How do guard cells open and close stomata

Why and how do guard cells open and close stomata? Dec 25, Explanation: When guard cells open CO2 gets in for the process of photosynthesis to take place.. Related questions How do I determine the molecular shape of a molecule?

What is the lewis structure for co2? What is the lewis structure for hcn? How is vsepr used to classify molecules? What are the units used for the ideal gas law? How does Charle's law relate to breathing? They also contain lots of mitochondria , which release energy from glucose during respiration in order to provide the energy needed for active transport.

The absorbed water is transported through the roots to the rest of the plant where it is used for different purposes:. Stomata are tiny holes found in the underside of leaves.

They control water loss and gas exchange by opening and closing. They allow water vapour and oxygen out of the leaf and carbon dioxide into the leaf.

Plants growing in drier conditions tend to have small numbers of tiny stomata and only on their lower leaf surface, to save water loss. Most plants regulate the size of stomata with guard cells.

Each stoma is surrounded by a pair of sausage-shaped guard cells. Stomata are specialized epidermal structures that are essential for plant survival and productivity. These structures consist of two guard cells around a pore. Every stoma is a molecular valve that acts in gas exchange, mainly CO 2 and O 2 , which is necessary for optimal photosynthesis and which restricts water loss by modulating the transpiration level.

The genes that are involved in the process of stomata development were crucial for the movement of plants from water to land during evolution since stomata facilitated gas exchange while limiting desiccation.

The stomatal morphogenesis pathway has been identified in detail in Arabidopsis thaliana through investigations of many mutants with an impaired stomatal pattern or with other morphological defects in their epidermal cells. Cell distribution and differentiation require a balance between proliferation and cell specification in time and space.

The differentiation of stomata is preceded by at least one asymmetric as well as a few symmetric cell divisions. It requires three different types of precursor cells: the meristemoid mother cell MMC , meristemoids and the guard mother cell GMC. The last step of stomatal development is the differentiation of the stoma itself within the structure of the guard cells. The number and pattern of stomata varies in different organs in A.

A common feature of patterning is that stomata are separated from each other by at least one epidermal cell. This pattern ensures the presence of neighbor cells for ion exchange, which is necessary for the regulation of the aperture width. Recent research has shown that the mode of action of stomata depends on the integration of environmental and intracellular signals.

Many environmental factors such as CO 2 concentration, biotic and abiotic stresses, and additionally different plant hormones, can modulate stomatal reaction. For plants that encounter dehydration stress, the most essential factor is the ability of stomata to close and thus prevent excess water loss. Opening and closing is achieved by the swelling and shrinking of the guard cells, which is driven by ion exchange; cytoskeleton reorganization and metabolite production; the modulation of gene expression and the posttranslational modification of proteins reviewed in Kim et al.

Swelling of the guard cells results in stomata opening since the content of ions and osmolites within them makes them bigger and thus able to move away from each other making the stomatal aperture larger.

In contrast, closing is an opposite mechanism and results in the shrinking of the guard cells when the efflux of ions occurs.

Stomatal closure is the earliest plant response to water deficit Schroeder et al. This rapid reaction is regulated by a complex network of signaling pathways, in which the major and the best-known player, abscisic acid ABA , acts in concert with jasmonates JA , ethylene, auxins, and cytokinins Nemhauser et al.

Generally, ABA and JA are positive regulators of stomatal closure, while auxin and cytokinins are positive regulators of stomatal opening. The mode of action of ethylene is ambiguous because it can act as a positive or negative regulator, depending on the tissue and conditions Nemhauser et al.

This paper presents a comprehensive review of the genetic and molecular basis of stomata action under the control of phytohormones, particularly when response to drought stress is considered.

The guard cell turgor is dynamically adjusted to environmental conditions and hormonal signals in order to facilitate the proper gas exchange and prevent excessive water loss. Mature guard cells do not have plasmodesmata and for this reason most influx and efflux of solutes occurs via ion channels, transporters, and pumps that are localized in the plasma membrane PM. The action of ion channels, transporters, and pumps that are essential for stomatal function is well documented and supported by molecular studies involving mutants in the genes encoding these protein.

The last one is transported from the apoplast by a nitrate transporter AtNRT1. The importance of NO 3 - uptake was confirmed by an analysis of an Arabidopsis clh1 mutant. The stomatal apertures of the chl1 mutant were smaller than those of the wild-type when nitrate was supplied.

