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Kazuo Shinozaki – One of the best experts on this subject based on the ideXlab platform.

  • antagonistic interaction between map kinase and protein phosphatase 2c in stress recovery
    Plant Science, 2006
    Co-Authors: Jeffrey Leung, Jerome Giraudat, Kazuo Shinozaki, Tsuyoshi Mizoguchi, Sofia Orfanidi, Francoise Chefdor, Tamas Meszaros, Susanne Bolte, Laszlo Bogre
    Abstract:

    The Arabidopsis mitogen-activated kinase MPK6 transmits a diversity of stress signals. However, much less is known about resetting mechanisms subsequent to the stress response. We show that MPK6 is a potential target of abscisic acid-insensitive 1 (ABI1), a protein phosphatase 2C that acts as a key element in attenuating abscisic acid (ABA)-dependent stress signaling. MPK6 can bind to ABI1 in vitro and in yeast, and that a complex containing these two proteins can be co-precipitated from transfected Arabidopsis cells. In whole plants, MPK6 is hyper-reactive to osmotic stress in mutants compromised in ABI1 activity, but not in its closest functional homolog ABI2. Moreover, conditional expression of a dominant-negative form of mpk6 renders plants hypersensitive to ABA. MPK6 enhances the synthesis of ethylene [1]. ABI1 may therefore down modulate ethylene as part of the resetting mechanism after stress.

  • the regulatory domain of srk2e ost1 snrk2 6 interacts with ABI1 and integrates abscisic acid aba and osmotic stress signals controlling stomatal closure in arabidopsis
    Journal of Biological Chemistry, 2006
    Co-Authors: Riichiro Yoshida, Taishi Umezawa, Tsuyoshi Mizoguchi, Seiji Takahashi, Fuminori Takahashi, Kazuo Shinozaki
    Abstract:

    ABI1 and ABI2 encode PP2C-type protein phosphatases and are thought to negatively regulate many aspects of abscisic acid (ABA) signaling, including stomatal closure in Arabidopsis. In contrast, SRK2E/OST1/SnRK2.6 encodes an Arabidopsis SnRK2 protein kinase and acts as a positive regulator in the ABA-induced stomatal closure. SRK2E/OST1 is activated by osmotic stress as well as by ABA, but the independence of the two activation processes has not yet been determined. Additionally, interaction between SRK2E/OST1 and PP2C-type phosphatases (ABI1 and ABI2) is not understood. In the present study, we demonstrated that the ABI1-1 mutation, but not the abi2-1 mutation, strongly inhibited ABA-dependent SRK2E/OST1 activation. In contrast, osmotic stress activated SRK2E/OST1 even in ABI1-1 and aba2-1 plants. The C-terminal regulatory domain of SRK2E/OST1 was required for its activation by both ABA and osmotic stress in Arabidopsis. The C-terminal domain was functionally divided into Domains I and II. Domain II was required only for the ABA-dependent activation of SRK2E/OST1, whereas Domain I was responsible for the ABA-independent activation. Full-length SRK2E/OST1 completely complemented the wilty phenotype of the srk2e mutant, but SRK2E/OST1 lacking Domain II did not. Domain II interacted with the ABI1 protein in a yeast two-hybrid assay. Our results suggested that the direct interaction between SRK2E/OST1 and ABI1 through Domain II plays a critical role in the control of stomatal closure.

  • Transcriptional Regulation of ABI3- and ABA-responsive Genes Including RD29B and RD29A in Seeds, Germinating Embryos, and Seedlings of Arabidopsis
    Plant Molecular Biology, 2006
    Co-Authors: Kazuo Nakashima, Kazuo Shinozaki, Yasunari Fujita, Koji Katsura, Kyonoshin Maruyama, Yoshihiro Narusaka, Motoaki Seki, Kazuko Yamaguchi-shinozaki
    Abstract:

