Auxin- and abscisic acid-dependent osmoregulation in protoplasts of Phaseolus vulgaris pulvini

Protoplasts isolated from the laminar pulvinus of Phaseolus vulgaris and ba thed in a medium containing KCI as the major salt were found to swell in re sponse to IAA and to shrink in response to ABA. The protoplasts of flexor c ells and those of extensor cells responded similarly. The results indicate that the cellular content of osmotic solutes is enhanced by IAA and reduced by ABA. The IAA-induced swelling was abolished when either the K+ or the C l- of the bathing medium was replaced by an impermeant ion or when the medi um was adjusted to neutral pH (instead of pH 6). The response was inhibited by vanadate. It is concluded that the swelling is caused by enhanced influ xes of K+ and Cl-, which probably occur through K+ channels and Cl-/H+ symp orters, respectively. The ABA-induced shrinking was inhibited by 5-nitro-2- (3-phenylpropylamino)benzoic acid, an anion-channel inhibitor, suggesting t hat it is caused by Cl- efflux through anion channels and chargebalancing K + efflux through outward-rectifying K+ channels. It appears that the two pl ant hormones act on pulvinar motor cells to regulate their turgor pressure, as they do in stomatal guard cells. The findings are discussed in relation to the pulvinar movements induced by environmental stimuli.