= Solution
\b[<Phosphorylation> converts ABA perception into regulated ion transport, coordinates calcium-dependent and calcium-independent inputs, and makes <stomatal closure> reversible.] Its role is distributed across a network of kinases, phosphatases and transport targets. ABA does not close the pore by directly supplying a mechanical force.
The opening and closing machinery is a pair of <guard cells> surrounding a <stoma>. Accumulation of osmotically active solutes favors water entry and increased <turgor pressure>; solute loss favors water exit and reduced aperture. During water stress, <abscisic acid> shifts the balance toward anion and potassium loss. Changes in transport can precede slower changes in <gene expression>, allowing the existing guard-cell machinery to respond rapidly.
The core ABA switch involves inhibition of an inhibitor. Without the activating ABA signal, selected clade-A <protein phosphatase 2C> enzymes, including <ABI1>, restrain ABA-responsive <SnRK2 kinases>. ABA binding to a <PYR-PYL-RCAR ABA receptor> promotes inhibition of these phosphatases. This permits activating kinase phosphorylation to persist and supports activation of <OST1>, also called <SnRK2.6>. Thus a hormone-dependent change in phosphatase activity changes the phosphorylation state and catalytic activity of downstream signaling proteins. The effect is specific to the relevant receptor, phosphatase and kinase combinations; it is not a claim that all phosphatases inhibit closure or all phosphorylation promotes it.
Biochemical reconstitution supports this causal ordering: defined receptor, phosphatase and kinase components can reconstruct ABA-dependent regulation without requiring an intact plant. This separates a sufficient core perception-and-relay module from the many parallel processes present in a stressed cell. However, a reconstructed molecular or transcriptional readout alone does not prove that the full mechanical response of a <stoma> has been rebuilt.<In vitro reconstitution of an abscisic acid signalling pathway by Hiroaki Fujii and others>[pp. 660–664, reconstituted ABA signaling components]{ref}
A crucial transport target is <SLAC1>. <OST1>-dependent <phosphorylation> activates this slow anion channel, allowing anions to leave the <guard cell>. Anion efflux depolarizes the <membrane potential>, favoring outward potassium current through <GORK>. Loss of these solutes reduces the osmotic drive retaining water, so water exits and <turgor pressure> falls. Activation of an anion channel can therefore initiate a larger electrical and osmotic response without the kinase phosphorylating every ion that moves. Suppression of opening-associated transport further reinforces the shift toward closure.
\Image[/past-exam-of-the-plant-sciences-course-of-the-university-of-cambridge/2024/paper-plm1-aba-switch.png]
{title=ABA relieves a phosphatase brake on kinase-dependent guard-cell transport}
{description=Original qualitative pathway schematic. Arrows indicate promotion; blunt-ended connections indicate inhibition. The core switch is separated from calcium-dependent convergence and the final electrical and osmotic consequences. It does not imply that every ABA response requires a detectable calcium elevation.}
Channel activation experiments provide an important bridge between the core ABA switch and physiology. Coexpression and electrophysiology showed that <OST1> activates <SLAC1>, whereas phosphatase action antagonizes activation; kinase activity and channel regulatory regions are therefore functionally important. These findings are stronger than simple colocalization. Together with guard-cell genetic and current measurements, they support a route from a phosphorylation-dependent channel change to altered aperture. A heterologous cell nevertheless lacks the complete transport and mechanical environment of a real <guard cell>.<Activity of guard cell anion channel SLAC1 is controlled by drought-stress signaling kinase-phosphatase pair by Dietmar Geiger and others>[Figures 1–4, channel currents and kinase–phosphatase regulation]{ref}
Calcium-dependent signaling converges on this machinery. <Calcium-dependent protein kinases> can activate <SLAC1>, and different kinases have different calcium sensitivities. Consequently, <phosphorylation> can combine hormone-dependent release of the phosphatase brake with a <calcium signature>. This is a convergence network, not a mandatory linear sequence in which ABA must always cause a measurable global calcium rise before any closure can occur.<Guard cell anion channel SLAC1 is regulated by CDPK protein kinases with distinct Ca2+ affinities by Dietmar Geiger and others>[pp. 8023–8028, kinase-specific calcium dependence and anion-channel regulation]{ref} Regulation of plant <NADPH oxidases> and <reactive oxygen species> can further couple kinase activity, <redox signalling> and calcium entry. The physiological contribution of these branches varies with conditions.
Reversal is as important as activation. Phosphatases, calcium clearance and renewed opening-associated transport reset the system when conditions change. Phosphorylation can alter activity, interactions and localization without waiting for protein replacement. \b[The outcome is a tunable balance of kinase and phosphatase activities that connects ABA-dependent sensing to solute flux, water movement and pore mechanics, while integrating additional environmental inputs.]
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