Researchers made a breakthrough in understanding how CO2 regulates water use in plants

Last year the Earth’s atmospheric CO₂ concentration passed 400 parts per million, whereas it was roughly 300 ppm only one hundred years ago. Although CO₂, the starting matter for sugars, is not all bad, such a sharp increase affects climate and plants. However, the mechanistic understanding on how CO₂ affects plant water use – the basic pillar of sustainable food production – remains obscure.

Studies led by the researchers from the University of Tartu, Institute of Technology identified the mechanism that plants use to manage their water use and growth in the changing levels of atmospheric CO₂. The results were published in PLOS Biology and Plant Cell, whereas the latter study was selected as one of the top stories published in 2016.

According to PhD 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 and atmospheric CO₂, the supply for the photosynthetic machinery, will enter the plant and 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 by that balance the availability of this substrate for photosynthesis and loss of water by evaporation.”

Guard cells, CO₂ and plant water use

The opening and closure 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 into 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,” said Kollist.

The discovery of the novel mechanism resulted from four projects that started independently from each other in different laboratories in different continents. Cooperation with Finnish, US, Chinese and German research groups had an important role and enabled to apply different methods to control emerging hypotheses. Doctoral student Kadri Tõldsepp who was in charge of biochemical experiments explained that they were the first to demonstrate the significance of a certain type of regulatory proteins (MAP kinases – mitogen-activated protein kinases) in stomatal response to CO₂. These regulators were found to control the function of another protein, HT1 that 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,” said Tõldsepp.

Studying stress tolerance of various forms of thale cress (Arabidopsis thaliana) paved the way to the discovery

Asking what are the genetic details that cause higher sensitivity to the air pollutant ozone of some natural accessions and mutants of thale cress led to the identification of the novel mechanism.

For example, PhD student Liina Jakobson, first author of the PLOS Biology paper, studied why the stomatal pores of the ozone-sensitive thale cress naturally growing 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 Cape Verde Islands more sensitive to environmental stress and we became 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,” explained Jakobson.

Professor Hannes Kollist added that in the light of increasing levels of CO₂ and changing climate it is very important to understand the molecular basis of basic processes in the biosphere. „It is particularly important to study these processes in plants – organisms that provide us food and oxygen, and are the source for a lot of chemicals used as pharmaceuticals, flavouring substances and building materials.”

EXCERPT. Consortia led by Estonian researchers made a breakthrough in understanding how CO₂ controls plant water use and growth. This information contributes to the development of knowledge-based agriculture and enables to breed water-saving crops, which can give high yield also in environments of increased CO₂ concentration.

Follow-up research aiming to apply the discovered mechanism in crops, for example in tomato and rice, has already been initiated. Further experiments addressing the structural features of the studied protein-protein interactions are also ongoing. The practical purpose of this study is to breed plants that grow better in stressful environmental conditions and to develop compounds that would enable to enhance the efficiency of plants’ water management.