Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Cell culture wikipedia , lookup
Organ-on-a-chip wikipedia , lookup
Cell encapsulation wikipedia , lookup
Extracellular matrix wikipedia , lookup
Tissue engineering wikipedia , lookup
G protein–coupled receptor wikipedia , lookup
Cellular differentiation wikipedia , lookup
Hedgehog signaling pathway wikipedia , lookup
List of types of proteins wikipedia , lookup
ABA signaling Byeong-ha Lee ABA can be made in all parts of plant with higher production in the leaves and the root cap (Zeevaart and Creelman, 1988). ABA is structurally carotenoid-derived. Thus, it is believed to be produced mainly in plastid with the final conversion of xanthoxin into ABA in the cytosol (Seo and Koshiba, 2002). Movement of ABA occurs in both the phloem and xylem as well as by diffusion between cells (Zeevaart and Creelman, 1988). Upon drought stress, ABA produced in roots is thought to move into leaves resulting in reduced transpiration. Unlike other plant hormones, ABA content in plants fluctuates dramatically with environmental conditions. What triggers ABA biosynthesis is not clear. It is believed that plasma membrane loosening from cell wall due to desiccation and other stresses is the signal for ABA biosynthesis, and consequently the ABA signaling. ABA signaling would be mediated by the receptor(s). Despite the intensive effort to identify ABA receptors, no such components have been reported. ABA is a weak acid and its protonated form can be easily diffusible through lipid bilayer (Kaiser and Hartung, 1981). Therefore, theoretically ABA receptor can be intracellular and/or extracellular. Nevertheless, there is some information about the cellular localization about ABA receptor. At first, it was believed that ABA perception takes place at the plasma membrane based on the two experiments; (1) In epidermal peels of Valerianella locusta, stomatal closure occurred in medium of not only pH5 but also pH8 where no ABA uptake was recorded (Hartung, 1983) and (2) Binding of ABA by intact Vicia faba guard cell protoplast is trypsin-sensitive (Hornberg and Weiler, 1984). However, the idea of extracellular ABA perception was challenged by the observations that the ABA-induced stomatal closure was achieved by microinjection of ABA into guard cell cytosol, indicating the existence of internal ABA receptors (Allan et al., 1994; Schwartz et al., 1994). Interestingly, it was also reported that injected ABA into cytosol does not inhibit the stomatal opening, whereas external ABA does inhibit the stomatal opening, suggesting the extracellular ABA receptor (Anderson et al., 1994). In addition, in suspension cell culture systems, ABA-protein conjugates that cannot enter the cells induced ion channel activity (Jeannette et al., 1999) and gene expression (Schultz and Quatrano, 1997; Jeannette et al., 1999), suggesting the existence of extracellular ABA receptor. Changes in ABA-induced ion efflux efficiency depending on external pH suggested both internal and external ABA perceptions are required for the full stomatal response (Macrobbie, 1995a). These results suggest the presence of both extracellular and intracellular ABA receptors. There is no simple defined ABA signaling and most ABA signaling is done in guard cell or aleurone cell. It is not certain that the signaling pathway in guard cells or aleurone cells can be applied to other cells in plants. Indeed, Ca2+ increase by ABA in guard cells is not observed in barley aleurone cells (Wang et al., 1996) although some common signaling modules appear to be present in both systems. Also, heterogeneous expression system study with Xenopus oocytes, reveled the different response to ABA in guard cells and mesophyll cells (Sutton et al., 2000). Thus, it is very primitive to draw conclusive ABA signaling that works for all cells. Discussion below is very idealized signaling mainly based on the guard cell signaling with some signaling components from other systems. Cytosolic Ca2+ has been implicated as a second messenger in ABA responses in a variety of plant cells. In guard cells, plasma membrane proton pumps and inwardrectifying K+ (K+in) channels are inhibited by this Ca2+ increase, whereas anion efflux channels are activated by Ca2+ elevation. Anion efflux from guard cells causes plasma membrane depolarization