If soil dries out, plants have to restrict their water consumption. To do this, they produce the hormone abscisic acid in their roots and send it along the vascular system to the leaves. Once there, the hormone works in conjunction with nitrate to ensure that the leaf pores, or stomata, close and thereby prevent the loss of any more valuable water through evaporation.
The main way in which plants lose water is through their stomata. Yet, these pores are indispensable, as without them plants would be unable to exchange carbon dioxide and other gases with the environment and, as a result, photosynthesis would not be possible, nor would growth. Plants therefore have to regulate the extent to which their stomata open to suit their needs.
Guard cells regulate leaf pore width
Whether stomata are closed or open depends on bean-shaped guard cells. These are found in the epidermis of the leaves, always with two lying opposite each other. When the guard cells are bulging with ions and water, they part and open the pore. As they go limp, the pore becomes smaller and smaller until finally this water vapor valve is completely closed.
During times of drought, the guard cells become the destination for the hormone abscisic acid. “Upon its arrival there, it binds to its receptor, which, in turn, regulates the anion channel SLAC1 via two enzymes,” says Professor Rainer Hedrich. As a consequence, ions and water flow out from the guard cells. This reduces the pressure somewhat, causing the pores to close and limiting the evaporation of water from the leaves.
Publication in “Science Signaling”
New findings relating to this regulatory mechanism are presented by Professor Rainer Hedrich and Dr. Dietmar Geiger from the Department of Molecular Plant Physiology and Biophysics at the University of Würzburg in the current issue of the renowned journal “Science Signaling”. They describe in detail the nature of the anion channel SLAC1 as well as that of the newly discovered anion channel SLAH3. What is special about this channel is that it requires both abscisic acid and nitrate for its activation.
Nitrate as a co-transmitter with abscisic acid
Nitrate is known mainly as a component of agricultural fertilizer. Plants draw nitrate from the soil, transport it to the leaves, and use it there as a source of nitrogen for protein production. This process really gains momentum when photosynthesis is at its peak because it supplies the carbon structure used by the plant as a basic building block for proteins. When photosynthesis is working well, the leaves are also capable of processing a lot of nitrate.
Nitrate finds its way from the roots to the leaves dissolved in water. The plant can tailor its nitrate replenishment to meet its needs by increasing or restricting the flow of water. It does this by opening or closing its valves, enabling it to regulate the pull that is exerted by water evaporating via the leaves right down into the roots.
Guard cells measure nitrate content in the leaf
“For this regulation to work, the guard cells must be capable of measuring the nitrate content in their vicinity,” says Professor Hedrich. If the nitrate content in the leaf rises sharply, this signals to the plant that it cannot process any more nitrate for the time being because photosynthesis is not working optimally. It is able to do without carbon dioxide at this time, and the stomata close, thereby conserving water. In this case, nitrate acts as an antiperspirant.
The Würzburg biophysicists have identified the anion channel SLAH3 as the sensor for the process. If the nitrate content in the guard cells exceeds a certain threshold and, at the same time, there is a critical quantity of abscisic acid, the channel is activated and sets the closure of the leaf pores in motion.
Anion channel as a multisensory regulator
Hedrich draws the following conclusion: “This anion channel is a multisensory interface. It measures the ratio of water consumption, nitrate content, and photosynthetic performance of the plant, integrates the measurements, and regulates the aperture of the stomata in response.” It enables the plant to keep the loss of water down to a minimum during times of drought without excessively restricting the photosynthetic performance at the same time.
"Stomatal Closure by Fast Abscisic Acid Signaling Is Mediated by the Guard Cell Anion Channel SLAH3 and the Receptor RCAR1”, Dietmar Geiger, Tobias Maierhofer, Khaled A.S. AL-Rasheid, Sönke Scherzer, Patrick Mumm, Anja Liese, Peter Ache, Christian Wellmann, Irene Marten, Erwin Grill, Tina Romeis und Rainer Hedrich, Science Signaling, 17. Mai 2011, Vol. 4, Issue 173, DOI: 10.1126/scisignal.2001346
Prof. Dr. Rainer Hedrich, T +49 (0)931 31-86100, firstname.lastname@example.org
Dr. Dietmar Geiger, T +49 (0)931 31-86105, email@example.com
Robert Emmerich | Uni Würzburg
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