Guard cell
Encyclopedia
Guard cells are specialized cells located in the Leaf
epidermis
of plants. Pairs of guard cells surround tiny stoma
tal airway pores (Figure 1). These tiny holes in the surface of leaves are necessary for gas exchange into and out of the plant; carbon dioxide
(CO2) enters the plant allowing the carbon fixation reactions of photosynthesis
to occur. Oxygen
(O2) exits the plant as a byproduct of photosynthesis. The opening and closing of the stoma
tal gas exchange holes is regulated by swelling and shrinking of the two surrounding guard cells (Figure 1). Due to the presence of the stoma
tal pores on plant leaf surfaces, water evaporates through the stomatal openings causing plants to lose water. Over 95% of water loss from plants can occur by evaporation (transpiration
) through the stomatal pores. Therefore, it is important for plants to be able to balance the amount of CO2 being brought into the plant with the amount of water escaping as a result of the open stomatal pores. Hence, the guard cells are the gate keepers of the plants ability to take in CO2 from the atmosphere for photosynthesis – while regulating how much water plants lose to the atmosphere. Opening and closure of the stomatal pore (Figure 1) is mediated by changes in the turgor pressure of the two guard cells. The turgor pressure of guard cells is controlled by movements of large quantities of ions and sugars into and out of the guard cells. When guard cells take up these solutes, the water potential
(Ψ) inside the cells decreases, causing osmotic
water flow into the guard cells. This leads to a turgor pressure increase causing swelling of the guard cells and the stomatal pores open (Figure 2). The ions that are taken up by guard cells are mainly potassium
(K+) ions and chloride
(Cl-) ions. In addition guard cells take up sugars that also contribute to opening of the stomatal pores.
of plants is mediated by several mechanisms that work together, including stabilizing and protecting the plant from damage caused by desiccation and also controlling how much water plants lose through the stomatal pores during drought. A plant hormone, abscisic acid
(ABA), is produced in response to drought. A major type of ABA receptor has been identified. Future research is needed to test if these receptors can be used to engineer drought tolerance in plants. The plant hormone ABA causes the stomatal pores to close in response to drought, which reduces plant water loss via transpiration to the atmosphere and allows plants to avoid or slow down water loss during droughts. The use of drought tolerant crop
plants would lead to a reduction in crop losses during droughts. Since guard cells control water loss of plants, the investigation on how stomatal opening and closure are regulated could lead to the development of plants with improved avoidance or slowing of desiccation and better water use efficiency.
s and pumps have been identified and shown to function in the uptake of ions and opening of stomatal apertures (Figure 2). Ion release from guard cells causes stomatal pore closing: Other ion channels have been identified that mediate release of ions from guard cells, which results in osmotic water efflux from guard cells due to osmosis
, shrinking of the guard cells, and closing of stomatal pores (Figures 1 and 2). Specialized potassium efflux channels participate in mediating release of potassium from guard cells. Anion channels were identified as important controllers of stomatal closing. Anion channels have several major functions in controlling stomatal closing: (a) They allow release of anions, such as chloride and malate
from guard cells, which is needed for stomatal closing. (b) Anion channels are activated by signals that cause stomatal closing, for example by intracellular calcium
and ABA. The resulting release of negatively charged anions from guard cells results in an electrical shift of the membrane to more positive voltages (depolarization
) at the intracellular surface of the guard cell plasma membrane. This electrical depolarization of guard cells leads to activation of the outward potassium channels and the release of potassium through these channels (Figure 2). At least two major types of anion channels have been characterized in the plasma membrane: S-type anion channels and R-type anion channels.
s are large intracellular storage organelle
s in plants cells. In addition to the ion channels in the plasma membrane, vacuolar ion channels have important functions in regulation of stomatal opening and closure because vacuoles can occupy up to 90% of guard cell’s volume. Therefore, a majority of ions are released from vacuoles when stomata are closed. Vacuolar K+ (VK) channels and fast vacuolar channels can mediate K+ release from vacuoles. Vacuolar K+ (VK) channels are activated by elevation in the intracellular calcium concentration. Another type of calcium-activated channel, is the slow vacuolar (SV) channel. SV channels have been shown to function as cation channels that are permeable to Ca2+ ions, but their exact functions are not yet known in plants.
