Arabidopsis possesses a superfamily of ATP-binding cassette (ABC) transporters. Among these, the multidrug resistance-associated protein AtMRP5/AtABCC5 regulates stomatal aperture and controls plasma membrane anion channels of guard cells. Remarkably, despite the prominent role of AtMRP5 in conferring partial drought insensitivity upon Arabidopsis, we know little of the biochemical function of AtMRP5. Our phylogenetic analysis showed that AtMRP5 is closely related to maize MRP4, mutation of which confers a low inositol hexakisphosphate kernel phenotype. We now show that insertion mutants of AtMRP5 display a low inositol hexakisphosphate phenotype in seed tissue and that this phenotype is associated with alterations of mineral cation and phosphate status. By heterologous expression in yeast, we demonstrate that AtMRP5 encodes a specific and high affinity ATP-dependent inositol hexakisphosphate transporter that is sensitive to inhibitors of ABC transporters. Moreover, complementation of the mrp5-1 insertion mutants of Arabidopsis with the AtMRP5 cDNA driven from a guard cell-specific promoter restores the sensitivity of the mutant to abscisic acid-mediated inhibition of stomatal opening. Additionally, we show that mutation of residues of the Walker B motif prevents restoring the multiple phenotypes associated with mrp5-1. Our findings highlight a novel function of plant ABC transporters that may be relevant to other kingdoms. They also extend the signaling repertoire of this ubiquitous inositol polyphosphate signaling molecule.Guard cells form pairs of cells, which are conjoined at their ends, in the epidermis of the aerial tissues of plants. The cells surround a central pore, the stoma, through which gas exchange occurs. The principal gases exchanged are CO 2 and water vapor, and the function of the stomatal complex may be considered as the maximization of CO 2 assimilation by photosynthesis for the minimization of water loss.Guard cells and hence the aperture of the central pore are sensitive to environmental factors including light, temperature, CO 2 , and ozone (1). Stomatal closure is initiated by the drought stress hormone abscisic acid (ABA).3 The closure of stomata is a result of a loss of turgor of the delimiting guard cells as a consequence of ion efflux, predominantly Cl Ϫ and K ϩ , and metabolic conversion of organic acids into starch (2).Although the molecular identity of genes encoding the outward and inward K ϩ conductances are known for Arabidopsis thaliana (3), it remains to be demonstrated whether the recently identified SLAC protein (4, 5) encodes the S-type anion channel or is a subunit thereof.The ATP-binding cassette family of membrane proteins is among the most ubiquitous and variable group of membrane proteins and is most commonly associated with membrane transport phenomena. The substrates transported are especially diverse, and consequently a major obstacle to the interpretation of ABC transporter function, particularly pertinent in the context of guard cell function, is the identificatio...

No abstract