Arabidopsis NPF4.6 and NPF5.1 Control Leaf Stomatal Aperture by Regulating Abscisic Acid Transport

The plant hormone abscisic acid (ABA) is actively synthesized in vascular tissues and transported to guard cells to promote stomatal closure. Although several transmembrane ABA transporters have been identified, how the movement of ABA within plants is regulated is not fully understood. In this study, we determined that Arabidopsis NPF4.6, previously identified as an ABA transporter expressed in vascular tissues, is also present in guard cells and positively regulates stomatal closure in leaves. We also found that mutants defective in NPF5.1 had a higher leaf surface temperature compared to the wild type. Additionally, NPF5.1 mediated cellular ABA uptake when expressed in a heterologous yeast system. Promoter activities of NPF5.1 were detected in several leaf cell types. Taken together, these observations indicate that NPF5.1 negatively regulates stomatal closure by regulating the amount of ABA that can be transported from vascular tissues to guard cells.

Cytoplasmic pH and Abscisic Acid Transport across Guard Cell Membranes

The cytoplasmic pH value of guard cells of Valerianella locusta was determined from the distribution of the weak acid [14C]ABA between the medium and the cell, which was shown to be 8.1 ± 0.1. Treatment of guard cells in media with low pH (pH 5.0) and different concentrations of sodium acetate (0 − 5 mol m−3) caused a considerable acidification of the cytoplasm (6.6 − 7.7). Using the compartmental analysis we determined also the fluxes of ABA at the plasma membrane of guard cells (Φω) and at the tonoplast of guard cells (Φᴄᴠ) under steady state conditions. The flux of ABA from the cytoplasm into the vacuoles (tonoplast flux) was dependent upon the cytoplasmic pH. The data give evidence that ABA− is transported across the tonoplast by a saturable transport component. The ABA flux at the plasma membrane seemed to be independent of the cytoplasmic pH.