title: Abscisic Acid Transport and Homeostasis in the Context of Stomatal Regulation.
abstract: The discovery of cytosolic ABA receptors is an important breakthrough in stomatal research; signaling via these receptors is involved in determining the basal stomatal conductance and stomatal responsiveness. However, the source of ABA in guard cells is still not fully understood. The level of ABA increases in guard cells by de novo synthesis, recycling from inactive conjugates via β-glucosidases BG1 and BG2 and by import, whereas it decreases by hydroxylation, conjugation, and export. ABA importers include the NRT1/PTR family protein AIT1, ATP-binding cassette protein ABCG40, and possibly ABCG22, whereas the DTX family member DTX50 and ABCG25 function as ABA exporters. Here, we review the proteins involved in ABA transport and homeostasis and their physiological role in stomatal regulation. Recent experiments suggest that functional redundancy probably exists among ABA transporters between vasculature and guard cells and ABA recycling proteins, as stomatal functioning remained intact in abcg22, abcg25, abcg40, ait1, and bg1bg2 mutants. Only the initial response to reduced air humidity was significantly delayed in abcg22. Considering the reports showing autonomous ABA synthesis in guard cells, we discuss that rapid stomatal responses to atmospheric factors might depend primarily on guard cell-synthesized ABA, whereas in the case of long-term soil water deficit, ABA synthesized in the vasculature might have a significant role.
authors.affiliation: Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia.
authors.forename: Ebe
authors.initials: E
authors.lastname: Merilo
authors.affiliation: Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia.
authors.forename: Pirko
authors.initials: P
authors.lastname: Jalakas
authors.affiliation: Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia.
authors.forename: Kristiina
authors.initials: K
authors.lastname: Laanemets
authors.affiliation: Division of Plant Biology, Department of Biosciences, University of Helsinki, P.O. Box 65 (Viikinkaari 1), FI-00014 Helsinki, Finland.
authors.forename: Omid
authors.initials: O
authors.lastname: Mohammadi
authors.affiliation: Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia.
authors.forename: Hanna
authors.initials: H
authors.lastname: Hõrak
authors.affiliation: Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia. Electronic address: hannes.kollist@ut.ee.
authors.forename: Hannes
authors.initials: H
authors.lastname: Kollist
authors.affiliation: Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia; Division of Plant Biology, Department of Biosciences, University of Helsinki, P.O. Box 65 (Viikinkaari 1), FI-00014 Helsinki, Finland.
authors.forename: Mikael
authors.initials: M
authors.lastname: Brosché
mesh_terms: D000040:Abscisic Acid / Q000378:metabolism*; D001692:Biological Transport; D006706:Homeostasis; D006900:Hydroxylation; D010940:Plant Proteins / Q000378:metabolism; D054046:Plant Stomata / Q000166:cytology / Q000378:metabolism*
chemical_list: D010940:Plant Proteins; D000040:Abscisic Acid
keywords: CO(2); abscisic acid; air humidity; guard cell; stomatal regulation; transport