that in turn suppresses K+in channel activities and activates outward-rectifying K+(K+out) channels. Both K+ and anion efflux from guard cells cause stomatal closure. The abi1-1 and abi2-1 mutants show reduced ABA-induced cytosolic Ca2+ elevation in guard cells (Allen et al., 1999), which may be the cause of abolished ABAinduced stomatal closure in the abi1 and abi2 mutants. Extracellular Ca2+ addition (which in turn causes increase Ca2+ in cytosol in guard cells) bypasses abi1-1 and abi2-1 mutation and brings about the normal anion channel activity and stomatal closure in abi11 and abi2-1. This result suggests that the dominant abi1 and abi2 mutations are located upstream of or close to cytosolic Ca2+ increases in ABA signaling in guard cells. In Arabidopsis guard cells, hydrolysable ATP treatment bring about more efficient activation of slow-type anion channel by Ca2+ increase. Also preincubation of guard cells with kinase inhibitors 2 M K-252a and 50 M staurosporine inhibited Ca2+ activation of anion channels. This suggests that the kinase activity is involved downstream of abi1 and abi2 phosphatases in connecting between Ca2+ and slow-type anion channels as positive regulator. The final step in activation of slow-type anion channels was shown to be achieved by high ATP concentration and suboptimal nanomolar cytosolic Ca2+ concentration (Schwartz et al., 1994), suggesting that final step for anion channel activation involves Ca2+ independent kinase activity. In aleurone cells, calcium-independent MAP kinase activity is also activated by ABA (Knetsch et al., 1996). Thus, in summary, abi1 and abi2 phosphatases are located upstream of or close to Ca2+ increase that is followed by phosphorylation events by kinase(s) with Ca2+ independent kinase activity at the final step for anion channel activation in guard cells. Although it has been studied in the guard cells, it may be, if not all, applied to other cell types as similar responses were observed in aleurone cells for example. However, it should be noted that in abi1 mutants, guard cell response to ABA was restored by simultaneous application of protein kinase inhibitors along with ABA (Pei et al., 1997) and in some cases, timing difference, concentration difference, cotreatment with ABA in the treatment of kinase inhibitor result in non-inhibition in ABA-induced anion channel activation in guard cells. These results suggest that different protein kinases are involved positively or negatively in ABA signaling. Overexpression of a constitutively activated Ca2+ dependent protein kinase in maize mesophyll protoplasts leads to the activation of ABA signaling, which can be partially reversed by expression of mutant abi1 phosphatase (Sheen, 1996; Sheen, 1998). This suggests that additional phosphorylation event upstream of abi1 might be present. Alternatively, different system has different ABA signaling pathway. Indeed, recent studies with heterogeneous expression of total mRNA from guard cells and mesophyll cells from Vicia faba revealed distinct ABA signaling pathway between two cell types (Sutton et al., 2000). ABA-induced Ca2+ increase is mediated by second messengers, IP3 and cADPR. cADPR has been shown to induce cytosolic Ca2+ increase and stomatal closure in Commelina guard cells (Leckie et al., 1998). Also in tomato hypocotyls cells, microinjected cADPR induced ABA-inducible gene expression that can be abolished by EGTA treatment (Wu et al., 1997). IP3 involvement was also implicated by the observation that AtPLC1 (phospholipase C) gene is ABA-inducible (Hirayama et al., 1995). IP3 was shown to cause cytosolic Ca2+ increase and stomatal closure (Gilroy et al., 1990). Consistent with this, ABA elevates IP3 levels and the PLC inhibitor, U-73122 suppresses ABA-induced stomatal closure suggesting IP3 involvement in ABA signaling by triggering Ca2+ increase in cytosol. However, inhibition of either cADPR or IP3 action by inhibitors cannot completely suppress ABA-induced stomatal closure (Leckie et al., 1998; Jacob et al., 1999; Staxen et al., 1999). Instead, treatment with both U-73122 and nicotinamide, an inhibitor of cADPR production completely inhibits ABA-induced stomatal closure (Jacob et al., 1999; MacRobbie, 2000), suggesting both IP3 and cADPR are required for ABA signaling in guard cells inducing cytosolic Ca2+ increase. Nevertheless, ABA-responsive promoters (RD29A promoter or KIN2 promoter hooked up to the GUS reporter gene) are activated in tomato hypocotyls by application of ABA, cADPR, or IP3 of which only IP3 effect is abolished with treatment of heparin, an inhibitor of IP3 receptor, suggesting that IP3 may not play a primary role in ABA signaling. Genetic evidence IP3 involvement in ABA signaling (gene induction and germination) can be found in the Arabidopsis fry1 mutant (Xiong et al., 2001a). Phospholipase D (PLD) has been implicated in ABA signaling in both guard cells (Jacob et al., 1999) and aleurone cells (Ritchie and Gilroy, 1998). The level of phosphatidic acid, product of PLD enzyme reaction transiently increases upon ABA treatment in Vicia guard cells (Jacob et al., 1999), leading to stomatal closure. Phosphatidic acid treatment does not bring about cytosolic Ca2+ increase and 1-butanol, an inhibitor of PLD activity cannot completely inhibit ABA-induced stomatal closure. Instead, near complete inhibition is achieved by 1-butanol and nicotinamide, an inhibitor of cADPR production. These results indicate that PLD-mediated ABA signaling is a parallel pathway of cADPR-mediated ABA signaling. Also in aleurone cells, phosphatidic acid induced the ABA response in the absence of ABA and ABA response was inhibited by 1-butanol (Ritchie and Gilroy, 1998). In microsomal membranes prepared from aleurone protoplasts that have ABAinduced PLD activity, the activation of PLD by ABA is associated with a plasma membrane-enriched fraction and is GTP-dependent. G protein agonist, GTPS is capable of stimulating PLD independently of ABA, while G protein antagonist, GDPS or pertussis toxin inhibits the PDL activation by ABA (Ritchie and Gilroy, 2000). These results suggest that in aleurone cells ABA signaling through PDL takes place at the plasma membrane and is mediated by G-protein activity although the identity of G protein remains to be solved. Similarly, G-protein involvement in stomatal opening inhibition by ABA was demonstrated with the Arabidopsis gpa1 mutants (Wang 2001 Science). Although GCR1, a heptahelical G protein-coupled receptor was proposed to be coupled with GPA1 (Gα protein) (Ellis and Miles, 2001), no linked signaling components have been experimentally determined. The ERA1 gene encodes the β-subunit of farnesyl transferase, an enzyme that catalyses the attachment of a 15-carbon farnesyl lipid to specific proteins for membrane localization (Cutler et al., 1996). Due to its nature, it may not function directly as a signaling component. Instead, it will play a role as a partner of signaling components. The era1 mutant is hypersensitive to ABA in germination, suggesting its negative role in ABA signaling. In both abi1 era1 and abi2 era1 double mutants, era1 suppresses the abi phenotype indicating that ERA1 may act on signaling components at or downstream of the ABI1 and ABI2 phosphatases. The ABI1 and ABI2 gene are expressed in all tissues examined so far (Leung et al., 1997). The ERA1 gene is expressed ubiquitously in all actively growing tissue (Pei et al., 1998; Ziegelhoffer et al., 2000). The cellular localizations of ERA1 have not been experimentally determined. ABA signal, mediated by receptor(s), Ca2+ increase by IP3 and cADPR, phosphatidic acid by PLD, G protein, and/or phosphorylation/dephosphorylation, should reach effector molecules such as ion channels in guard cells and/or nucleus for ABAinduced gene expression. Mutations in ABI3, ABI4, and ABI5 genes caused reduced seed dormancy and reduced sensitivity to ABA in germination but no obvious defects were observed in vegetative tissue, suggesting that their functions might be confined in seeds development. However, gene expression analysis revealed that these three genes are expressed not only during seed development but also in vegetative tissue to a limited degree though. Also, ectopic expression of ABI3 or ABI4 led the transgenic Arabidopsis plants to ABA hypersensitivity in vegetative tissue. These results indicate that they may play a role in vegetative ABA response as well. In consistence with this, another abi4 alleles was isolated from screening designed sugar-insensitive seedling growth mutants, which suggests, in a different point of view, the cross-talk between ABA signaling and sugar