Proline level is partly under the control of abscisic acid in canola leaf discs during recovery from hyper-osmotic stress

The relationships between the changes induced in abscisic acid (ABA) and proline contents were investigated in canola ( Brassica napus L.) leaf discs (CLDs) subjected to sequential hyper- and hypo-osmotic treatments. Changes in ABA content were found to precede that of proline, suggesting that ABA could exert regulatory effects on both osmo-induced proline accumulation and its subsequent mobilization during recovery. When exogenously supplied in the light during recovery, ABA caused inhibition of the apparent rate of proline mobilization with an I50 of about 20 μ M . Under dark conditions, the ABA effect was not observed. This is in accordance with the fact that this effect could be suppressed in the light while using photosynthesis inhibitors. The ABA effect was partly restored under dark conditions when the CLDs were supplied with NADPH at high concentration. Under both conditions, ABA exerted its effect when supplied together with cycloheximide, suggesting that it does not rely on protein synthesis. When turgid CLDs, which heavily absorbed exogenously supplied l -proline, were incubated in the light on the reference medium with ABA, proline mobilization was also restricted, provided ABA was supplied during the period of proline uptake. Surprisingly, in CLDs that had recovered their full turgor after application of successive hyper- and hypo-osmotic treatments, application of ABA led to a very high accumulation of proline. In contrast, in freshly cut turgid CLDs incubated in presence of ABA, the basic proline level was only weakly enhanced. Thus, the apparent inhibitory effect of ABA on proline mobilization could be mediated through both an activation of biosynthesis and an inhibition of catabolism of this amino acid via light-dependent processes that remain to be elucidated.