Based on the physiological studies of ABA function to date, the translocation and communication of this phytohormone between cells, organs and tissues play important roles in whole plant physiological responses (Schachtman and Goodger 2008
, Wilkinson and Davies 2010
). For example, ABA is a key regulator of leaf stomatal conductance: under drought conditions, ABA concentrations increase in the apoplast, leading to stomatal closure (Schachtman and Goodger 2008
, Wilkinson and Davies 2010
). ABA is predominantly biosynthesized and metabolized in vascular tissues, but acts in the stomatal responses of distant guard cells (Cheng et al. 2002
, Koiwai et al. 2004
, Okamoto et al. 2009
). Indeed, some reports suggest that there are systemic and dynamic changes in gene expression related to ABA or stress responses (Christmann et al. 2007
, Endo et al. 2008
). Most genes and factors identified so far in ABA signaling are mainly involved in ABA intracellular regulation (Hirayama and Shinozaki 2007
, Hirayama and Shinozaki 2010
); however, ABA intercellular regulation is not well studied in any plant species. Thus, the molecular basis of ABA transport needs to be investigated to understand whole plant ABA intercellular communication.
Recently, it was reported that one of the ATP-binding cassette (ABC) transporter genes, AtABCG25, encodes a protein responsible for ABA transport and response in Arabidopsis (Kuromori et al. 2010
). The atabcg25
mutants were originally isolated by genetically screening for ABA sensitivity during the greening of cotyledons. AtABCG25 was expressed mainly in vascular tissues, where ABA is predominantly biosynthesized. The fluorescent protein-fused AtABCG25 was localized at the plasma membrane in plant cells.
The ABC transporter is conserved in many model species from E. coli
to humans and was reported to transport various metabolites or signaling molecules, involving phytohormones, in an ATP-dependent manner (Higgins 1992
, Rea 2007
, Nagashima et al. 2008
). In membrane vesicles derived from AtABCG25-expressing insect cells, AtABCG25 exhibited ATP-dependent ABA transport activity. Furthermore, the AtABCG25-overexpressing plants had higher leaf temperatures, implying an influence on stomatal regulation. These results suggest that AtABCG25 is an exporter of ABA through the plasma membrane and is involved in the intercellular ABA signaling pathway.
Another ABC transporter in Arabidopsis, AtABCG40, was independently reported to function as an ABA importer in plant cells (Kang et al. 2010
mutants were selected by testing seed germination and stomatal movements in 13 of 15 Arabidopsis ABC transporter gene knockout mutants (atabcg29–atabcg41
). AtABCG40 was expressed in the leaves of young plantlets and in primary and lateral roots; in leaves, the expression was the highest in guard cells. Plasma membrane localization was shown by ABCG40::sGFP expression driven by the native promoter in Arabidopsis guard cells. In addition, uptake of ABA by yeast and BY2 cells expressing AtABCG40 increased, whereas ABA uptake by the protoplasts of atabcg40
plants decreased, compared with control cells. In loss-of- function atabcg40
mutants, the stomata closed more slowly in response to ABA, resulting in reduced drought tolerance. In response to exogenous ABA, the up-regulation of ABA- inducible genes was strongly delayed in atabcg40
plants, indicating that ABCG40 is necessary for timely responses to ABA. These results suggest that AtABCG40 is an importer of ABA through plasma membranes, and integrates ABA- dependent signaling and transport processes.
In both cases, stereospecificity for transport was shown by experiments using ABA stereoisomers. Interestingly, the Km
saturation kinetics of ATP-dependent ABA transport do not differ greatly (260
nM and 1
μM for AtABCG25 and AtABCG40, respectively), although different assay systems were used to calculate activity (Kang et al. 2010
, Kuromori et al. 2010
). These findings strongly suggest that AtABCG25 and AtABCG40 are responsible for active control of ABA transport between plant cells. From two reports (Kang et al. 2010
, Kuromori et al. 2010
), a simple model can be proposed: ABA is exported from ABA-biosynthesizing cells to the apoplastic area, and then imported from the apoplast into guard cells ().
Fig. 7 Schematic view of hypothetical ABA intercellular transmission. This diagram is an Arabidopsis leaf section showing two distinct cell types: vascular tissues, including vascular parenchyma cells, and guard cells on the leaf epidermis. AtABCG25 might function (more ...)
This model is also consistent with recent reports that some ABA receptors that trigger ABA signaling in cells are soluble and localized to the cytosol (Ma et al. 2009
, Park et al. 2009
), and suggests the potential importance of an ABA transporter that could deliver ABA in a regulated fashion to initiate rapid and controlled responses to various stress conditions (Kang et al. 2010
). The investigation of the ABA transport mechanism has just begun and it provides a novel impetus for examining ABA intercellular regulation.