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

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. 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. Water is indispensable for plant activities such as photosynthesis, respiration, and growth. To overcome the serious problems caused by water-deficit conditions, plants have developed unique systems to prevent water loss from, and to retain water inside, their cells. Abscisic acid (ABA) 3The abbreviations used are: ABA, abscisic acid; ABI, ABA insensitive; OS, osmotic stress; SnRK2, Snf1-related protein kinase 2; GFP, green fluorescent protein. 3The abbreviations used are: ABA, abscisic acid; ABI, ABA insensitive; OS, osmotic stress; SnRK2, Snf1-related protein kinase 2; GFP, green fluorescent protein. is a stress-related plant hormone and plays a significant role in the adaptation to water stress (1Bray E.A. Trends Plan. Sci. 1997; 2: 48-53Abstract Full Text PDF Scopus (1157) Google Scholar, 2Bonetta D. McCourt P. Trends Plant Sci. 1998; 3: 231-235Abstract Full Text Full Text PDF Scopus (104) Google Scholar, 3Shinozaki K. Yamaguchi-Shinozaki K. Seki M. Curr. Opin. Plant Biol. 2003; 6: 410-417Crossref PubMed Scopus (1406) Google Scholar, 4Yamaguchi-Shinozaki K. Shinozaki K. Trends Plant Sci. 2005; 10: 88-94Abstract Full Text Full Text PDF PubMed Scopus (988) Google Scholar, 5Finkelstein R.R. Gampala S.S.L. Rock C.D. Plant Cell. 2002; 14: S15-S45Crossref PubMed Scopus (1659) Google Scholar, 6Zhu J.K. Annu. Rev. Plant Physiol. Plant Mol. Biol. 2002; 53: 247-573Crossref PubMed Scopus (4216) Google Scholar). Much is known concerning the function of ABA in the adaptive process, and many factors have been identified in ABA signaling. Protein phosphorylation is one of the best characterized events involved in ABA signaling. However, to understand the major process of ABA signaling, we need to characterize the detailed functions of each signaling molecule. Through a combination of biochemical and bioinformatics approaches, we found the ABA-specific protein kinase activities in Arabidopsis T87 cells and identified their genes as SnRK2 (SNF1-related protein kinase 2) (7Yoshida R. Hobo T. Ichimura K. Mizoguchi T. Takahashi F. Alonso J. Ecker J.R. Shinozaki K. Plant Cell Physiol. 2002; 43: 1473-1483Crossref PubMed Scopus (426) Google Scholar). SRK2E is one of the Arabidopsis SnRK2 specifically activated by ABA (7Yoshida R. Hobo T. Ichimura K. Mizoguchi T. Takahashi F. Alonso J. Ecker J.R. Shinozaki K. Plant Cell Physiol. 2002; 43: 1473-1483Crossref PubMed Scopus (426) Google Scholar). Loss of SRK2E function causes an ABA-insensitive phenotype in stomatal closure (7Yoshida R. Hobo T. Ichimura K. Mizoguchi T. Takahashi F. Alonso J. Ecker J.R. Shinozaki K. Plant Cell Physiol. 2002; 43: 1473-1483Crossref PubMed Scopus (426) Google Scholar). This srk2e mutant cannot cope with a rapid humidity decrease and results in a wilty phenotype. On the other hand, a drought-sensitive mutant, open stomata 1 (ost1), was identified by a unique system using infrared thermography, and the OST1 gene was found to be identical to SRK2E (At4g33950) (8Mustilli A.C. Merlot S. Vavasseur A. Fenzi F. Giraudat J. Plant Cell. 2002; 14: 3089-3099Crossref PubMed Scopus (847) Google Scholar). Reactive oxygen species have been proposed to function as second messengers for ABA signaling in stomata cells and to activate Ca2+ channels (9Pei Z.M. Murata Y. Benning G. Thomine S. Klusener B. Allen G.J. Grill E. Schloeder J.I. Nature. 2000; 406: 731-734Crossref PubMed Scopus (1595) Google Scholar). SRK2E/OST1 may act upstream of reactive oxygen species production (8Mustilli A.C. Merlot S. Vavasseur A. Fenzi F. Giraudat J. Plant Cell. 2002; 14: 3089-3099Crossref PubMed Scopus (847) Google Scholar). Thus, SRK2E/OST1/SnRK2.6 is now regarded as a major, positive regulator of ABA signaling in Arabidopsis. The importance of SnRK2 in water stress signaling has been extensively studied in many species. The genes that have been described are wheat ABA-induced protein kinase 1 PKABA1 (10Anderberg R.J. Walker-Simmons M.K. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 10183-10187Crossref PubMed Scopus (245) Google Scholar), fava bean ABA-activated serine-threonine protein kinase AAPK (11Li J. Wang X.Q. Watson M.B. Assmann S.M. Science. 2000; 287: 300-303Crossref PubMed Scopus (364) Google Scholar), soybean protein kinase SPK1 and SPK2 (12Monks D.E. Aghoram K. Courtney P.D. DeWald D.B. Dewey R.E. Plant Cell. 2001; 13: 1205-1219Crossref PubMed Scopus (100) Google Scholar), tobacco osmotic stress-activated protein kinase NtOSAK (13Mikolajczyk M. Awotunde O.S. Muszynska G. Klessig D.F. Dobrowolska G. Plant Cell. 2000; 12: 165-178Crossref PubMed Scopus (251) Google Scholar, 14Kelner A. Pekala I. Kaczanowski S. Muszynska G. Hardie D.G. Dobrowolska G. Plant Physiol. 2004; 136: 3255-3265Crossref PubMed Scopus (65) Google Scholar), rice osmotic stress/ABA-activated protein kinase SAPK (15Kobayashi Y. Yamamoto S. Minami H. Kagaya Y. Hottori T. Plant Cell. 2004; 16: 1163-1177Crossref PubMed Scopus (332) Google Scholar), and those in Arabidopsis (16Boudsocq M. Barbier-Brygoo H. Lauriere C. J. Biol. Chem. 2004; 279: 41758-41766Abstract Full Text Full Text PDF PubMed Scopus (338) Google Scholar). The SnRK2 gene family is unique to plants and consists of 10 genes in Arabidopsis (17Hrabak E.M. Chan C.W.M. Gribskov M. Harper J.F. Choi J.H. Halford N. Kudia J. Luan S. Nimmo H.G. Sussman M.R. Thomas M. Walker-Simmons K. Zhu J.K. Harmon A.C. Plant Physiol. 2003; 132: 666-680Crossref PubMed Scopus (741) Google Scholar). Recently, we demonstrated that SRK2C/OSKL4/SnRK2.8 was activated by osmotic stress and significantly increased the drought tolerance of Arabidopsis plants through activating stress-responsive gene expression in transgenic overexpressors (18Umezawa T. Yoshida R. Maruyama K. Yamaguchi-Shinozaki K. Shinozaki K. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 17306-17311Crossref PubMed Scopus (267) Google Scholar). The manner in which stomata cells sense humidity changes to activate stomatal closure remains unknown. There are apparently at least two pathways in drought stress signaling that target stomatal closure (19Luan S. Plant Cell Environ. 2002; 25: 229-237Crossref PubMed Scopus (143) Google Scholar). One is ABA dependent; the other is ABA independent, namely, an osmosensing pathway. Osmoregulation of inward K+ channels has been found to function in stomata cells, which provoked the presence of the osmosensing pathway in plant cells (20Liu K. Luan S. Plant Cell. 1998; 10: 1957-1970Crossref PubMed Scopus (2306) Google Scholar). However, little is known about the nature of the ABA-independent osmosensing pathway in stomatal closure. In the present study, we examined whether ABA-dependent or ABA-independent pathways, or both, function in the activation of SRK2E/OST1 to close stomata cells under low humidity stress. Using Arabidopsis T87 cultured cells we demonstrated that not only ABA, but also osmotic stress (OS), activates SRK2E/OST1 in vivo (7Yoshida R. Hobo T. Ichimura K. Mizoguchi T. Takahashi F. Alonso J. Ecker J.R. Shinozaki K. Plant Cell Physiol. 2002; 43: 1473-1483Crossref PubMed Scopus (426) Google Scholar). An experiment using ABA-insensitive or ABA-deficient mutants revealed that the OS-dependent activation of SRK2E/OST1 was not mediated by ABA, implying that two independent pathways exist and function in SRK2E/OST1 signaling. Domain analysis of the SRK2E/OST1 protein indicated that its C-terminal region plays an important role in controlling both ABA- and OS-dependent activation of SRK2E/OST1. A yeast two-hybrid analysis showed direct physical binding between SRK2E/OST1 and ABI1. Interestingly, the binding site was located to the C terminus of SRK2E/OST1 that was needed for ABA response. The