, the investigation of signal processing in single guard cells has become open to the power of genetics
. Cytosolic and nuclear proteins and chemical messengers that function in stomatal movements have been identified that mediate the transduction of environmental signals thus controlling CO2 intake into plants and plant water loss. Research on guard cell signal transduction mechanisms is producing an understanding of how plants can improve their response to drought stress by reducing plant water loss.
Guard cells also provide an excellent model for basic studies on how a cell integrates numerous kinds of input signals to produce a response (stomatal opening or closing). These responses require coordination of numerous cell biological processes in guard cells, including signal reception, ion channel and pump regulation, membrane trafficking, transcription
, cytoskeletal
rearrangements and more. A challenge for future research is to assign the functions of some of the identified proteins to these diverse cell biological processes.
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
epidermis
Epidermis (botany)
The epidermis is a single-layered group of cells that covers plants' leaves, flowers, roots and stems. It forms a boundary between the plant and the external environment. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds,...
of plants. Pairs of guard cells surround tiny stoma
Stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...
tal airway pores (Figure 1). These tiny holes in the surface of leaves are necessary for gas exchange into and out of the plant; carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(CO2) enters the plant allowing the carbon fixation reactions of photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
to occur. Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O2) exits the plant as a byproduct of photosynthesis. The opening and closing of the stoma
Stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...
tal gas exchange holes is regulated by swelling and shrinking of the two surrounding guard cells (Figure 1). Due to the presence of the stoma
Stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...
tal pores on plant leaf surfaces, water evaporates through the stomatal openings causing plants to lose water. Over 95% of water loss from plants can occur by evaporation (transpiration
Transpiration
Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...
) through the stomatal pores. Therefore, it is important for plants to be able to balance the amount of CO2 being brought into the plant with the amount of water escaping as a result of the open stomatal pores. Hence, the guard cells are the gate keepers of the plants ability to take in CO2 from the atmosphere for photosynthesis – while regulating how much water plants lose to the atmosphere. Opening and closure of the stomatal pore (Figure 1) is mediated by changes in the turgor pressure of the two guard cells. The turgor pressure of guard cells is controlled by movements of large quantities of ions and sugars into and out of the guard cells. When guard cells take up these solutes, the water potential
Water potential
Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as surface tension...
(Ψ) inside the cells decreases, causing osmotic
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
water flow into the guard cells. This leads to a turgor pressure increase causing swelling of the guard cells and the stomatal pores open (Figure 2). The ions that are taken up by guard cells are mainly potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
(K+) ions and chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
(Cl-) ions. In addition guard cells take up sugars that also contribute to opening of the stomatal pores.
Water loss and water use efficiency
Water stress (drought and salt stress) is one of the major environmental problems causing severe losses in agriculture and in nature. Drought toleranceDrought tolerance
Drought tolerance refers to the degree to which a plant is adapted to arid or drought conditions. Desiccation tolerance is an extreme degree of drought tolerance...
of plants is mediated by several mechanisms that work together, including stabilizing and protecting the plant from damage caused by desiccation and also controlling how much water plants lose through the stomatal pores during drought. A plant hormone, abscisic acid
Abscisic acid
Abscisic acid , also known as abscisin II and dormin, is a plant hormone. ABA functions in many plant developmental processes, including bud dormancy. It is degraded by the enzyme -abscisic acid 8'-hydroxylase.-Function:...