    ABA-responsive elements (ABREs) are cis -acting elements and basic leucine zipper (bZIP)-type ABRE-binding protproteins (AREBs) are transcriptional activators that function in the expression of RD29B in vegetative tissue of Arabidopsis in response to abscisic acid (ABA) treatment. Dehydration-responsive elements (DREs) function as coupling elements of ABRE in the expression of RD29A in response to ABA. Expression analysis using abi3 and abi5 mutants showed that ABI3 and ABI5 play important roles in the expression of RD29B in seeds. Base-substitution analysis showed that two ABREs function strongly and one ABRE coupled with DRE functions weakly in the expression of RD29A in embryos. In a transient transactivation experiment, ABI3, ABI5 and AREB1 activated transcription of a GUS reporter gene driven by the RD29B promoter strongly but these proteins activated the transcription driven by the RD29A promoter weakly. In 35S :: ABI3 Arabidopsis plants, the expression of RD29B was up-regulated strongly, but that of RD29A was up-regulated weakly. These results indicate that the expression of RD29B having ABREs in the promoter is up-regulated strongly by ABI3, whereas that of RD29A having one ABRE coupled with DREs in the promoter is up-regulated weakly by ABI3. We compared the expression of 7000 Arabidopsis genes in response to ABA treatment during germination and in the vegetative growth stage, and that in 35S :: ABI3 plants using a full-length cDNA micrmicroarray. The expression of ABI3- and/or ABA-responsive genes and cis -elements in the promoters are discussed.

Julian I. Schroeder – One of the best experts on this subject based on the ideXlab platform.

  • reconstitution of abscisic acid activation of slac1 anion channel by cpk6 and ost1 kinases and branched ABI1 pp2c phosphatase action
    Proceedings of the National Academy of Sciences of the United States of America, 2012
    Co-Authors: Benjamin Brandt, Majid Ghassemian, Dennis E Brodsky, Shaowu Xue, Juntaro Negi, Koh Iba, Jaakko Kangasjarvi, Aaron B Stephan, Julian I. Schroeder
    Abstract:

    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 Ca2+-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 (down-regulation). 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.

  • triple loss of function of protein phosphatases type 2c leads to partial constitutive response to endogenous abscisic acid
    Plant Physiology, 2009
    Co-Authors: Silvia Rubio, Julian I. Schroeder, Americo Rodrigues, Angela Saez, Marie B Dizon, Alexander Galle, Taehoun Kim, Julia Santiago, Jaume Flexas, Pedro L Rodriguez
    Abstract:

    The phytohormone abscisic acid (ABA) is a key regulator of plant growth and development as well as plant responses to situations of decreased water availability. Protein phosphatases type 2C (PP2Cs) from group A, which includes the ABI1/HAB1 and PP2CA branches, are key negative regulators of ABA signaling. Specifically, HAB1, ABI1, ABI2, and PP2CA have been shown to affect both seed and vegetative responses to ABA. To further understand their contribution to ABA signaling and to unravel possible genetic interactions and functional redundancy among them, we have generated different combinations of double and triple mutants impaired in these PP2Cs. Interestingly, hab1-1pp2ca-1 and ABI1-2pp2ca-1 double mutants showed reduced water loss and enhanced resistance to drought stress, which further supports the role of PP2CA in vegetative responses to ABA. Two triple hab1-1ABI1-2abi2-2 and hab1-1ABI1-2pp2ca-1 mutants were generated, which showed an extreme response to exogenous ABA, impaired growth, and partial constitutive response to endogenous ABA. Thus, transcriptomic analysis revealed a partial up-regulation/down-regulation of a subset of ABA-responsive genes in both triple mutants in the absence of exogenous ABA. Comparison of ABA responses in the different pp2c mutants showed that a progressive increase in ABA sensitivity could be obtained through combined inactivation of these PP2Cs. These results indicate that ABA response is finely tuned by the integrated action of these genes, which is required to prevent a constitutive response to endogenous ABA that might have a deleterious effect on growth and development in the absence of environmental stress.