signaling. ABI3, ABI4, and ABI5 genes encode transcription factors with the B3 domain, the AP2 domain, and the bZIP domain, respectively. In contrast to the fact that the AP2 domain in ABI4 and bZIP domain in ABI5 have DNA binding and dimerization activity, ABI3 cannot specifically bind to DNA in vitro, although ABI3 can activate the transcription in vivo. This suggests ABI3 may interact with other proteins to gain DNA binding activity. These ABI3, ABI4, and ABI5 genes function in a combinatorial network, rather than a regulatory hierarchy, controlling seed development and ABA responses. Evidences are (1) physiologically, mutations in the genes show similar defect in seed development, (2) abi3/abi4 or abi3/abi5 double mutants display only slightly more ABAresistant phenotype than the single mutant, and (3) ectopic expression of ABI3 or ABI4 resulted in ABA hypersensitivity in vegetative tissue. In fact, in yeast two hybrid system, rice OSVP1 (ABI3 homolog) and TRAB1 (ABI5 homolog) interact with each other. Similarly, it was shown that ABI3 interact directly with ABI5 and ABI5 can also bind to itself. Using transient gene expression in rice protoplasts, the functional interactions of ABI5 with ABA signaling effectors such as VP1 and ABI1 have been shown (Gampala et al., 2002). Co-transformation with ABI5 results in specific transactivation of the ABAinducible promoters. This ABI5-mediated transactivation is inhibited by overexpression of abi1-1 protein phosphatase and by 1-butanol suggesting ABI1 interacts with ABI1mediated and PLD-mediated early ABA signaling modules. Furthermore, ABI5 interacts synergistically with ABA and co-expressed VP1, indicating that ABI5 is involved in ABA-regulated transcription mediated by VP1. These results provide the links between early ABA signaling and transcriptional activities through ABI5 and VP1 transcription factors. Many proteins with bZIP domain have been reported for their capacity of binding to ABA-responsive element (ABRE) found in the promoters of ABA-responsive genes, which contain (C/T)ACGTGGC consensus sequence. Using this consensus sequence in yeast one hybrid system, several bZIP factors (ABRE-binding factors; ABF) were isolated. ABF genes are induced by ABA and their activity to induce an ABA-responsible gene is inhibited in abi1-1 mutant. However, no functional evidence for the bZIP factors in ABA signaling has been revealed until very recent. Transgenic Arabidopsis plants overexpressing ABF3 and ABF4 display ABA hypersensitivity in germination, root growth, and stomatal opening. Some ABA-inducible genes are more upregulated in ABF3/4 overexpression lines than in the wild type, while the expression of some ABArepressible gene were reduced more in the ABF3/4 overexpression lines. The expression of ABI1 and ABI2 genes were also enhanced in the ABF3/4 overexpression lines, especially in the ABF3 lines, suggesting that ABI1 (and ABI2) might be regulated by ABF3 (and ABF4). Similarly, overexpression of ABI5 brings about hypersensitivity to ABA in germination inhibition and better water-retention capacity in vegetative tissue. Many signaling components in ABA signaling pathway need to be identified. Genomics approach will be helpful for this ABA signaling components hunting. Global gene expression profile by microarray will provide a map to develop the strategy. If possible, protein profile would be of a great help. ABA perception by receptor(s) is an most early event in ABA signaling. Therefore, if we can get gene and protein expression changes in this early event, then probability for cloning receptors will be much higher. In addition, as ABA induces a pleiotropic response in plant, focusing on the most upstream signaling event may be necessary. For this, abi1 mutant will provide very good tool. abi1 mutant is defective in cytosol Ca2+ increase and consequently displays abolished ABAinduced stomatal closure. In addition, ABF ability to induce ABA-responsible gene is inhibited in abi1 mutant. As abi1 acts on upstream or at Ca+ increase in ABA signaling, aib1 can only provide the narrow window of ABA early signaling which we want to focus on. ABA receptors are believed to exist both inside and outside of the cells and mesophyll cells and guard cells may have distinct ABA signaling pathways as shown in Vicia faba (Sutton et al., 