Stomata

Abstract

Stomata are cellular pores on the aerial surface of plants. They open to allow the uptake of carbon dioxide and close to limit water loss, and thus are essential for plant growth and homeostasis. Stomata, which are common to almost all land plants, represent a critical evolutionary innovation of early land plants. A stoma is formed by a pair of guard cells, which are the final products of a specialised cell lineage. Mature guard cells mediate stomatal opening and closing by regulating the fluxes of ions, and hence water, in and out of the cells. To maximise fitness in the ever‐changing environments, both the development and movement of the stomatal guard cells are highly responsive to diverse environmental signals. These stomatal responses will likely play a key role in plant adaptation and agricultural production in the face of global climate change.

Key Concepts

  • Stomata are small adjustable pores on the leave surface that enable gas exchange.
  • Open stomata allow the uptake of carbon dioxide for photosynthesis, while closed stomata prevent excessive loss of water.
  • During evolution, stomata enabled plants to survive the drier environments on land.
  • The development of stomata involves several cell fate transition steps and is driven by cell‐type‐specific master transcription factors.
  • Stomatal movements are controlled by a pair of guard cells, which change their cell volume through ion‐driven uptake and release of water.
  • Environmental signals regulate both the development and movement of guard cells, allowing plants to best adapt to their surroundings.
  • Stomata are likely to be important in the adaption of plants to global warming.

Keywords: stomata; guard cells; stomatal development; ion transport; environmental regulation; global climate change; agriculture

Figure 1. Structure and distribution of stomata in dicots and monocots. Images of stomata obtained by scanning electron microscopy (a) or transmitted light microscopy (b, c and d). (a) Leaf surface of Nicotiana alamata consisting of jigsaw‐like epidermal pavement cells (marked #1) surrounding stomatal pores (#2) and leaf hairs (trichomes, #3). Reproduced from http://remf.dartmouth.edu/images/botanicalLeafSEM/index.html. (b) Stoma of tobacco (N. tabacum cv. Samsun). The pore (marked #1) is surrounded by two kidney‐shaped guard cells (#2). Note the round structure in the middle of the cells which contains the nucleus and chloroplasts at the periphery of the cells. (c) Stomatal complex of maize (Zea mays). The pore (marked #1) is surrounded by two dumbbell‐shaped guard cells (#2), which are in turn flanked by two companion cells (#3). Reproduced with permission from P Mumm, University of Würzburg. (d) Leaf surface of rice (Oryza sativa). The monocot stomata (pseudo‐coloured in green), unlike those in dicots, are distributed linearly along specific cell files.
Figure 2. Simplified diagram of the development of stomatal guard cells in Arabidopsis. The stomatal lineage starts when a protodermal cell (grey) commits and becomes a meristemoid mother cell (pink), which divides asymmetrically and produces a smaller daughter cell, the meristemoid (red), and a larger daughter cell, the stomatal lineage ground cell (SLGC; beige). Meristemoids can self‐renew, and SLGCs can also divide asymmetrically and yield more meristemoids (not shown). A meristemoid will eventually develop into a guard mother cell (orange), which undergoes a single symmetrical division to form a pair of guard cells (green). Three closely related bHLH transcription factors, SPEECHLESS (SPCH), MUTE and FAMA, which form heterodimer with either ICE1/SCRM or its close homologue SCRM2 (not shown), act successively as the master regulators in driving the cell fate transition steps.
Figure 3. Three‐dimensional representation of guard cells in closed (left) and open (right) stomata. Guard cells force stomatal opening by an osmotically driven increase of their cell volume. The increase in volume causes the guard cells to bend and change the shape of their cross section from a flat oval to a circle. Reproduced with permission from Roelfsema and Hedrich 2005 © John Wiley and Sons Ltd.
Figure 4. Simplified diagram of the ion transport‐driven stomatal movement. During stomatal opening, the proton pumps (H+‐ATPase) on the plasma membrane transport protons out of the guard cells, causing an H+ gradient as well as an electrical charge difference (membrane potential) across the membrane. The membrane potential drives an influx of potassium (K+) through K+‐uptake ion channels. Anions, such as nitrate (NO3) and chloride (Cl), are taken up into the cytosol through cotransport with H+. The ions are further transported into the vacuole, in which the V‐type H+‐ATPase (not shown) generates an H+ gradient and vacuolar membrane potential. Anion transport into the vacuole, through anion channels, is supported by the electrical potential, whereas K+ uptake requires H+ cotransport (not shown). Stomatal closure is associated with the efflux of K+ from the vacuole, which also degrades the membrane potential. Owing to the depolarised vacuolar membrane, potential anions (A) are released into the cytosol. At the plasma membrane, the activation of anion channels (S‐ and R‐type) leads to an efflux of anions into the guard cell wall and depolarisation at the plasma membrane. The depolarised plasma membrane induces the export of K+ through the K+‐efflux channels.
Figure 5. Simplified model of the Abscisic Acid (ABA) signal transduction pathway in guard cells. The stress hormone ABA is perceived by the PYR/PYL/RCAR family of ABA receptors. The activated receptors repress the PP2C phosphatases, which are negative regulator of ABA signalling. Their repression leads to activation of SnRKs, including OST1, which induces the ion transporters SLAC1 but represses KAT1. The action of OST1, together with other cellular events (see text), results in stomatal closure. The core ABA signalling pathway is highlighted on the grey background.
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Further Reading

Vatén A and Bergmann DC (2012) Mechanisms of stomatal development: an evolutionary view. EvoDevo 3: 11.

Willmer C and Fricker M (1996) Stomata, 2nd edn. London, UK: Chapman & Hall.

Zeiger E , Farquhar GD and Cowan IR (eds) (1987) Stomatal Function. Stanford, CA: Stanford University Press.

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Lau, On Sun(Mar 2017) Stomata. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002075.pub3]