possible mechanisms involved in the activation of SRK2E/OST1 in the response to ABA and OS were discussed in relation to stomata closure. Plant Growth Conditions—The Columbia and Landsberg erecta (Ler) ecotypes of Arabidopsis thaliana were used. The seeds of wild-type (Columbia, Ler), srk2e, and transgenic plants were sterilized with 70% ethanol and 1% antiformin and sown on Gamborg’s B5 medium agar plates (21Valvekens D. Van Montagu M. Van Lijsebettens M. Proc. Natl. Acad. Sci. U. S. A. 1988; 85: 5536-5540Crossref PubMed Scopus (1139) Google 1% were under of at for of and transgenic plants and cells were by each to green fluorescent protein by the in I. M. H. M. T. Y. Y. S. R. S. K. A. H. H. Y. Y. Plant Cell Physiol. PubMed Scopus Google or T. C. S. Seki M. M. M. Yamaguchi-Shinozaki K. Shinozaki K. Plant J. 2002; PubMed Scopus Google Scholar). The was into to wild-type and srk2e plants and T87 cells. of and abi1-1 was by abi1-1 with abi2-1 was by abi2-1 with and aba2-1 was by aba2-1 with In each the was to and plants were by a combination of To abi1-1 and tolerance to both ABA and were the To tolerance to both and were the and Water Loss humidity and water loss were as described (7Yoshida R. Hobo T. Ichimura K. Mizoguchi T. Takahashi F. Alonso J. Ecker J.R. Shinozaki K. Plant Cell Physiol. 2002; 43: 1473-1483Crossref PubMed Scopus (426) Google Scholar). ABA, OS, and in Arabidopsis T87 and in T87 cells was as described (7Yoshida R. Hobo T. Ichimura K. Mizoguchi T. Takahashi F. Alonso J. Ecker J.R. Shinozaki K. Plant Cell Physiol. 2002; 43: 1473-1483Crossref PubMed Scopus (426) Google Scholar). the OS in T87 cells, 1 and of were to the cells in a in of and the of the T87 cells, of were on The ABA and OS of Arabidopsis plants were by the plant in of ABA and or Protein and and kinase were as described (7Yoshida R. Hobo T. Ichimura K. Mizoguchi T. Takahashi F. Alonso J. Ecker J.R. Shinozaki K. Plant Cell Physiol. 2002; 43: 1473-1483Crossref PubMed Scopus (426) Google Scholar). and and analysis were as described (7Yoshida R. Hobo T. Ichimura K. Mizoguchi T. Takahashi F. Alonso J. Ecker J.R. Shinozaki K. Plant Cell Physiol. 2002; 43: 1473-1483Crossref PubMed Scopus (426) Google that the were used in for and for the two-hybrid each in was by and into the or interaction were into and on medium lacking and in the presence of The was to the activated SRK2E as in Arabidopsis in a manner as (8Mustilli A.C. Merlot S. Vavasseur A. Fenzi F. Giraudat J. Plant Cell. 2002; 14: 3089-3099Crossref PubMed Scopus (847) Google Scholar). A low of ABA was to activate in ABA activated two protein and in wild-type and was not in srk2e, that to SRK2E The kinase activities were not in with ABA even genes were in the under the C and results suggested that the activation of the was Using the transgenic we examined whether low humidity activated SRK2E/OST1. was activated by low humidity in The kinase was also activated by low humidity in wild-type but not in the srk2e To whether OS activated SRK2E/OST1 in we Arabidopsis T87 cells with only ABA, but also OS, such as and activated the in Arabidopsis cells did not activate The ABA-dependent of SRK2E/OST1 by abi1-1 but by the of ABI1 and ABI2 in the activation of SRK2E/OST1 by ABA, we the in of abi1-1 and abi2-1 and aba2-1 was used as a ABA activated the in wild-type but not in abi1-1 plants In contrast, abi2-1 and aba2-1 did not the activation of the by ABA results indicated that two ABI1 and have in ABA signaling in of of SRK2E/OST1. The and the OS-dependent of the role of ABA signaling in the OS-dependent activation of SRK2E/OST1, we the in and with low humidity and of the and aba2-1 significantly the OS-dependent activation of results indicated that ABA signaling is not required for the OS-dependent activation of SRK2E/OST1 and that ABA and OS may activate kinase in Arabidopsis plants. water loss in both abi1-1 and aba2-1 mutants but the OS-dependent activation was in mutants results the role of the ABA-dependent pathway in SRK2E/OST1 activation in stomatal closure (7Yoshida R. Hobo T. Ichimura K. Mizoguchi T. Takahashi F. Alonso J. Ecker J.R. Shinozaki K. Plant Cell Physiol. 