2000). Therefore, ABA perception location (extracellular and intercellur) and tissue type (guard cells and mesophyll cells) should be considered for ABA receptor(s) cloning. Guard cells from abi1 should be the best tissue for ABA receptor (or ABA signaling components) cloning. To distinguish the extracellular and intercellular receptors, ABA-protein conjugates treatment or ABA microinjection may be required although for the genomics approach ABA microinjection of tons of guard cells seems impossible. Global gene and protein expression profile obtained with abi1 after ABA treatment (control = abi1 without ABA treatment) will show smaller number (that we can handle) of very specific genes with high probability of them being ABA most upstream signaling components including ABA receptor(s). This is based on the assumption that ABA signaling components are most likely ABA-inducible. In fact, ABA receptors might not be ABA-inducible or at least they should be constitutively present to some extent, so they can perceive ABA signaling. Even in the case ABA receptor is not inducible, we might get some signaling molecules. As receptor should interact with these signaling molecules, yeast two hybrid experiment can be applied to fish out the receptor candidates. ABA receptor candidates can be tested their ABA binding ability to facilitate the cloning. Once the candidate genes are selected, one should find the receptor gene knock-out Arabidopsis mutants. Then, ABA response such as stomatal closure or ABAinducible gene expression in the knock-outs should be tested accordingly. Recently, expression studies in Xenopus oocytes showed a successful ABA response (Sutton et al., 2000). In there mesophyll mRNA-injected oocytes responded to ABA in a similar way as mesophyll cells. Further, induced knock-out by introducing a specific complementary RNA (cRNA) resulted in specific defect. This system can be employed for ABA signaling component cloning or functional test of the candidate gene. ABA-responsible oocytes with a cRNA of candidate gene may present more functional information. When this system is utilized with genomics, the outcome should be synergistically informative. References Allan, A.C., Fricker, M.D., Ward, J.L., Beale, M.H., and Trewavas, A.J. (1994). Two transduction pathways mediate rapid effects of abscisic acid in Commelina guard cells. Plant Cell 6, 1319-1328. Allen, G.J., Kuchitsu, K., Chu, S.P., Murata, Y., and Schroeder, J.I. (1999). Arabidopsis abi1-1 and abi2-1 phosphatase mutations reduce abscisic acid-induced cytoplasmic calcium rises in guard cells. Plant Cell 11, 1785-1798. Anderson, B.E., Ward, J.M., and Schroeder, J.I. (1994). Evidence for an Extracellular Reception Site for Abscisic-Acid in Commelina Guard-Cells. Plant Physiology 104, 1177-1183. Cutler, S., Ghassemian, M., Bonetta, D., Cooney, S., and McCourt, P. (1996). A protein farnesyl transferase involved in abscisic acid signal transduction in Arabidopsis. Science 273, 1239-1241. Ellis, B.E., and Miles, G.P. (2001). One for all? Science 292, 2022-2023. Gampala, S.S.L., Finkelstein, R.R., Sun, S.S.M., and Rock, C.D. (2002). ABI5 interacts with abscisic acid signaling effectors in rice protoplasts. J. Biol. Chem. 277, 1689-1694. Gilroy, S., Read, N.D., and Trewavas, A.J. (1990). Elevation of Cytoplasmic Calcium by Caged Calcium or Caged Inositol Trisphosphate Initiates Stomatal Closure. Nature 346, 769-771. Hartung, W. (1983). The Site of Action of Abscisic-Acid at the Guard-Cell Plasmalemma of Valerianella-Locusta. Plant Cell Environ. 6, 427-428. Hirayama, T., Ohto, C., Mizoguchi, T., and Shinozaki, K. (1995). A Gene Encoding a Phosphatidylinositol-Specific Phospholipase-C Is Induced by Dehydration and Salt Stress in Arabidopsis- Thaliana. Proc. Natl. Acad. Sci. U. S. A. 92, 3903-3907. Hornberg, C., and Weiler, E.W. (1984). High-Affinity Binding-Sites for Abscisic-Acid on the Plasmalemma of Vicia-Faba Guard-Cells. Nature 310, 321-324. Jacob, T., Ritchie, S., Assmann, S.M., and Gilroy, S. (1999). Abscisic acid signal transduction in guard cells is mediated by phospholipase D activity. Proc. Natl. Acad. Sci. U. S. A. 96, 12192-12197. Jeannette, E., Rona, J.P., Bardat, F., Cornel, D., Sotta, B., and Miginiac, E. (1999). Induction of RAB18 gene expression and activation of K+ outward rectifying channels depend on