Guard Cell Metabolism


Guard cells are found in leaf epidermis as a pair of cells surrounding the stomatal pore. Stomatal aperture is regulated by the metabolism of guard cells according to the prevailing environmental condition. Guard cell metabolism is thus adapted to support stomatal movement, which demands a high control of osmolytes content within the cell as well as the need to balance the exchange of ions and metabolites between the cytoplasm and the apoplastic space. Guard cells have several characteristics of sink cells. This is probably an alternative given the low photosynthetic rate and the few number of chloroplasts found in these cells. The glycolysis, the tricarboxylic acid cycle and the degradation of starch and lipids seem to be activated in the light to support stomatal opening. This indicates that guard cell metabolism is differentially regulated compared to mesophyll cells. The particularities of guard cell metabolism are reviewed here.

Key Concepts

  • Stomatal aperture controls water use efficiency by modulating CO2 and H2O exchanges between the leaf and the atmosphere.
  • Changes in osmolyte concentration within guard cells regulate stomatal aperture.
  • Signals from mesophyll cells are known to alter stomatal movements.
  • Malate imported from mesophyll cells or produced within guard cells is involved on stomatal movement regulation.
  • Guard cells have several characteristics of sink cells.
  • It seems likely that photosynthesis in guard cell chloroplasts is mainly related to provide ATP and NADPH required for turgor control rather than producing carbohydrates.
  • Lipids, starch and sucrose are degraded during light‐induced stomatal opening.
  • Mitochondria metabolism is important to maintain guard cell energetic demands.

Keywords: glycolysis; malate; photosynthesis; respiration; stomatal movement; sucrose; tricarboxylic acid cycle

Figure 1. (a) Guard cell metabolism during day and night conditions. During the day, guard cell photosynthesis and mitochondria provide the ATP needed to maintain the activity of plasma membrane H+‐ATPases, which is important to create a H+ gradient across guard cell plasma membrane. This H+ gradient is dissipated by the transport of ions between the apoplastic space and the symplast of guard cells during stomatal opening. Phototropins, blue light receptors located in plasma membrane of guard cells, stimulate H+‐ATPase pump. Phototropin signalling is also a mechanism to stimulate starch and lipid (TAG) degradation. By contrast to mesophyll cells, in which respiration is light inhibited, guard cells seem to degrade starch, lipids and sucrose to sustain mitochondrial metabolism and glutamine biosynthesis. In this vein, guard cells have higher anaplerotic CO2 fixation mediated by PEPc, when compared to mesophyll cells. Given the low RubisCO‐mediated photosynthetic capacity of guard cells, sucrose may also be imported from neighbouring mesophyll cells. Therefore, sucrose is a putative signal from mesophyll cells that connects mesophyll photosynthetic process with stomatal movement. Another important metabolite for guard cell metabolism is malate, which can be imported from mesophyll cells and synthesised within guard cells from starch breakdown and PEPc fixation. Malate accumulation in the vacuole acts as a counter‐ion of K+. During the night, the content of guard cell osmolyte molecules such as malate and sugars seems to be converted into starch. Furthermore, sucrose accumulation at the apoplastic space and its degradation within guard cells has been reported as mechanisms to induce stomatal closure. Thus, the transport of sucrose mesophyll cells may also contribute to stomatal closure during stomatal closure conditions. However, these hypotheses have yet to be tested. (b) Diel course of starch synthesis and degradation in both mesophyll and guard cells. Whilst mesophyll cells have a linear pattern of starch synthesis in the light and starch degradation in the night, guard cells linearly degrade starch at the beginning of the diel course and accumulate starch in the rest of the day and during the beginning of the night. For this, different isoforms of AMY and BAM enzymes seems to be involved in starch degradation in mesophyll and guard cells (a).


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Further Reading

Daloso DM , dos Anjos L and Fernie AR (2016) Roles of sucrose in guard cell regulation. New Phytologist 211: 809–818.

Daloso DM , Medeiros DB , dos Anjos L , et al. (2017) Metabolism within the specialized guard cells of plants. New Phytologist 216: 1018–1033.

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Lima VF , Medeiros DB , Dos Anjos L , et al. (2018) Toward multifaceted roles of sucrose in the regulation of stomatal movement. Plant Signaling & Behavior 00: 1–8.

Misra BB , Acharya BR , Granot D , Assmann SM and Chen S (2015) The guard cell metabolome: functions in stomatal movement and global food security. Frontiers in Plant Science 6: 1–13.

Santelia D and Lawson T (2016) Rethinking guard cell metabolism. Plant Physiology 172: 1371–1392.

Santelia D and Lunn JE (2017) Transitory starch metabolism in guard cells: unique features for a unique function. Plant Physiology 174: 539–549.

Vavasseur A and Raghavendra AS (2005) Guard cell metabolism and CO2 sensing. New Phytologist 165: 665–682.

Zeiger E , Talbott LD , Frechilla S , Srivastava A and Zhu J (2002) The guard cell chloroplast: a perspective for the twenty‐first century. New Phytologist 153: 415–424.

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Cândido‐Sobrinho, Silvio A, Lima, Valéria F, and Daloso, Danilo M(Mar 2019) Guard Cell Metabolism. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0028343]