  • enhancement of abscisic acid sensitivity and reduction of water consumption in arabidopsis by combined inactivation of the protein phosphatases type 2c ABI1 and hab1
    Plant Physiology, 2006
    Co-Authors: Angela Saez, Julian I. Schroeder, Nadia Robert, Mohammad H Maktabi, Ramon Serrano, Pedro L Rodriguez
    Abstract:

    Abscisic acid (ABA) plays a key role in plant responses to abiotic stress, particularly drought stress. A wide number of ABA-hypersensitive mutants is known, however, only a few of them resist/avoid drought stress. In this work we have generated ABA-hypersensitive drought-avoidant mutants by simultaneous inactivation of two negative regulators of ABA signaling, i.e. the protein phosphatases type 2C (PP2Cs) ABA-INSENSITIVE1 (ABI1) and HYPERSENSITIVE TO ABA1 (HAB1). Two new recessive loss-of-function alleles of ABI1, ABI1-2 and ABI1-3, were identified in an Arabidopsis (Arabidopsis thaliana) T-DNA collection. These mutants showed enhanced responses to ABA both in seed and vegetative tissues, but only a limited effect on plant drought avoidance. In contrast, generation of double hab1-1 ABI1-2 and hab1-1 ABI1-3 mutants strongly increased plant responsiveness to ABA. Thus, both hab1-1 ABI1-2 and hab1-1 ABI1-3 were particularly sensitive to ABA-mediated inhibition of seed germgermination. Additionally, vegetative responses to ABA were reinforced in the double mutants, which showed a strong hypersensitivity to ABA in growth assays, stomatal closure, and induction of ABA-responsive genes. Transpirational water loss under drought conditions was noticeably reduced in the double mutants as compared to single parental mutants, which resulted in reduced water consumption of whole plants. Taken together, these results reveal cooperative negative regulation of ABA signaling by ABI1 and HAB1 and suggest that fine tuning of ABA signaling can be attained through combined action of PP2Cs. Finally, these results suggest that combined inactivation of specific PP2Cs involved in ABA signaling could provide an approach for improving crop performance under drought stress conditions.

Riichiro Yoshida – One of the best experts on this subject based on the ideXlab platform.

  • the regulatory domain of srk2e ost1 snrk2 6 interacts with ABI1 and integrates abscisic acid aba and osmotic stress signals controlling stomatal closure in arabidopsis
    Journal of Biological Chemistry, 2006
    Co-Authors: Riichiro Yoshida, Taishi Umezawa, Tsuyoshi Mizoguchi, Seiji Takahashi, Fuminori Takahashi, Kazuo Shinozaki
    Abstract:

    ABI1 and ABI2 encode PP2C-type protein phosphatases and are thought to negatively regulate many aspects of abscisic acid (ABA) signaling, including stomatal closure in Arabidopsis. In contrast, SRK2E/OST1/SnRK2.6 encodes an Arabidopsis SnRK2 protein kinase and acts as a positive regulator in the ABA-induced stomatal closure. SRK2E/OST1 is activated by osmotic stress as well as by ABA, but the independence of the two activation processes has not yet been determined. Additionally, interaction between SRK2E/OST1 and PP2C-type phosphatases (ABI1 and ABI2) is not understood. In the present study, we demonstrated that the ABI1-1 mutation, but not the abi2-1 mutation, strongly inhibited ABA-dependent SRK2E/OST1 activation. In contrast, osmotic stress activated SRK2E/OST1 even in ABI1-1 and aba2-1 plants. The C-terminal regulatory domain of SRK2E/OST1 was required for its activation by both ABA and osmotic stress in Arabidopsis. The C-terminal domain was functionally divided into Domains I and II. Domain II was required only for the ABA-dependent activation of SRK2E/OST1, whereas Domain I was responsible for the ABA-independent activation. Full-length SRK2E/OST1 completely complemented the wilty phenotype of the srk2e mutant, but SRK2E/OST1 lacking Domain II did not. Domain II interacted with the ABI1 protein in a yeast two-hybrid assay. Our results suggested that the direct interaction between SRK2E/OST1 and ABI1 through Domain II plays a critical role in the control of stomatal closure.

  • The regulatory domain of SRK2E/OST1/SnRK2.6 interacts with ABI1 and integrates abscisic acid (ABA) and osmotic stress signals controlling stomatal closure in Arabidopsis
    The Journal of biological chemistry, 2005
    Co-Authors: Riichiro Yoshida, Taishi Umezawa, Tsuyoshi Mizoguchi, Seiji Takahashi, Fuminori Takahashi, Kazuo Shinozaki
    Abstract:

