Cytosolic abscisic acid activates guard cell anion channels without preceding Ca ²⁺ signals

Abstract

The phytohormone abscisic acid (ABA) reports on the water status of the plant and induces stomatal closure. Guard cell anion channels play a central role in this response, because they mediate anion efflux, and in turn, cause a depolarization-induced K ؉ release. We recorded early steps in ABA signaling, introducing multibarreled microelectrodes in guard cells of intact plants. Upon external ABA treatment, anion channels transiently activated after a lag phase of Ϸ2 min. As expected for a cytosolic ABA receptor, iontophoretic ABA loading into the cytoplasm initiated a rise in anion current without delay. These ABA responses could be elicited repetitively at resting and at largely depolarized potentials (e.g., 0 mV), ruling out signal transduction by means of hyperpolarizationactivated calcium channels. Likewise, ABA stimulation did not induce a rise in the cytosolic free-calcium concentration. However, the presence of Ϸ100 nM background Ca 2؉ was required for anion channel function, because the action of ABA on anion channels was repressed after loading of the Ca 2؉ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N ,N -tetraacetate. The chain of events appears very direct, because none of the tested putative ABA-signaling intermediates (inositol 1,4,5 trisphosphate, inositol hexakisphosphate, nicotinic acid adenine dinucleotide phosphate, and cyclic ADP-ribose), could mimic ABA as anion channel activator. In patchclamp experiments, cytosolic ABA also evoked anion current transients carried by R-and S-type anion channels. The response was dose-dependent with half-maximum activation at 2.6 M ABA. Our studies point to an ABA pathway initiated by ABA binding to a cytosolic receptor that within seconds activates anion channels, and in turn, leads to depolarization of the plasma membrane. stomatal closure T he plant hormone abscisic acid (ABA) provides a developmental signal serving as a chemical switch between, for instance, dormancy and growth of seeds and buds (1). Furthermore, this sesquiterpene is involved in the transmission of environmental changes like drought, saline, and cold periods into stress-adaptation processes (2). Based on the time scale of the individual ABA responses, they can be subdivided into fast (seconds up to minutes) and slow (hours up to days, or even months) signaling processes. Stomatal closure represents the fastest ABA response known so far, characterized by a half-time of Ϸ5 min, and presumably does not involve gene activation. Fast stomatal closure is accomplished by the release of potassium and chloride via voltage-dependent ion channels and by metabolic degradation of the major organic anion malate (3, 4).In search for ABA signaling intermediates, guard cells have been challenged with well characterized modulators operating in signal transduction pathways of animal cells (5-8). To a large extent, the respective substances were injected into guard cells of excised epidermal peels, and the corresponding responses were monitored (9-12). After microinjection, photolysis of caged inositol 1,4,...

Guard cell anion channel SLAC1 is regulated by CDPK protein kinases with distinct Ca 2+ affinities

“…Discussion ABA-activation of guard cell anion channels in leaves of intact plants takes place under two different scenarios: one is Ca 2+ -independent (8,9), whereas the other is associated with changes in cytosolic Ca 2+ levels (7,33). Very recently we could show that the Ca 2+ independent protein kinase OST1 activates SLAC1 derived anion currents upon ABA treatment in guard cells (6).…”

“…This ABA pathway resembles the Ca 2+ -independent activation of SLAC-type anion currents in guard cells. ABA signal transduction, however, has been shown to activate guard cell anion channels in a calcium-independent as well as -dependent manner (7)(8)(9)(10). This became evident in abi1-1 mutant plants, where anion channels do not respond to ABA anymore (11) but still activate with calcium (12).…”

Activity of guard cell anion channel SLAC1 is controlled by drought-stress signaling kinase-phosphatase pair

“…The initial steps in ABA signal transduction have been shown to activate guard cell anion channels in a calcium-dependent as well as -independent manner (10)(11)(12)(13). Elements of these ABA signaling pathways, however, have been identified by genetic screens revealing ABA-insensitive and open-stomata plant mutants with deregulated guard cell volume control (14)(15)(16)(17).…”

ABA signalling is fine-tuned by antagonistic HAB1 variants

“…A bscisic acid (ABA) is essential for plant adaptation to abiotic stresses, such as drought [1][2][3][4] . The identification of the PYR/PYLs/RCAR protein family (hereafter referred to as PYLs) as ABA receptors has led to a breakthrough in ABA signalling.…”