2002; 43: 1473-1483Crossref PubMed Scopus (426) Google Scholar, A.C. Merlot S. Vavasseur A. Fenzi F. Giraudat J. Plant Cell. 2002; 14: 3089-3099Crossref PubMed Scopus (847) Google Scholar). Domain of the SRK2E/OST1 Arabidopsis SnRK2 gene has a kinase domain that is to the yeast protein kinase Thomas M. M. Plant Protein in Plant Scholar). and SnRK2 not have regulatory but to have an important in their or C-terminal the C-terminal of 10 Arabidopsis SnRK2 and found two one is the of the kinase domain and the other is the of Domain I Domain I is in 10 SnRK2 Domain II be into two in Arabidopsis SnRK2 as Thomas M. M. Plant Protein in Plant Scholar). One has an region as and the other has a region as SRK2E/OST1 to the with other SnRK2 and and in T87 cells as a protein and examined their to ABA and to GFP, were activated by both ABA and OS in a manner to also the response to ABA and OS in which to the by using transgenic cells. The was strongly activated by OS, but not by ABA as (16Boudsocq M. Barbier-Brygoo H. Lauriere C. J. Biol. Chem. 2004; 279: 41758-41766Abstract Full Text Full Text PDF PubMed Scopus (338) Google Scholar). Domain II was well plant Arabidopsis SRK2E/OST1, fava bean AAPK (11Li J. Wang X.Q. Watson M.B. Assmann S.M. Science. 2000; 287: 300-303Crossref PubMed Scopus (364) Google Scholar), and rice (15Kobayashi Y. Yamamoto S. Minami H. Kagaya Y. Hottori T. Plant Cell. 2004; 16: 1163-1177Crossref PubMed Scopus (332) Google Scholar). AAPK and have been to be activated by ABA (11Li J. Wang X.Q. Watson M.B. Assmann S.M. Science. 2000; 287: 300-303Crossref PubMed Scopus (364) Google Scholar, Y. Yamamoto S. Minami H. Kagaya Y. Hottori T. Plant Cell. 2004; 16: 1163-1177Crossref PubMed Scopus (332) Google Scholar). results suggested that Domain II is critical for ABA-induced activation of SRK2E/OST1 and that other to the Thus, we two of of SRK2E/OST1 and as in T87 cells to the role of Domain II in the activation also the SRK2E/OST1 in T87 cells. of on SRK2E/OST1 activities were by kinase ABA- and OS-dependent activities but were not completely in SRK2E/OST1 a C-terminal region of was by the ABA-dependent activation was specifically whereas the OS-dependent activation was or The SRK2E/OST1 kinase lacking both Domains I and II completely to ABA and that SRK2E/OST1 may be activated by two independent pathways, ABA- and OS-dependent of the C-terminal Domain in the of the C-terminal region is indispensable for both ABA- and OS-dependent activation of SRK2E/OST1, we examined the of C terminus on the of the wilty phenotype in in srk2e plants a wilty phenotype in to srk2e plants. the water loss in plants. complemented the wilty phenotype of srk2e, but did not completely the wilty phenotype This SRK2E/OST1 to retain function of SRK2E/OST1, as the of water loss was between that of srk2e plants and the transgenic srk2e plants complemented with SRK2E/OST1. However, the of water loss in srk2e plants the as that in srk2e plants the of water To the of ABA on the C terminus we the transgenic plants with ABA to the water A of water loss was in the srk2e plants as with the srk2e plants The of low humidity on the activation of the SRK2E/OST1 was examined in the transgenic plants. The kinase demonstrated that was to to the OS results suggested that low humidity or SRK2E/OST1 the stress to stomata closure and that the C-terminal Domain II region may be responsible for both ABA-dependent activation of SRK2E/OST1 and stomatal closure. between the of SRK2E/OST1 and yeast two-hybrid was to the between SRK2E/OST1 and well known factors identified in ABA signaling, ABI1 J. M. D. F. Giraudat J. Science. PubMed Scopus Google Scholar), ABI2 J. Merlot S. Giraudat J. Plant Cell. 1997; PubMed Scopus Google Scholar), E. Y. A. J. 2001; PubMed Scopus Google Scholar), and X.Q. H. S.M. Science. 