    ABI1 and ABI2 encode PP2C-type protein phosphatases and are thought to negatively regulate many aspects of abscisic acid (ABA) signaling, including stomatal closure in Arabidopsis. In contrast, SRK2E/OST1/SnRK2.6 encodes an Arabidopsis SnRK2 protein kinase and acts as a positive regulator in the ABA-induced stomatal closure. SRK2E/OST1 is activated by osmotic stress as well as by ABA, but the independence of the two activation processes has not yet been determined. Additionally, interaction between SRK2E/OST1 and PP2C-type phosphatases (ABI1 and ABI2) is not understood. In the present study, we demonstrated that the ABI1-1 mutation, but not the abi2-1 mutation, strongly inhibited ABA-dependent SRK2E/OST1 activation. In contrast, osmotic stress activated SRK2E/OST1 even in ABI1-1 and aba2-1 plants. The C-terminal regulatory domain of SRK2E/OST1 was required for its activation by both ABA and osmotic stress in Arabidopsis. The C-terminal domain was functionally divided into Domains I and II. Domain II was required only for the ABA-dependent activation of SRK2E/OST1, whereas Domain I was responsible for the ABA-independent activation. Full-length SRK2E/OST1 completely complemented the wilty phenotype of the srk2e mutant, but SRK2E/OST1 lacking Domain II did not. Domain II interacted with the ABI1 protein in a yeast two-hybrid assay. Our results suggested that the direct interaction between SRK2E/OST1 and ABI1 through Domain II plays a critical role in the control of stomatal closure.

Tsuyoshi Mizoguchi – One of the best experts on this subject based on the ideXlab platform.

  • antagonistic interaction between map kinase and protein phosphatase 2c in stress recovery
    Plant Science, 2006
    Co-Authors: Jeffrey Leung, Jerome Giraudat, Kazuo Shinozaki, Tsuyoshi Mizoguchi, Sofia Orfanidi, Francoise Chefdor, Tamas Meszaros, Susanne Bolte, Laszlo Bogre
    Abstract:

    The Arabidopsis mitogen-activated kinase MPK6 transmits a diversity of stress signals. However, much less is known about resetting mechanisms subsequent to the stress response. We show that MPK6 is a potential target of abscisic acid-insensitive 1 (ABI1), a protein phosphatase 2C that acts as a key element in attenuating abscisic acid (ABA)-dependent stress signaling. MPK6 can bind to ABI1 in vitro and in yeast, and that a complex containing these two proteins can be co-precipitated from transfected Arabidopsis cells. In whole plants, MPK6 is hyper-reactive to osmotic stress in mutants compromised in ABI1 activity, but not in its closest functional homolog ABI2. Moreover, conditional expression of a dominant-negative form of mpk6 renders plants hypersensitive to ABA. MPK6 enhances the synthesis of ethylene [1]. ABI1 may therefore down modulate ethylene as part of the resetting mechanism after stress.

  • the regulatory domain of srk2e ost1 snrk2 6 interacts with ABI1 and integrates abscisic acid aba and osmotic stress signals controlling stomatal closure in arabidopsis
    Journal of Biological Chemistry, 2006
    Co-Authors: Riichiro Yoshida, Taishi Umezawa, Tsuyoshi Mizoguchi, Seiji Takahashi, Fuminori Takahashi, Kazuo Shinozaki
    Abstract:

    ABI1 and ABI2 encode PP2C-type protein phosphatases and are thought to negatively regulate many aspects of abscisic acid (ABA) signaling, including stomatal closure in Arabidopsis. In contrast, SRK2E/OST1/SnRK2.6 encodes an Arabidopsis SnRK2 protein kinase and acts as a positive regulator in the ABA-induced stomatal closure. SRK2E/OST1 is activated by osmotic stress as well as by ABA, but the independence of the two activation processes has not yet been determined. Additionally, interaction between SRK2E/OST1 and PP2C-type phosphatases (ABI1 and ABI2) is not understood. In the present study, we demonstrated that the ABI1-1 mutation, but not the abi2-1 mutation, strongly inhibited ABA-dependent SRK2E/OST1 activation. In contrast, osmotic stress activated SRK2E/OST1 even in ABI1-1 and aba2-1 plants. The C-terminal regulatory domain of SRK2E/OST1 was required for its activation by both ABA and osmotic stress in Arabidopsis. The C-terminal domain was functionally divided into Domains I and II. Domain II was required only for the ABA-dependent activation of SRK2E/OST1, whereas Domain I was responsible for the ABA-independent activation. Full-length SRK2E/OST1 completely complemented the wilty phenotype of the srk2e mutant, but SRK2E/OST1 lacking Domain II did not. Domain II interacted with the ABI1 protein in a yeast two-hybrid assay. Our results suggested that the direct interaction between SRK2E/OST1 and ABI1 through Domain II plays a critical role in the control of stomatal closure.