Furthermore, the chl1 mutant was drought tolerant Guo et al. Ions supplied into the guard cells together with water transported via aquaporins generate the turgor that are necessary to keep stomata open Figure 1 A.

Figure 1. Regulation of ion channels, pumps, and transporters localized in the plasma membrane of the guard cells during stomatal opening and closure. Ions supplied into the guard cells together with water transported via aquaporins generate the turgor that is needed to keep stomata opened. Taken together, the efflux of solutes from the guard cells leads to a reduced turgor and stomatal closure Figure 1 B.

Abscisic acid has been postulated as a main regulator of stomatal movements but its proper functioning depends on the appropriate level of biologically active ABA within the plant cells. The exception, not fully recognized yet, is ABA signal transduction pathway. Although ABA has been the focus of many research groups since the early 90s, there are still many questions in regards to the function of the proteins involved in ABA signaling, protein interactions or the impact of the components of signalosome on specific physiological responses.

Therefore, with the progress in studies on ABA signaling, the state of knowledge and the already known interaction web should be updated and verified. Abscisic acid is synthesized in the plastids and cytosol, mainly in the vascular parenchyma cells but also in the guard cells, through the cleavage of a C40 carotenoid precursor, followed by a two-step conversion of the intermediate xanthoxin into ABA via ABA-aldehyde Taylor et al. The pathway begins with isopentenyl pyrophosphate IPP , which is the biological isoprene unit and the precursor of all terpenoids, as well as many plant hormones.

The next step is the epoxidation of zeaxanthin and antheraxanthin into violaxanthin, which is then catalyzed by zeaxanthin epoxidase ZEP Marin et al. After a series of violaxanthin modifications that are controlled by the enzyme ABA4, violaxanthin is converted into 9-cis-epoxycarotenoid North et al.

Oxidative cleavage of the major epoxycarotenoid 9-cis-neoxanthin by the 9-cis-epoxycarotenoid dioxygenase NCED yields a C15 intermediate — xanthoxin Schwartz et al. This step is the last one that occurs in plastids. Xanthoxin is exported to the cytoplasm where a two-step reaction via ABA-aldehyde occurs. The appropriate level of active ABA is achieved not only through the biosynthesis and catabolism reactions performed by CYPA cytochrome P, family , subfamily A, polypeptide 1, 2, 3, 4 Kushiro et al.

ABA can be inactivated at the C-1 hydroxyl group by different chemical compounds that form various conjugates and that accumulate in vacuoles or in the apoplastic space Dietz et al. Lee et al. Their findings showed that ABA deconjugation plays a significant role in providing an ABA pool that allows plants to adjust to changing physiological and environmental conditions Figure 2 C.

Figure 2. Abscisic acid biosynthesis, catabolism, deconjugation, transport, and signaling. The balance between active and inactive ABA in the cell is achieved not only by the regulation of biosynthesis and catabolism but also by ABA conjugation and deconjugation. The ability of ABA to move long distances allows it to serve as a critical stress messenger. Kuromori et al. The gene encoding this transporter is mainly expressed in the guard cells.

ABA delivery to the guard cells promotes a cascade of reactions that lead to stomatal closure and that inhibit stomatal opening in order to prevent water loss Figure 2 D.

The ABA mode of action is linked to diurnal stomatal movements. It has been proposed that this link is based on both the molecular connections between ABA and circadian-clock pathways and on ABA biosynthesis and response to light reviewed in Tallman, Although several studies have been carried out linking the diurnal cycle with ABA signaling, there is still a need for further research that would clarify this connection.

It has been confirmed that the elevated ABA levels in the dark phase of the day are responsible for stomatal closure but, on the other hand, the molecular basis of the sensing CO 2 molecules by guard cells is still not well understood. This part of investigations still needs confirmation through the use of well-established methods. In darkness, stomata are closed.

During the night, elevated levels of CO 2 in the leaves were observed due to respiration. It has been proved that CO 2 has a positive effect on the stomatal closure process. Figure 3. The role of ABA in the diurnal regulation of stomatal movements.

As a result of these processes, elevated levels of ABA are present in the guard cells. In the dawn B , the first light promotes ABA catabolism processes and the level of ABA biosynthesis decreases, which leads to a decreased concentration of active ABA in the guard cells. The accumulation of sugars such as glucose, fructose and sucrose has been reported during the light phase of the day Talbott and Zeiger, In the midday, ABA is delivered to the apoplast around the guard cells through the xylem transpiration stream and the guard cells are regulated by steady-state ABA concentrations Figure 3 B.

