Regulation of Phorbol 12-Myristate 13-Acetate in the Gravitropic Response and Ethylene Production in Primary Roots of Maize

Phorbol 12-myristate 13-acetate (PMA), a known tumor-promoting phorbol ester, activates the signal transduction enzyme protein kinase C (PKC) in animal cells. We investigated the effect of PMA on the regulation of gravitropism via ethylene production in primary roots of maize. PMA stimulated root growth and the gravitropic response in a concentration-dependent manner at 10-6 M and 10-4 M over 8 hrs. These effects were prevented by treatment with staurosporine (STA), a potent inhibitor of PKC. These results support the possibility that the gravitropic response might be regulated through protein kinases that are involved in the signal transduction system. Ethylene is known to play a role in the regulation of root growth and gravitropism. Ethylene production was increased by about 26% and 37 % of the control rate in response to 10-6 M and 10-4 M PMA, respectively. PMA also stimulated the activity of ACC synthase (ACS), which converts the S-adenosyl-L-methionine (AdoMet) to 1-ami-nocyclopropane-1-carboxylic acid (ACC) in the ethylene production pathway. These effects on ethylen