.”
To make the figures add up without having to chop down the last remaining tropical rainforests, we will need to make much more efficient use of the available farmland, says Klein Lankhorst. Improving photosynthesis plays a key role here. Traditional plant breeding methods have helped global yields grow by 1 per cent per annum but many crops are now close to their ceiling. In countries such as China and South Korea, for instance, rice yields have not risen for ten years. European wheat yields have also hardly budged for a number of years. “It is high time for a new approach. Photosynthesis is the only yield property that has never been the target of breeding programmes.”
Some plants resume photosynthesizing much faster than others. Plant breeders can make use of that
Veins in a leaf. Photo: Shutterstock
CO2 REDUCTION THANKS TO SMART STORAGE
How can we increase photosynthesis rates without disrupting key processes in the plant?
To what extent can more efficient plants contribute directly to a better climate by taking more CO2 out of the air? “That’s not how it works,” says Klein Lankhorst. “Because when we eat the harvested products, we breathe out that same CO2. And CO2 is released too when the residues from the harvest lie in the field rotting. We call this the carbon cycle.”
It only really helps if you don’t use the plant-based biomass for food but instead remove it from the system for a longer period, for example in the form of bioplastics or green construction materials. “We are already experimenting with bio-asphalt, where bitumen from petroleum is replaced as a binding agent by lignin from wood. That reduces the use of fossil raw materials and you take the carbon sequestered in the lignin out of the carbon cycle.”
PHOTOSYNTHESIS PROCESS BROKEN DOWN INTO PARTS
Klein Lankhorst is the programme developer for the photosynthesis theme in Wageningen’s Plant Sciences Group, which has a large number of Wageningen experts working together on photosynthesis. “Photosynthesis is very complex. It is not the kind of plant property that you can improve just like that. Hundreds of genes are involved. Thanks to breakthroughs in genomics, bio-informatics and the automated screening of large numbers of plants for their external characteristics (known as phenotyping), we are now able to break down the photosynthesis process into different parts. Each of those parts can then be improved.”
How can photosynthesis be beefed up? The process has various weak points, each of which plant breeders could work on improving. For example, they could let plants deal more efficiently with fluctuating light conditions. In nature, the incident light is constantly changing: clouds pass in front of the sun, or leaves flutter in the wind and cast a fleeting shadow on the leaves below.
As the light becomes more intense, photosynthesis slows down. It is as if the plant puts on sunglasses. When the light dims, the ‘sunglasses’ come off again. However, some plant species have been found to resume their photosynthesis much more quickly and efficiently than others. Plant breeders can make use of that fact. In experiments in which tobacco plants were given three new genes that let photosynthesis resume more efficiently, plant yields increased by 15 per cent.
Left: The Phenovator, a robot for measuring photosynthesis. Photo: Laya Zindel
Right: The Phenovator produces very precise measurements of the photosynthesis of 1440 plants several times a day. Photo: Tom Theeuwen
CHANGES WITHOUT GENETIC MANIPULATION
A second option is the Rubisco enzyme, which plays a key role in photosynthesis. The enzyme is rather slipshod as one fifth of the time it grabs an oxygen molecule instead of a carbon dioxide molecule. Such mistakes result in toxins. Getting rid of them costs the plant needless energy, with slower growth as a result. In January this year, American scientists made the news when they managed to take genes from a pumpkin plant and insert them in the tobacco plant. The beefed-up tobacco plant was able to get rid of the toxins better, lost less energy and grew to become 40 per cent heavier.
Improvements in the photosynthesis boosted tobacco plant yields. Photo: Shutterstock
“Here in Wageningen, we are now busy investigating whether you can also cross-breed similar genes into crops using modern plant breeding techniques without the need for genetic manipulation”, Klein Lankhorst explains. “Wageningen researchers are experts not only in the physics of photosynthesis but also in biodiversity and plant physiology. This interdisciplinary approach means we are able to incorporate a variety of perspectives in our photosynthesis research. Many of the genes involved in photosynthesis are now known. A key question now is which genes we can best use to increase photosynthesis rates without disrupting other important processes in the plant.”
THE EUROPEAN RESEARCH PROGRAMME PHOTOSYNTHESIS 2.0
While a field of potatoes only converts at most 0.5 per cent of the incident sunlight into biomass, some wild plants will use 4 to 5 per cent of the sunlight for their photosynthesis. Examples are desert plants, which germinate, flower and produce seeds in the space of a few weeks, as well as shortpod mustard, a wild relative of rape. Question 1: What is their secret? Question 2: How can agriculture make use of that? To get answers to those questions, scientists are looking hard for EU funding via the framework programmes. The Photosynthesis 2.0 initiative was started in 2016. The intention is to get a major European research programme up and running, probably with over 100 partners. As a first step, the European Commission has earmarked three million euros for drawing up a roadmap: what shape should such a huge programme take and what does society think of this endeavour? Klein Lankhorst is coordinating the compilation of the roadmap.
EXTREMELY PRECISE MEASUREMENTS OF LIGHT CAPTURE
The Wageningen researcher Emilie Wientjes was recently given a Vidi grant by the Netherlands Organization for Scientific Research (NWO) to study the efficiency of photosynthesis. “The energy in light is meant to drive photosynthesis but some of it is always lost through fluorescence. The more efficiently a leaf uses light, the faster the fluorescence disappears,” explains Wientjes. “We can use short laser pulses to determine precisely when the leaf captures light and we can measure to a few trillionths of a second how fast this light is used for photosynthesis. This will let us compare the natural variation between plant species in the efficiency of light capture, for example.” This knowledge could help scientists improve photosynthesis in crops.