RESPONSE OF CULTURED MAIZE CELLS TO (-ABSCISIC ACID, (-)-ABSCISIC ACID, AND THEIR METABOLITES())

The metabolism and effects of (+)-S- and (-)-R-abscisic acid (ABA) and some metabolites were studied in maize (Zea mays L. cv Black Mexican Sweet) suspension-cultured cells. Time-course studies of metabolite fo rmation were performed in both cells and medium via analytical high-pe rformance liquid chromatography. Metabolites were isolated and identif ied using physical and chemical methods. At 10 mu M concentration and 28 degrees C, (+)-ABA was metabolized within 24 h, yielding natural (- )-phaseic acid [(-)PA] as the major product. The unnatural enantiomer (-)-ABA was less than 50% metabolized within 24 h and gave primarily ( -)-7'hydroxyABA [(-)-7'-HOABA], together with (+)-PA and ABA glucose e ster. The distribution of metabolites in cells and medium was differen t, reflecting different sites of metabolism and membrane permeabilitie s of conjugated and nonconjugated metabolites. The results imply that (+)-ABA was oxidized to (-)-PA inside the cell, whereas (-)-ABA was co nverted to (-)-7'-HOABA at the cell surface. Growth of maize cells was inhibited by both (+)- and (-)ABA, with only weak contributions from their metabolites. The concentration of (+)-ABA that caused a 50% inhi bition of growth of maize cells was approximately 1 mu M, whereas that for its metabolite (-)-PA was approximately 50 mu M. (-)-ABA was less active than (+)-ABA, with 50% growth inhibition observed at about 10 mu M. (-)-7'-HOABA was only weakly active, with 50% inhibition caused by approximately 500 mu M. Time-course studies of medium pH indicated that (+)-ABA caused a transient pH increase (+0.3 units) at 6 h after addition that was not observed in controls or in samples treated with (-)-PA. The effect of (-)-ABA on medium pH was marginal. No racemizati on at C-1' of (+)-ABA, (-)-ABA, or metabolites was observed during the studies.