In the evening, ABA biosynthesis outweighs the ABA catabolism in the guard cells, which leads to stomatal closure for review, see Tallman, Under drought stress conditions, ABA would reach a concentration high enough to cause ion efflux and an inhibition of sugar uptake by the guard cells in the midday, thus reducing the apertures for the rest of the day. In order to define the role of ABA in stress response, the action of several components of the pathways mentioned were tested in response to stress.

It has been shown that ABA concentrations can increase up to fold in response to drought stress Outlaw, A significant increase in NCED transcript levels can be detected within 15—30 min after leaf detachment or dehydration treatment Qin and Zeevaart, ; Thompson et al.

Cheng et al. An immunohistochemical analysis, using antibodies raised against AtNCED3, revealed that protein is accumulated in the leaf vascular parenchyma cells in response to drought stress. This was not detected in non-stressed conditions. These data indicate that drought-induced ABA biosynthesis occurs primarily in the vascular parenchyma cells and that vascular-derived ABA might trigger stomatal closure via the transport to the guard cells Endo et al.

AtNCED3 expression is upregulated by drought conditions across the species observed and decreases after rehydration. Drought, like the dark part of a diurnal cycle, also promotes the deconjugation of the ABA-glucose ester ABA-GE , which is stored in the vacuoles of leaf cells and also circulates in the plant Xu et al.

ABA delivery to the guard cells via ABCG transporters, such as AGCG22 that was mentioned above, promotes a cascade of reactions that lead to stomatal closure and that inhibit stomatal opening in order to prevent water loss Figure 2. Although its function is clear and confirmed by advanced molecular analysis, there is still a need to explain the impact of single components, such as kinases, on the regulation of the ion channels or the proton pump e.

On the other hand, the interaction between ABA regulated kinases SnRK2s and S-type anion channels, and the potassium inwardly rectifying channels, described below, has been well established and documented. Kinases are able to regulate the activity of ion channels and the proton pump. SLAC1 encodes the anion-conducting subunit of an S-type anion channel. Increased SLAC1 activity causes an efflux of anions which results in depolarization of the membrane as a consequence of phosphorylation by SnRK.

Together, these events lead to a reduction of the turgor, which results in stomatal closure in response to ABA as a major signal of drought Figure 3 A. Mori et al. Figure 4. ABA regulation of stomatal closure during drought stress. Together, these events lead to a decrease in the turgor of the guard cells and to stomatal closure under drought conditions.

The sequence of events, which is explained in detail in the main text and presented in green in the figure, is the core of the reactions that are induced or inhibited by different proteins that are activated by ABA. Blue arrows indicate activation, while red blunt ended lines indicate inhibition. Several of the genes involved in the processes described above and more are presented in Table 1 together with a description of mutant phenotypes.

Table 1. Selected genes involved in the regulation of stomatal movement under stress. Mutants in OST1 showed a wilty phenotype in water deficit conditions because of the impairment of stomatal closure and ROS production Mustilli et al. Exogenously applied NO donors triggered stomatal closure, whereas the application of an NO scavenger inhibited ABA-induced stomatal closure Neill et al.

There is some evidence that both H 2 O 2 and NO actions in the guard cells require calcium. Jasmonates are lipid-derived phytohormones that are involved in the regulation of vegetative and reproductive growth and the defense response against abiotic stress Katsir et al. JA biosynthesis is induced by stress conditions Wasternack, and many genes related to JA signaling are regulated by drought stress Huang et al.

The positive role of JA in the regulation of stomatal closure was observed in many studies Gehring et al. Similar to the ABA signaling pathway, JA signaling has been under intense investigation, particularly in relation to stress response. With the progress in research, many new components and their roles in JA-mediated stress response will be identified.

Although the interaction between ABA and JA signaling pathways in stomata function has been established, there is still a need for further investigation and identification of the nodes linking these two signaling pathways, such as CPK6, which is described below. Figure 5. Me-JA regulated stomatal closure during drought stress. Munemasa et al.

