BASIS FOR CHANGES IN THE AUXIN-SENSITIVITY OF AVENA-SATIVA (OAT) LEAF-SHEATH PULVINI DURING THE GRAVITROPIC RESPONSE

During the gravitropic response, auxin-sensitivity of the lower flanks of leaf-sheath pulvini of Avena sativa (oat) is at least 1000-fold hi gher than those of the upper flanks and non-gravistimulated pulvini. W hen the pulvini are treated with 1 mM Ca2+, a 10-fold increase in auxi n-sensitivity of the pulvini is observed. Related to this difference i n auxin-sensitivity, in vitro activation of the vanadate-sensitive H+- ATPase by IAA was observed. Results show that the activation of the H-ATPase by IAA. is probably mediated by soluble protein factors and th at the H+-ATPase prepared from the lower flanks is activated by IAA wi th a 1000-fold higher auxin-sensitivity as compared with that from the upper flanks of the graviresponding pulvini. Ammonium sulfate fractio nation experiments show that these soluble protein factors are in the 30 to 60 % fraction. Auxin-binding assays reveal that lower flanks con tain more high-affinity soluble auxin-binding sites (kD; on the order of 10(-9) M) and less low-affinity soluble auxin-binding sites (kD; on the order of 10(-6) M) than upper flanks. It is concluded that differ ential auxin-sensitivity of graviresponding oat-shoot pulvini is achie ved by the modulation of affinities of auxin-binding sites in upper an d lower flanks of the pulvini, that Ca2+ is involved in such modulatio n, and that one of the probable cellular functions of these auxin bind ing sites is the activation of the proton pump on the plasma membranes .