The plant hormone abscisic acid (ABA) is produced in response to abiotic stresses and mediates stomatal closure in response to drought via recently identified ABA receptors (pyrabactin resistance/regulatory component of ABA receptor; PYR/RCAR). SLAC1 encodes a central guard cell S-type anion channel that mediates ABA-induced stomatal closure. Coexpression of the calcium-dependent protein kinase 21 (CPK21), CPK23, or the Open Stomata 1 kinase (OST1) activates SLAC1 anion currents. However, reconstitution of ABA activation of any plant ion channel has not yet been attained. Whether the known core ABA signaling components are sufficient for ABA activation of SLAC1 anion channels or whether additional components are required remains unknown. The Ca 2+ -dependent protein kinase CPK6 is known to function in vivo in ABA-induced stomatal closure. Here we show that CPK6 robustly activates SLAC1-mediated currents and phosphorylates the SLAC1 N terminus. A phosphorylation site (S59) in SLAC1, crucial for CPK6 activation, was identified. The group A PP2Cs ABI1, ABI2, and PP2CA down-regulated CPK6-mediated SLAC1 activity in oocytes. Unexpectedly, ABI1 directly dephosphorylated the N terminus of SLAC1, indicating an alternate branched early ABA signaling core in which ABI1 targets SLAC1 directly (downregulation). Furthermore, here we have successfully reconstituted ABA-induced activation of SLAC1 channels in oocytes using the ABA receptor pyrabactin resistant 1 (PYR1) and PP2C phosphatases with two alternate signaling cores including either CPK6 or OST1. Point mutations in ABI1 disrupting PYR1-ABI1 interaction abolished ABA signal transduction. Moreover, by addition of CPK6, a functional ABA signal transduction core from ABA receptors to ion channel activation was reconstituted without a SnRK2 kinase.Arabidopsis | chloride channel T he perception of the phytohormone abscisic acid (ABA) is achieved by the recently discovered 14-member START protein family of ABA receptors named pyrabactin resistance (PYR), or regulatory component of ABA receptor (RCAR) (1, 2). PYR/RCARs have been shown to bind to clade A PP2Cs and inhibit the activity of these PP2Cs in the presence of ABA (1-5). Structural studies show that PYR1, PYL1, and PYL2 function as ABA receptors, with ABA binding in a protein cavity that locks down the ABA molecule (6-10).ABA reduces transpirational water loss of plants by inducing stomatal closure (11). ABA can cause an increase in guard cell intracellular Ca 2+ concentration (12-17), which leads to the down-regulation of inward-rectifying K + channels and activation of both slow-sustained (S-type) and rapid-transient (R-type) anion channels (18)(19)(20). Previous findings have led to the model that S-type anion channels play a key role in controlling stomatal closure (18,21,22). slac1 mutant plants have greatly reduced S-type anion channel activity (23) and display impaired stomatal closure in response to ABA, elevated CO 2 , ozone, reactive oxygen species, calcium, and reduced humidity, underlining that SLAC1 repres...

Drought stress triggers an increase in the level of the plant hormone abscisic acid (ABA), which initiates a signaling cascade to close stomata and reduce water loss. Recent studies have revealed that guard cells control cytosolic ABA concentration through the concerted actions of biosynthesis, catabolism as well as transport across membranes. Substantial progress has been made at understanding the molecular mechanisms of how the ABA signaling core module PYR/PYL/RCAR-PP2C-SnRK2 controls the activity of anion channels and thereby stomatal aperture. In this review, we focus on our current mechanistic understanding of ABA signaling in guard cells including the role of the second messenger Ca2+ as well as crosstalk with biotic stress responses.

A central question is how specificity in cellular responses to the eukaryotic second messenger Ca2+ is achieved. Plant guard cells, that form stomatal pores for gas exchange, provide a powerful system for in depth investigation of Ca2+-signaling specificity in plants. In intact guard cells, abscisic acid (ABA) enhances (primes) the Ca2+-sensitivity of downstream signaling events that result in activation of S-type anion channels during stomatal closure, providing a specificity mechanism in Ca2+-signaling. However, the underlying genetic and biochemical mechanisms remain unknown. Here we show impairment of ABA signal transduction in stomata of calcium-dependent protein kinase quadruple mutant plants. Interestingly, protein phosphatase 2Cs prevent non-specific Ca2+-signaling. Moreover, we demonstrate an unexpected interdependence of the Ca2+-dependent and Ca2+-independent ABA-signaling branches and the in planta requirement of simultaneous phosphorylation at two key phosphorylation sites in SLAC1. We identify novel mechanisms ensuring specificity and robustness within stomatal Ca2+-signaling on a cellular, genetic, and biochemical level.DOI: http://dx.doi.org/10.7554/eLife.03599.001

Plants constantly renew during their life cycle and thus require to shed senescent and damaged organs. Floral abscission is controlled by the leucine-rich repeat receptor kinase (LRR-RK) HAESA and the peptide hormone IDA. It is unknown how expression of IDA in the abscission zone leads to HAESA activation. Here we show that IDA is sensed directly by the HAESA ectodomain. Crystal structures of HAESA in complex with IDA reveal a hormone binding pocket that accommodates an active dodecamer peptide. A central hydroxyproline residue anchors IDA to the receptor. The HAESA co-receptor SERK1, a positive regulator of the floral abscission pathway, allows for high-affinity sensing of the peptide hormone by binding to an Arg-His-Asn motif in IDA. This sequence pattern is conserved among diverse plant peptides, suggesting that plant peptide hormone receptors may share a common ligand binding mode and activation mechanism.DOI: http://dx.doi.org/10.7554/eLife.15075.001