In coi1 coronatine insensitive 1 and cpk6 mutants, the activation of S-type anion channels was disrupted Munemasa et al. Geiger et al. Figure 6. Hormonal crosstalk in the regulation of stomatal closure and opening during water stress. The regulation of stomatal opening and closure is not only regulated by ABA, whose role is dominant, but also by other phytohormones. Jasmonates JA and brassinosteroids BR induce stomatal closure and inhibit stomatal opening under drought conditions, whereas the role of other hormones is ambiguous.

Cytokinins CK and auxins AUX in low physiological concentrations promote stomatal opening while in high concentrations, they are able to inhibit this process. The role of ethylene ET is the most curious. It can stimulate the closing and opening of the stomata. The details are described in the text.

Suhita et al. This suggests that jasmonate-induced changes in stomatal movements require endogenous ABA. In order to clarify this hypothesis, Hossain et al. In the wild-type, 0. Ethylene is a gaseous phytohormone that is involved in the regulation of numerous plant processes such as seed germination, root-hair growth, leaf and flower senescence and abscission, fruit ripening, nodulation, and plant responses to stresses Bleecker and Kende, It has been observed that ethylene can influence stomatal response via crosstalk with ABA; however, reports on its effect have been contradictory.

Ethylene has been linked to the promotion of both stomatal closure Pallas and Kays, and stomatal opening Madhavan et al. These contradictory effects need to be verified. One possible reason could be related to the methods used for stomatal observation that use detached leaves. Experiments with detached leaves do not always reflect the real response to stress or other applied factors in plants. Tanaka et al. This was clear evidence that ethylene repressed ABA action in stomatal closure.

In a drought stressed eto1 ethylene overproducer 1 mutant, stomata closed more slowly and were less sensitive to ABA than in the drought-treated wild type Tanaka et al. In order to elucidate the interaction between ethylene and ABA during stomatal response, epidermal peels from the wild-type and eto1 were treated with ABA, ethylene, and both phytohormones.

When ethylene was applied independently of ABA, it induced H 2 O 2 synthesis within 30 min of the treatment. When ethylene was applied to the ABA-pretreated wild-type epidermal peels, an inhibition of stomatal closure was observed Tanaka et al. Desikan et al. There have been some studies that revealed both increased and decreased ethylene production in response to drought stress. However, most of them described experiments with detached leaves, which may not reflect the response of intact plants under drought conditions Morgan et al.

Generally, elevated ABA concentrations limit the production of ethylene; and therefore a dramatic increase of ABA concentration during water stress probably causes a reduction in the production of ethylene Sharp, The physiological mechanism of ethylene inhibition of the ABA-mediated stomatal closure may be related to the function of ethylene as a factor that ensures a minimum carbon dioxide supply for photosynthesis by keeping stomata half-opened under the stress conditions Leung and Giraudat, ; Tanaka et al.

Auxins and cytokinins are major phytohormones that are involved in processes related to plant growth and development such as cell division, growth and organogenesis, vascular differentiation, lateral root initiation as well as gravi- and phototropism Berleth and Sachs, Auxins typically play a positive role in stomatal opening but high concentrations of auxin can inhibit stomatal opening Lohse and Hedrich, ; Figure 6. The impact of cytokinins on stomatal movements is also ambiguous.

It has been shown that an increased cytokinin concentration in xylem sap promotes stomatal opening and decreases sensitivity to ABA. However, stomatal response to exogenously applied cytokinins depends on the concentration and cytokinin species Figure 6. Generally, exogenous cytokinins and auxins can inhibit ABA-induced stomatal closure in diverse species Stoll et al.

Brassinosteroids BR are polyhydroxylated steroidal phytohormones that are involved in seed germination, stem elongation, vascular differentiation, and fruit ripening Clouse and Sasse, ; Steber and McCourt, ; Symons et al. Together, these results suggest that there is an interaction between BR and ABA in drought response that is related to stomatal closure. Many factors that are responsible for the regulation of stomatal movements have been already identified, such as components of ABA and other phytohormone signaling pathways.

However, further analyses of the networks of protein interactions, the co-expression of genes, metabolic factors, etc. Taking into account that phytohormone pathways are still under intensive investigations and there are still many gaps to be elucidated, many of the already established interactions may be changed as further progress in research is achieved.

There are ambiguous reports in regards to the role of some phytohormones, such as ethylene, auxins, or cytokinins, in the regulation of stomatal movement that need to be clarified. In addition, the interaction between the diurnal cycle and ABA pathway should be further investigated in order to achieve a full understanding of this process.