Estonian-led groups of researchers made a breakthrough in understanding how CO2 controls the water use and growth of plants. This information will contribute to the development of knowledge-based agriculture, and makes it possible to breed water-saving crops, which can keep up high yields also in environments with increased CO2 concentration.
Plants are fascinating organisms – in the presence of light and water they will use atmospheric carbon dioxide (CO2) to make food, flavour, and many raw materials for our everyday life.
In global terms, the availability of fresh water is the key factor that limits plant growth and crop yield. But still very little is known about how CO2 regulates water use in plants. This is where four groups led by researchers of the University of Tartu made significant contributions in 2016. The findings are important for breeding water-saving crops that promise high yields also in environments with high CO2 levels.
Last year the Earth’s atmospheric CO2 concentration passed 400 parts per million, while there were roughly 300 ppm just one hundred years ago. Although CO2, the starting matter for sugars, is not all bad, such a sharp increase affects the climate as well as plants. However, the mechanistic understanding of how CO2 affects plant water use – the basic pillar of sustainable food production – remains obscure.
Studies led by researchers of the University of Tartu’s Institute of Technology identified the mechanism plants use to manage their water use and growth in the changing levels of atmospheric CO2. Studies were published in PLOS Biology, and Plant Cell. The latter study was selected as one of the top stories of the publication in 2016.
According to doctoral student Hanna Hõrak, first author of the Plant Cell paper, plants breathe and sweat through stomata, microscopic openings on the surfaces of leaves and stems. “In the morning, the stomata open, atmospheric CO2, the supply for the photosynthetic machinery, will enter the plant, and the production of sugars will start. In the course of this, oxygen, the basis of all animal life, is released,” Hõrak explained.
“The interior of plants is wet, thus when the stomatal pores open, water evaporates from the leaf into the drier atmosphere via a process called transpiration. During drought, plants may wither and die, and to avoid this, plants close their stomata and restrict their transpiration. Plants can also sense the concentration of CO2, and with it they can balance its availability for photosynthesis and loss of water by evaporation.”
Guard cells, CO2 and plant water use
The opening and closing of stomata is regulated by special cells, guard cells that form the stomatal pore. As these cells swell, stomata will open and, vice versa, close as the cells shrink. This is driven by accumulation and release of ions and water in the guard cells. Earlier research of the group, led by Professor Hannes Kollist and others, had shown that for CO2–induced closure of the stomata, a special ion channel has to be activated. “However, there are still major gaps in understanding molecular switches that control activation of this channel in response to changes in CO2 concentration,” Kollist said.
The discovery of the novel mechanism resulted from the work of four projects that started independently in different laboratories on different continents. Cooperation with Finnish, American, Chinese, and German research groups played an important role and made it possible to apply different methods to control emerging hypotheses.
Doctoral student Kadri Tõldsepp, who was in charge of her group’s biochemical experiments, explained that they were the first to demonstrate the significance of a certain type of regulatory protein (MAP kinases – mitogen-activated protein kinases) in stomatal response to CO2. These regulators were found to control the function of another protein, HT1, which is the key regulator of CO2-sensing in guard cells. “Such a control mechanism makes it possible for regulatory proteins to activate ion channels, and to cause stomatal closure if the CO2 concentration is high,” Tõldsepp said.
Studying stress tolerance of various forms of thale cress paved the way to discovery
Asking what the genetic details were that caused higher sensitivity of some natural accessions and mutants of thale cress to the air pollutant ozone led to the identification of the mechanism.
PhD student Liina Jakobson, first author of the PLOS Biology paper, studied why the stomatal pores of the ozone-sensitive thale cress as it naturally grows on the Cape Verde Islands were more open, and did not respond normally to changes in CO2 levels. “We wanted to understand what the genetic differences are that make thale cress plants from the Cape Verde Islands more sensitive to environmental stress, and we were particularly excited when it turned out that these differences are caused by natural mutations in genes that regulate plant water use at different CO2 concentrations,” Jakobson explained.
Professor Hannes Kollist added that in the light of increasing levels of CO2 and the changing climate, it was very important to understand the molecular basis of processes in the biosphere. “It is particularly important to study these processes in plants – organisms that provide us with food and oxygen, and are the source for a lot of chemicals used as pharmaceuticals, flavouring substances, and building materials,” the professor said.
Research that aims to apply the discovered mechanism in crops, for example in tomato and rice, has already begun. Further experiments addressing the structural features of the studied protein-protein interactions are already ongoing, hopefully leading to the practical application of breeding plants that grow better in stressful environmental conditions, and developing compounds that would enable us to enhance the efficiency of plants’ water management.
Editor: Editor: Dario Cavegn