Question

Which region serves primarily to regulate the stomatal opening?

Answer 1

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. During the opening of the stomata, the H+-ATPase pump mediates the efflux of H+ from the guard cells. In plants, H+-ATPases belong to the multi-gene family of the P-type ATPases, with 11 genes in Arabidopsis, which are all expressed in the guard cells (Ueno et al., 2005). In the guard cells, the action of H+-ATPase activity is positively regulated by blue light and auxins, whereas Ca2+ and ABA act as negative regulators. The efflux of H+ hyperpolarizes the PM and leads to K+ uptake via activation of inward K+ rectifying channels, such as KAT1 (potassium channel in Arabidopsis thaliana 1), KAT2 (potassium channel in Arabidopsis thaliana 2), and AKT1 (Arabidopsis thaliana K+ transporter 1) (Schachtman et al., 1992; Pilot et al., 2001; Szyroki et al., 2001). Another signal that activates the influx of K+ via K+ channels is the acidification of the apoplast as a result of H+ extrusion from the guard cells. K+ uptake is balanced by counter-ions, mainly Cl− obtained from the apoplast, malate2− that is derived from starch breakdown or . The last one is transported from the apoplast by a nitrate transporter AtNRT1.1 (CHL1) [nitrate transporter 1 (chlorina1)]. The importance of 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., 2003). Ions supplied into the guard cells together with water transported via aquaporins generate the turgor that are necessary to keep stomata open (Figure ​(Figure11A).

Figure 1
Figure 1
Regulation of ion channels, pumps, and transporters localized in the plasma membrane of the guard cells during stomatal opening and closure. During stomatal opening (A) H+-ATPase pumps H+ from the guard cells and hyperpolarizes the membrane, which