Metabolism of 1-Aminocyclopropane-1-Carboxylic Acid in Etiolated Maize Seedlings Grown under Mechanical Impedance

We investigated the metabolism of 1-aminocyclopropane-1-carboxylic acid (ACC) in etiolated maize (Zea mays L.) seedlings subjected to mechanical impedance by applying pressure to the growing medium. Total concentrations of ACC varied little in unimpeded seedlings, but impeded organs accumulated ACC. Roots had consistently higher concentrations of ACC than shoots or seeds, regardless of treatment. The concentration of ACC in the roots increased more than 100% during the first hour of treatment irrespective of the pressure applied; in shoots, total ACC concentration increased 46% at either low or high pressure during the first hour of treatment. The bulk of ACC synthesized under impeded and unimpeded conditions was present in a conjugated form, presumably, 1-(malonylamino)-cyclopropane-1-carboxylic acid. However, 1-(malonylamino)-cyclopropane-1-carboxylic acid increased 73% over controls after 10 hours at 25 kilopascals of pressure. Unimpeded tissue had about 77% ACC as the conjugate and 17% as free ACC, and less than 6% was used in ethylene production. Increased amounts of ACC were converted into ethylene under stress. In vivo ACC synthase activity in roots became six and seven times higher only 1 hour after initiation of treatment at 25 and 100 kilopascals of pressure, respectively, and remained high for at least 6 hours. However, the immediate and massive conjugation of mechanically induced ACC suggests that ACC N-malonyltransferase may play an important role in the regulation of mechanically induced ethylene production. After 8 hours, in vivo activity of the ethylene-forming enzyme complex increased 100 and 50% above normal level at 100 and 25 kilopascals, respectively. Furthermore, ethylene-forming enzyme complex activity was significantly greater at 100 kilopascals than in controls as early as 1 hour after treatment initiation. These data suggest that regulation of ethylene production under mechanical impedance involves the concerted action of ACC synthase, the ethylene-forming enzyme complex, and ACC N-malonyltransferase.