an extracellular perception of ABA in Arabidopsis thaliana suspension cells. Plant Journal 18, 13-22. Kaiser, W.M., and Hartung, W. (1981). Uptake and Release of Abscisic-Acid by Isolated Photoautotrophic Mesophyll-Cells, Depending on Ph Gradients. Plant Physiology 68, 202-206. Knetsch, M.L.W., Wang, M., SnaarJagalska, B.E., and HeimovaaraDijkstra, S. (1996). Abscisic acid induces mitogen-activated protein kinase activation in barley aleurone protoplasts. Plant Cell 8, 1061-1067. Leckie, C.P., McAinsh, M.R., Allen, G.J., Sanders, D., and Hetherington, A.M. (1998). Abscisic acid-induced stomatal closure mediated by cyclic ADP- ribose. Proc. Natl. Acad. Sci. U. S. A. 95, 15837-15842. Leung, J., Merlot, S., and Giraudat, J. (1997). The Arabidopsis ABSCISIC ACID-INSENSITIVE2 (ABI2) and ABI1 genes encode homologous protein phosphatases 2C involved in abscisic acid signal transduction. Plant Cell 9, 759-771. Macrobbie, E.A.C. (1995a). Aba-Induced Ion Efflux in Stomatal Guard-Cells Multiple Actions of Aba inside and Outside the Cell. Plant Journal 7, 565-576. MacRobbie, E.A.C. (2000). ABA activates multiple Ca2+ fluxes in stomatal guard cells, triggering vacuolar K+(Rb+) release. Proc. Natl. Acad. Sci. U. S. A. 97, 12361-12368. Pei, Z.M., Ghassemian, M., Kwak, C.M., McCourt, P., and Schroeder, J.I. (1998). Role of farnesyltransferase in ABA regulation of guard cell anion channels and plant water loss. Science 282, 287-290. Pei, Z.M., Kuchitsu, K., Ward, J.M., Schwarz, M., and Schroeder, J.I. (1997). Differential abscisic acid regulation of guard cell slow anion channels in Arabidopsis wild-type and abi1 and abi2 mutants. Plant Cell 9, 409-423. Ritchie, S., and Gilroy, S. (1998). Abscisic acid signal transduction in the barley aleurone is mediated by phospholipase D activity. Proc. Natl. Acad. Sci. U. S. A. 95, 2697-2702. Ritchie, S., and Gilroy, S. (2000). Abscisic acid stimulation of phospholipase D in the barley aleurone is G-protein-mediated and localized to the plasma membrane. Plant Physiology 124, 693-702. Schultz, T.F., and Quatrano, R.S. (1997). Evidence for surface perception of abscisic acid by rice suspension cells as assayed by Em gene expression. Plant Sci. 130, 63-71. Schwartz, A., Wu, W.H., Tucker, E.B., and Assmann, S.M. (1994). Inhibition of Inward K+ Channels and Stomatal Response by Abscisic-Acid - an Intracellular Locus of Phytohormone Action. Proc. Natl. Acad. Sci. U. S. A. 91, 4019-4023. Seo, M., and Koshiba, T. (2002). Complex regulation of ABA biosynthesis in plants. Trends Plant Sci. 7, 41-48. Sheen, J. (1996). Ca2+-dependent protein kinases and stress signal transduction in plants. Science 274, 1900-1902. Sheen, J. (1998). Mutational analysis of protein phosphatase 2C involved in abscisic acid signal transduction in higher plants. Proc. Natl. Acad. Sci. U. S. A. 95, 975980. Staxen, I., Pical, C., Montgomery, L.T., Gray, J.E., Hetherington, A.M., and McAinsh, M.R. (1999). Abscisic acid induces oscillations in guard-cell cytosolic free calcium that involve phosphoinositide-specific phospholipase C. Proc. Natl. Acad. Sci. U. S. A. 96, 1779-1784. Sutton, F., Paul, S.S., Wang, X.Q., and Assmann, S.M. (2000). Distinct abscisic acid signaling pathways for modulation of guard cell versus mesophyll cell potassium channels revealed by expression studies in Xenopus laevis oocytes. Plant Physiology 124, 223-230. Wang, M., Oppedijk, B.J., Lu, X., VanDuijn, B., and Schilperoort, R.A. (1996). Apoptosis in barley aleurone during germination and its inhibition by abscisic acid. Plant Mol.Biol. 32, 1125-1134. Wu, Y., Kuzma, J., Marechal, E., Graeff, R., Lee, H.C., Foster, R., and Chua, N.H. (1997). Abscisic acid signaling through cyclic ADP-Ribose in plants. Science 278, 2126-2130. Xiong, L., Lee, B.-h., Ishitani, M., Lee, H., Zhang, C.Q., and Zhu, J.K. (2001a). FIERY1 encoding an inositol polyphosphate 1-phosphatase is a negative regulator of abscisic acid and stress signaling in Arabidopsis. Genes Dev. 15, 1971-1984. Zeevaart, J.A.D., and Creelman, R.A. (1988). Metabolism and Physiology of Abscisic-Acid. Annu. Rev. Plant Physiol. Plant Molec. Biol. 39, 439-473. Ziegelhoffer, E.C., Medrano, L.J., and Meyerowitz, E.M. (2000). Cloning of the Arabidopsis WIGGUM gene identifies a role for farnesylation in meristem development. Proc. Natl. Acad. Sci. U. S. A. 97, 7633-+.