FIGURE 2.

Schematic illustration of ion channels, aquaporins (AQPs) and transporters activated by drought or ABA and controlling stomatal closure. The accumulation of cytoplasmic Ca²⁺ ([Ca²⁺]_cyt), is possible due to Ca²⁺-permeable channels (yellow) localized in the PM (e.g., AtCNGC5, AtCNGC6, AtGLR3.1) and the tonoplast (AtTPC1). Increasing the amount of calcium activates S-type (AtSLAC1 and AtSLAH3) and R-type (AtQUAC1) channels, which are indicated in green. AtABCC5/MRP5 (orange), localized to the vacuolar membrane (VM), is a regulator of Ca²⁺-permeable and S-type channels. The actions of the S- and R-type channels induce membrane depolarization and activate K⁺ flow through AtGORK and AtKUP6 (pink) from guard cells. The release of K⁺ from the vacuole is mediated by the AtTPK1 channel (pink). Additionally, stomatal closure is regulated by the action of the AtZIFL1.3 isoform, indicated in pink. AQPs are responsible for the outflow of water (e.g., VfPIP1), shown in blue. EcS, extracellular space; Cpl, cytoplasm; Vac, vacuole.