2001; PubMed Scopus Google Scholar). binding was between SRK2E/OST1 and ABI1 The abi1-1 the binding to SRK2E/OST1 to the wild-type ABI1 Domain analysis indicated that the C-terminal Domain II involved in the ABA-dependent response was required and for the interaction with ABI1 A and ABI1 did not to the C-terminal region of that not to ABA (16Boudsocq M. Barbier-Brygoo H. Lauriere C. J. Biol. Chem. 2004; 279: 41758-41766Abstract Full Text Full Text PDF PubMed Scopus (338) Google Scholar). SRK2E/OST1/SnRK2.6 plays in the to OS and ABA in stomatal closure. SRK2E/OST1/SnRK2.6 with of the C-terminal regulatory were used to the of kinase in the stomatal closure by low humidity in Arabidopsis. analysis of the of the with the and and the yeast two-hybrid analysis to that the ABA-dependent and pathways between low humidity and stomatal closure in Arabidopsis. also critical of Domain II in the C-terminal regulatory region of SRK2E/OST1 in the interaction with ABI1 and activation by ABA in stomata closure. ABA-dependent and between and in Arabidopsis T87 cells, OS, such as and activated SRK2E/OST1 in as as ABA did OS ABA through or Arabidopsis gene expression S. M. T. M. Seki M. T. S. Y. Yamaguchi-Shinozaki K. Shinozaki K. Plant J. 2001; PubMed Google Scholar, M. T. Trends Plant Sci. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar), in the present the ABA of SRK2E/OST1 was examined using Arabidopsis and In the abi1-1 mutant, OS activated SRK2E/OST1, but ABA did not In aba2-1 and both ABA and OS activated SRK2E/OST1. results showed that OS activates SRK2E/OST1 in Arabidopsis and that are at least two independent pathways to activate SRK2E/OST1, the ABA-dependent and only ABA and OS, but also low activate SRK2E/OST1 and prevent rapid water loss in Arabidopsis. humidity in water loss in abi1-1 and aba2-1 in wild-type plants abi1-1 the ABA-dependent activation of SRK2E/OST1, kinase activation was even in abi1-1 and aba2-1 plants with the wilty phenotype The srk2e a wilty phenotype to low humidity stress (7Yoshida R. Hobo T. Ichimura K. Mizoguchi T. Takahashi F. Alonso J. Ecker J.R. Shinozaki K. Plant Cell Physiol. 2002; 43: 1473-1483Crossref PubMed Scopus (426) Google Scholar). results suggested that the ABA-dependent and pathways between low humidity and stomatal closure as described in and discussed of Domains I and II in the of proposed that SRK2E/OST1 may a critical role in ABA-dependent stomatal closure (7Yoshida R. Hobo T. Ichimura K. Mizoguchi T. Takahashi F. Alonso J. Ecker J.R. Shinozaki K. Plant Cell Physiol. 2002; 43: 1473-1483Crossref PubMed Scopus (426) Google Scholar). of SRK2E/OST1 is important to the of signaling and in upstream including of There are regulatory in SnRK2 for the in their C-terminal Thomas M. M. Plant Protein in Plant Scholar). Using T87 cells, we found two required for the activation of SRK2E/OST1 and one is the Domain II region involved in ABA-dependent activation and the other is the Domain I region that functions in the ABA-independent pathway The Domain I region a the SnRK2 This region may be important for the OS-dependent and ABA-independent activation of the SnRK2 of Domain II specifically the ABA-dependent activation of SRK2E in Arabidopsis The importance of the C terminus was also demonstrated in rice SnRK2 (15Kobayashi Y. Yamamoto S. Minami H. Kagaya Y. Hottori T. Plant Cell. 2004; 16: 1163-1177Crossref PubMed Scopus (332) Google Scholar). ABA activates but not in A kinase by of the C terminus of both Domains I and for the region of was activated by ABA (15Kobayashi Y. Yamamoto S. Minami H. Kagaya Y. Hottori T. Plant Cell. 2004; 16: 1163-1177Crossref PubMed Scopus (332) Google Scholar). In we the region to Domain II. Domain II is in the ABA-activated in plant species and may