CHARACTERIZATION OF AN ABSCISIC-ACID CARRIER IN SUSPENSION-CULTURED BARLEY CELLS

The uptake of [H-3]-abscisic acid in barley (Hordeum vulgare L. cv. He artland) cell cultures was found to be mediated through both non-satur able and saturable components. The kinetic parameters of the saturable component, determined at pH 4.5 and 21 degrees C, showed a K-m for na tural or (+)-ABA of 1.3+/-0.7 mu M and an apparent V-max of 7.0+/-2.8 nmol g(-1) cells h(-1). The carrier showed a strong preference for the natural enantiomer of ABA as compared to the unnatural one. Other sub stances tested, e.g. amino acids, organic acids, and other growth regu lators, did not appear to interfere with the carrier-mediated uptake o f ABA. At low external concentrations of ABA (below 2.0 mu M), the sat urable component was greater than the diffusion component. Similarly, between pH 4.0 and 6.0, the saturable uptake was responsible for more than 50% of the total uptake. The carrier may be important in vivo for mediating uptake when endogenous levels of ABA are tow (c. 1 mu M). T he carrier specificity was evident in inhibition experiments done with ABA analogues. Our data showed that the carrier could accommodate sma ll modifications in the ABA structure. Four analogues were able to com pete efficiently with (+)-ABA for the binding site of the carrier. Thr ee of these competitors were of the (+)-series. Only one (-)-analogue, (-)-ABA, was able to inhibit markedly the saturable uptake of (+)-ABA . The induction of the ABA-responsive gene WCS120 (Houde ef al., 1992) presented stricter requirements for the ABA molecule than the carrier , although with a similar preference for the (+)-analogues. Besides ()-ABA itself, only two of the analogues tested, both (+)-series, were able to induce the WCS120 gene after a 24 h incubation period. The abs ence of correlation between the activity of the analogues as ABA inhib itors in the carrier system, and their capacity to induce the WCS120 g ene tend to suggest that the carrier is not directly involved in the s ignal transduction pathway leading to the induction of this specific g ene.