PERCEPTION OF GIBBERELLIN AND ABSCISIC-ACID AT THE EXTERNAL FACE OF THE PLASMA-MEMBRANE OF BARLEY (HORDEUM-VULGARE L) ALEURONE PROTOPLASTS

The response of protoplasts isolated from aleurone layers of barley (H ordeum vulgare L. cv Himalaya) to internally and externally applied ho rmone was analyzed to localize the site of perception of the hormonal signal. Protoplasts responded to externally applied gibberellic acid ( GA(3)) with increased synthesis and secretion of alpha-amylase, transi ent expression of the glucuronidase reporter gene fused to the hormone -responsive elements of the alpha-amylase promoter, and the vacuolatio n typical of GA(3)-treated aleurone cells. When up to 250 mu M GA(3) w as microinjected into the protoplast cytoplasm, none of these response s were observed. This did not reflect damage to the protoplasts during the microinjection procedure, since microinjected protoplasts remaine d responsive to externally applied hormone. Nor did it reflect loss of microinjected GA(3) from the protoplast, since 50% of microinjected [ H-3]GA(20) was retained by protoplasts for at least 24 h. Externally a pplied abscisic acid (ABA) could reverse the stimulation of alpha-amyl ase synthesis and secretion, whereas microinjecting up to 250 mu M ABA was ineffective at antagonizing the stimulatory effect of GA(3). Thes e results suggest that the site of perception of GA(3) and ABA in the barley aleurone protoplast is on the external face of the plasma membr ane.