  • The regulatory domain of SRK2E/OST1/SnRK2.6 interacts with ABI1 and integrates abscisic acid (ABA) and osmotic stress signals controlling stomatal closure in Arabidopsis
    The Journal of biological chemistry, 2005
    Co-Authors: Riichiro Yoshida, Taishi Umezawa, Tsuyoshi Mizoguchi, Seiji Takahashi, Fuminori Takahashi, Kazuo Shinozaki
    Abstract:

    ABI1 and ABI2 encode PP2C-type protein phosphatases and are thought to negatively regulate many aspects of abscisic acid (ABA) signaling, including stomatal closure in Arabidopsis. In contrast, SRK2E/OST1/SnRK2.6 encodes an Arabidopsis SnRK2 protein kinase and acts as a positive regulator in the ABA-induced stomatal closure. SRK2E/OST1 is activated by osmotic stress as well as by ABA, but the independence of the two activation processes has not yet been determined. Additionally, interaction between SRK2E/OST1 and PP2C-type phosphatases (ABI1 and ABI2) is not understood. In the present study, we demonstrated that the ABI1-1 mutation, but not the abi2-1 mutation, strongly inhibited ABA-dependent SRK2E/OST1 activation. In contrast, osmotic stress activated SRK2E/OST1 even in ABI1-1 and aba2-1 plants. The C-terminal regulatory domain of SRK2E/OST1 was required for its activation by both ABA and osmotic stress in Arabidopsis. The C-terminal domain was functionally divided into Domains I and II. Domain II was required only for the ABA-dependent activation of SRK2E/OST1, whereas Domain I was responsible for the ABA-independent activation. Full-length SRK2E/OST1 completely complemented the wilty phenotype of the srk2e mutant, but SRK2E/OST1 lacking Domain II did not. Domain II interacted with the ABI1 protein in a yeast two-hybrid assay. Our results suggested that the direct interaction between SRK2E/OST1 and ABI1 through Domain II plays a critical role in the control of stomatal closure.

Pedro L Rodriguez – One of the best experts on this subject based on the ideXlab platform.

  • HRS1 acts as a negative regulator of abscisic acid signaling to promote timely germination of Arabidopsis seeds.
    PLOS ONE, 2012
    Co-Authors: Chongming Wu, Juanjuan Feng, Ran Wang, Huixia Yang, Pedro L Rodriguez, Daowen Wang
    Abstract:

    In this work, we conducted functional analysis of Arabidopsis HRS1 gene in order to provide new insights into the mechanisms governing seed germgermination. Compared with wild type (WT) control, HRS1 knockout mutant (hrs1-1) exhibited significant germination delays on either normal medium or those supplemented with abscisic acid (ABA) or sodium chloride (NaCl), with the magnitude of the delay being substantially larger on the latter media. The hypersensitivity of hrs1-1 germination to ABA and NaCl required ABI3, ABI4 and ABI5, and was aggravated in the double mutant hrs1-1ABI1-2 and triple mutant hrs1-1hab1-1ABI1-2, indicating that HRS1 acts as a negative regulator of ABA signaling during seed germgermination. Consistent with this notion, HRS1 expression was found in the embryo axis, and was regulated both temporally and spatially, during seed germgermination. Further analysis showed that the delay of hrs1-1 germination under normal conditions was associated with reduction in the elongation of the cells located in the lower hypocotyl (LH) and transition zone (TZ) of embryo axis. Interestingly, the germination rate of hrs1-1 was more severely reduced by the inhibitor of cell elongation, and more significantly decreased by the suppressors of plasmalemma H+-ATPase activity, than that of WT control. The plasmalemma H+-ATPase activity in the germinating seeds of hrs1-1 was substantially lower than that exhibited by WT control, and fusicoccin, an activator of this pump, corrected the transient germination delay of hrs1-1. Together, our data suggest that HRS1 may be needed for suppressing ABA signaling in germinating embryo axis, which promotes the timely germination of Arabidopsis seeds probably by facilitating the proper function of plasmalemma H+-ATPase and the efficient elongation of LH and TZ cells.

