Guard cell

Paired cells that control the stomatal aperture

Guard cells are specialized plant cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore. The stomatal pores are largest when water is freely available and the guard cells become turgid, and closed when water availability is critically low and the guard cells become flaccid. Photosynthesis depends on the diffusion of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), produced as a byproduct of photosynthesis, exits the plant via the stomata. When the stomata are open, water is lost by evaporation and must be replaced via the transpiration stream, with water taken up by the roots. Plants must balance the amount of CO2 absorbed from the air with the water loss through the stomatal pores, and this is achieved by both active and passive control of guard cell turgor pressure and stomatal pore size.

Guard cells are cells surrounding each stoma. They help to regulate the rate of transpiration by opening and closing the stomata. Light is the main trigger for the opening or closing. Each guard cell has a relatively thick and thinner cuticle on the pore side and a thin one opposite it. As water enters the cell, the thin side bulges outward like a balloon and draws the thick side along with it, forming a crescent; the combined crescents form the opening of the pore. Guard cells contain phototropin proteins which are serine and threonine kinases with blue light photoreceptor activity. Phototrophins contain two light, oxygen, and voltage sensor (LOV) domains, and are part of the PAS domain superfamily. The phototropins trigger many responses such as phototropism, chloroplast movement and leaf expansion as well as stomatal opening. Serine and threonine are then phosphorylated within the protein, which induces H+ ATPase activity.) and differently oriented cellulose microfibers, causing them to bend outward when they are turgid, which in turn, causes stomata to open. Stomata close when there is an osmotic loss of water, occurring from the loss of K+ to neighboring cells, mainly potassium (K+) ions.

Water stress (drought and salt stress) is one of the major environmental problems causing severe losses in agriculture and in nature. 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 (ABA), is produced in response to drought. A major type of ABA receptor has been identified. 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 is regulated could lead to the development of plants with improved avoidance or slowing of desiccation and better water use efficiency.

Ion uptake into guard cells causes stomatal opening: The opening of gas exchange pores requires the uptake of potassium ions into guard cells. Potassium channels and pumps have been identified and shown to function in the uptake of ions and opening of stomatal apertures. 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: 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. At least two major types of anion channels have been characterized in the plasma membrane: S type anion channels and R type anion channels.

Vacuoles are large intracellular storage organelles 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. Vascuolar 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. SV channels have been shown to function as cation channels that are permeable to Ca2+ ions,

Guard cells control gas exchange and ion exchange through opening and closing. K+ is one ion that flows both into and out of the cell, causing a positive charge to develop. Malate is one of the main anions used to counteract this positive charge, and it is moved through the AtALMT6 ion channel. AtALMT6 is an aluminum activated malate transporter that is found in guard cells, specifically in the vacuoles. This transport channel was found to cause either an influx or efflux of malate depending on the concentrations of calcium. 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.

During the development of plant leaves, the specialized guard cells differentiate from "guard mother cells". The density of the stomatal pores in leaves is regulated by environmental signals, including increasing atmospheric CO2 concentration, which reduces the density of stomatal pores in the surface of leaves in many plant species by presently unknown mechanisms. The genetics of stomatal development can be directly studied by imaging of the leaf epidermis using a microscope. Several major control proteins that function in a pathway mediating the development of guard cells and the stomatal pores have been identified.