Overexpression of auxin-binding protein enhances the sensitivity of guard cells to auxin

To explore the role of auxin-binding= protein (ABP1) in planta, a number of transgenic tobacco (Nicotiana tabacum) lines were generated. The wild-type KDEL endoplasmic reticulum targeting signal was mutated to HDEL, another c ommon retention sequence in plants, and to KEQL or KDELGL to compromise its activity. The auxin-binding kinetics of these forms of ABP1 were found to be similar to those of ABP1 purified from maize (Zea mays). To test for a p hysiological response mediated by auxin, intact guard cells of the transgen ic plants were impaled with double-barreled microelectrodes, and auxin-depe ndent changes in K+ currents were recorded under voltage clamp. Exogenous a uxin affected inwardly and outwardly rectifying K+ currents in a dose-depen dent manner. Auxin sensitivity was markedly enhanced in all plants overexpr essing ABP1, irrespective of the form present. Immunogold electron microsco py was used to investigate the localization of ABP1 in the transgenic plant s. All forms were detected in the endoplasmic reticulum and the KEQL and KD ELGL forms passed further across the Golgi stacks than KDEL and HDEL forms. However, neither electron microscopy nor silver-enhanced immunogold epipol arization microscopy revealed differences in cell surface ABP1 abundance fo r any of the plants, including control plants, which indicated that overexp ression of ABP1 alone was sufficient to confer increased sensitivity to add ed auxin. Jones et al. ([1998] Science 282: 1114-1117) found increased cell expansion in transgenic plants overexpressing wild-type ABP1. Single cell recordings extend this observation, with the demonstration that the auxin s ensitivity of guard cell K+ currents is mediated, at least in part, by ABP1 .