  • triple loss of function of protein phosphatases type 2c leads to partial constitutive response to endogenous abscisic acid
    Plant Physiology, 2009
    Co-Authors: Silvia Rubio, Julian I. Schroeder, Americo Rodrigues, Angela Saez, Marie B Dizon, Alexander Galle, Taehoun Kim, Julia Santiago, Jaume Flexas, Pedro L Rodriguez
    Abstract:

    The phytohormone abscisic acid (ABA) is a key regulator of plant growth and development as well as plant responses to situations of decreased water availability. Protein phosphatases type 2C (PP2Cs) from group A, which includes the ABI1/HAB1 and PP2CA branches, are key negative regulators of ABA signaling. Specifically, HAB1, ABI1, ABI2, and PP2CA have been shown to affect both seed and vegetative responses to ABA. To further understand their contribution to ABA signaling and to unravel possible genetic interactions and functional redundancy among them, we have generated different combinations of double and triple mutants impaired in these PP2Cs. Interestingly, hab1-1pp2ca-1 and ABI1-2pp2ca-1 double mutants showed reduced water loss and enhanced resistance to drought stress, which further supports the role of PP2CA in vegetative responses to ABA. Two triple hab1-1ABI1-2abi2-2 and hab1-1ABI1-2pp2ca-1 mutants were generated, which showed an extreme response to exogenous ABA, impaired growth, and partial constitutive response to endogenous ABA. Thus, transcriptomic analysis revealed a partial up-regulation/down-regulation of a subset of ABA-responsive genes in both triple mutants in the absence of exogenous ABA. Comparison of ABA responses in the different pp2c mutants showed that a progressive increase in ABA sensitivity could be obtained through combined inactivation of these PP2Cs. These results indicate that ABA response is finely tuned by the integrated action of these genes, which is required to prevent a constitutive response to endogenous ABA that might have a deleterious effect on growth and development in the absence of environmental stress.

  • enhancement of abscisic acid sensitivity and reduction of water consumption in arabidopsis by combined inactivation of the protein phosphatases type 2c ABI1 and hab1
    Plant Physiology, 2006
    Co-Authors: Angela Saez, Julian I. Schroeder, Nadia Robert, Mohammad H Maktabi, Ramon Serrano, Pedro L Rodriguez
    Abstract:

    Abscisic acid (ABA) plays a key role in plant responses to abiotic stress, particularly drought stress. A wide number of ABA-hypersensitive mutants is known, however, only a few of them resist/avoid drought stress. In this work we have generated ABA-hypersensitive drought-avoidant mutants by simultaneous inactivation of two negative regulators of ABA signaling, i.e. the protein phosphatases type 2C (PP2Cs) ABA-INSENSITIVE1 (ABI1) and HYPERSENSITIVE TO ABA1 (HAB1). Two new recessive loss-of-function alleles of ABI1, ABI1-2 and ABI1-3, were identified in an Arabidopsis (Arabidopsis thaliana) T-DNA collection. These mutants showed enhanced responses to ABA both in seed and vegetative tissues, but only a limited effect on plant drought avoidance. In contrast, generation of double hab1-1 ABI1-2 and hab1-1 ABI1-3 mutants strongly increased plant responsiveness to ABA. Thus, both hab1-1 ABI1-2 and hab1-1 ABI1-3 were particularly sensitive to ABA-mediated inhibition of seed germination. Additionally, vegetative responses to ABA were reinforced in the double mutants, which showed a strong hypersensitivity to ABA in growth assays, stomatal closure, and induction of ABA-responsive genes. Transpirational water loss under drought conditions was noticeably reduced in the double mutants as compared to single parental mutants, which resulted in reduced water consumption of whole plants. Taken together, these results reveal cooperative negative regulation of ABA signaling by ABI1 and HAB1 and suggest that fine tuning of ABA signaling can be attained through combined action of PP2Cs. Finally, these results suggest that combined inactivation of specific PP2Cs involved in ABA signaling could provide an approach for improving crop performance under drought stress conditions.