(ABA), is produced in response to drought. A major type of ABA receptor has been identified. Future research is needed to test if these receptors can be used to engineer drought tolerance in plants. The plant hormone ABA causes the stomatal pores to close in response to drought, which reduces plant water loss via transpiration to the atmosphere and allows plants to avoid or slow down water loss during droughts. The use of drought tolerant crop
Crop
Crop may refer to:* Crop, a plant grown and harvested for agricultural use* Crop , part of the alimentary tract of some animals* Crop , a modified whip used in horseback riding or disciplining humans...
plants would lead to a reduction in crop losses during droughts. Since guard cells control water loss of plants, the investigation on how stomatal opening and closure are regulated could lead to the development of plants with improved avoidance or slowing of desiccation and better water use efficiency.
Ion uptake and release
Ion uptake into guard cells causes stomatal opening: The opening of gas exchange pores requires the uptake of potassium ions into guard cells. Potassium channelPotassium channel
In the field of cell biology, potassium channels are the most widely distributed type of ion channel and are found in virtually all living organisms. They form potassium-selective pores that span cell membranes...
s and pumps have been identified and shown to function in the uptake of ions and opening of stomatal apertures (Figure 2). Ion release from guard cells causes stomatal pore closing: Other ion channels have been identified that mediate release of ions from guard cells, which results in osmotic water efflux from guard cells due to osmosis
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
, shrinking of the guard cells, and closing of stomatal pores (Figures 1 and 2). Specialized potassium efflux channels participate in mediating release of potassium from guard cells. Anion channels were identified as important controllers of stomatal closing. Anion channels have several major functions in controlling stomatal closing: (a) They allow release of anions, such as chloride and malate
Malate
Malate is the ionized form of malic acid. It is an important chemical compound in biochemistry. In the C4 carbon fixation process, malate is a source of CO2 in the Calvin cycle....
from guard cells, which is needed for stomatal closing. (b) Anion channels are activated by signals that cause stomatal closing, for example by intracellular calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
and ABA. The resulting release of negatively charged anions from guard cells results in an electrical shift of the membrane to more positive voltages (depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
) at the intracellular surface of the guard cell plasma membrane. This electrical depolarization of guard cells leads to activation of the outward potassium channels and the release of potassium through these channels (Figure 2). At least two major types of anion channels have been characterized in the plasma membrane: S-type anion channels and R-type anion channels.
Vacuolar ion transport
VacuoleVacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
s are large intracellular storage organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s in plants cells. In addition to the ion channels in the plasma membrane, vacuolar ion channels have important functions in regulation of stomatal opening and closure because vacuoles can occupy up to 90% of guard cell’s volume. Therefore, a majority of ions are released from vacuoles when stomata are closed. Vacuolar K+ (VK) channels and fast vacuolar channels can mediate K+ release from vacuoles. Vacuolar K+ (VK) channels are activated by elevation in the intracellular calcium concentration. Another type of calcium-activated channel, is the slow vacuolar (SV) channel. SV channels have been shown to function as cation channels that are permeable to Ca2+ ions, but their exact functions are not yet known in plants.
Signal transduction
Guard cells perceive and process environmental and endogenous stimuli such as light, humidity, CO2, temperature, drought, and plant hormones to trigger cellular responses resulting in stomatal opening or closure. These signal transduction pathways determine for example how quickly a plant will lose water during a drought period. Guard cells have become a model for single cell signaling. Using Arabidopsis thalianaArabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...
, the investigation of signal processing in single guard cells has become open to the power of genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
. Cytosolic and nuclear proteins and chemical messengers that function in stomatal movements have been identified that mediate the transduction of environmental signals thus controlling CO2 intake into plants and plant water loss. Research on guard cell signal transduction mechanisms is producing an understanding of how plants can improve their response to drought stress by reducing plant water loss.
Guard cells also provide an excellent model for basic studies on how a cell integrates numerous kinds of input signals to produce a response (stomatal opening or closing). These responses require coordination of numerous cell biological processes in guard cells, including signal reception, ion channel and pump regulation, membrane trafficking, transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
, cytoskeletal
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
rearrangements and more. A challenge for future research is to assign the functions of some of the identified proteins to these diverse cell biological processes.