Abscisic acid activates ATPase in developing apple fruit especially in fruit phloem cells
Abscisic acid (ABA) improves the sink strength by promoting the phloem unloading and regulating the assimilate metabolism in the economic sink organs of crops, although its mechanism remains unknown. The present experiment, using the techniques of the in vivo injection of ABA into the intact apple fruit attached to a growing apple tree and the in vivo incubation of the fruit tissue in the ABA-contained medium, showed that ABA strongly activated the fruit ATPase especially P-ATPase, of which the activity was doubled by ABA treatment. This ATPase activation was shown to be in vivo tissue-dependent. The ABA-induced P-ATPase activation was fruit developmental stage-, ABA dose-, medium pH- and incubation time-dependent. Physiological active (+)ABA was shown more effective to stimulate P-ATPase activity than (+/–)ABA, and two ABA analogues (–)ABA and trans-ABA, had no effect on P-ATPase activation, indicating that only physiologically active cis(+)ABA can induce the enzyme activation, and so the ABA-induced effects are stereospecific. The protein synthesis inhibitor cycloheximide was shown to have no effect on P-ATPase activation by ABA, suggesting that synthesis of new proteins was not involved in the enzyme activation. The cytochemical assay revealed that P-ATPase was activated by ABA in both the phloem and its surrounding flesh parenchyma cells, and that the most strongly P-ATPase activation was observed in the plasma membrane of sieve element/companion cell complex. These data suggest that the improvement of phloem unloading by ABA previously reported in this fruit as in other crop sink organs may be attributed, at least partly, to the ABA-induced ATPase activation especially in phloem cells.