function as an also that the of Domain II significantly increased the OS-dependent activation of This suggested that Domain II may Domain I and the OS-dependent kinase SRK2E/OST1 Domain II only complemented the wilty phenotype of srk2e but the of SRK2E was activated by low humidity in a kinase assay. results also that the ABA-dependent and pathways in between low humidity and stomatal closure. signaling pathways may be required for stomatal closure in response to water stress. of the between SRK2E and ABI1 in the by in found that SRK2E/OST1 interacted with but not with other factors involved in ABA signaling, including a ABI2 The abi1-1 mutation, but not the abi2-1 mutation, inhibited the ABA-dependent activation of SRK2E/OST1 results indicated that ABI1 and ABI2 have in the signaling in of the control of SRK2E in Arabidopsis as in did not to the C terminus of that the in Domain II of the such as SRK2E/OST1, and ABI1 interacted with a A. T. M. B. Grill E. J. 2002; PubMed Scopus Google Scholar), and a protein kinase of C. J. Wang Proc. Natl. Acad. Sci. U. S. A. 2004; 101: PubMed Scopus Google Scholar). acid has been to with ABI1 and its M. Y. U. Zhu J.K. Proc. Natl. Acad. Sci. U. S. A. 2003; PubMed Scopus Google Scholar). The abi1-1 the binding to SRK2E/OST1, as found in the between and ABI1 A. T. M. B. Grill E. J. 2002; PubMed Scopus Google or and ABI2 Y. D. U. Zhu J.K. Cell. 2002; 3: Full Text Full Text PDF PubMed Scopus Google Scholar). of the SnRK2 family may activate and be activated by and upstream using their C-terminal Domain II. on the present we that ABI1 may have two in the ABA-dependent pathway to control stomatal as in ABI1 and ABI2 may function as of ABA signaling in of II. ABI1 may have a role in I of the ABA-dependent pathway as In ABI1 with Domain II of SRK2E/OST1. the wilty of srk2e and abi1-1 and the binding of the abi1-1 protein to we a in which ABI1 functions as a positive regulator in the activation of SRK2E by low humidity ABI1 functions as a regulator for the interaction between the abi1-1 protein and SRK2E in SRK2E Domain II is required for interaction with ABI1 and activation by ABA SRK2E Domain II was activated by low humidity but only complemented the wilty phenotype of srk2e and in the ABA-dependent and pathways may be required for the of the wilty phenotype of srk2e, as described An is that Domain II may be required for an interaction with a in stomatal closure by low humidity a mutant of with a and found that the ABA-dependent activation of SRK2E was not by the not The abi1-1 as a in the ABA signaling F. N. C. N. Giraudat J. Plant Cell. PubMed Scopus Google The abi1-1 protein may a regulator protein or a of SRK2E and function as a by interaction of the with Domain II of The activation of and to be by protein phosphorylation (15Kobayashi Y. Yamamoto S. Minami H. Kagaya Y. Hottori T. Plant Cell. 2004; 16: 1163-1177Crossref PubMed Scopus (332) Google Scholar). protein phosphorylation may in the activation of SRK2E/OST1. ABI1 encodes a PP2C-type protein J. M. D. F. Giraudat J. Science. PubMed Scopus Google Scholar). One possible for the interaction between SRK2E and ABI1 is that the may regulate the by This is not the abi1-1 protein has a binding to Domain II and F. N. C. N. Giraudat J. Plant Cell. PubMed Scopus Google wild-type ABI1. The ABA-dependent activation of SRK2E was inhibited by abi1-1 the that ABI1 may not be involved in the of The upstream that activate yeast or to the SnRK2 family were identified and characterized N. R.R. Mol. Cell. Biol. 2003; PubMed Scopus Google Scholar, A. K. D. U. T. M. D. Curr. Biol. 2003; Full Text Full Text PDF Scopus Google Scholar, A. D. M. Proc. Natl. Acad. Sci. U. S. A. 2003; PubMed Scopus Google Scholar). Thus, to understand the of ABA or OS signaling, is to the upstream that activate SRK2E/OST1. are to the genes that to the SRK2E/OST1 C terminus and regulate its ABA-specific response. for