Potentiation of proline accumulation in oilseed rape leaf discs exogenously supplied with combinations of PEG and cryoprotective agents is associated with overproduction of ABA

Physiological processes involved in the control of the magnitude of the stress-induced proline (Pro) response of higher plants are not fully understood. Here we are dealing with Pro accumulation by rape leaf discs (RLDs) treated in vitro, under light conditions, with dual unusual systems containing low concentrations of the non-permeant polymer PEG 6000 (PEG) added with readily permeant cryoprotective agents (CPAs). At osmotically active concentrations, dimethylsulfoxide (DMSO), glycerol, 1,3-propanediol, ethylene glycol and dimethylformamide (DMF) behaved as very poor inducers of the Pro response when provided alone. On the contrary when all those substances (apart DMF) were supplied at onset of treatments, in combination with PEG, Pro levels greatly exceeded what could be predicted through simple additivity of the effects of those substances provided individually. This suggested potentiation effects at the level of some component(s) of Pro metabolism involved in stress-induced Pro accumulation. We have also demonstrated that exogenous ABA could substitute for DMSO (but not PEG), this other binary system also inducing (through potentiation) high rate of Pro accumulation. The strinking similarity between the responses induced with PEG + DMSO and PEG + ABA suggested that DMSO induced increases in the endogenous amount of ABA which might be, at its turn, acting as exogenously supplied ABA. Additional amounts of ABA actually accumulated in leaf discs treated with the dual system PEG + DMSO. Other CPAs tested in this study, except dimethylformamide (DMF), might also be acting on the same way. Potentiating effects, associated with enhanced amount of ABA, were also found to result from combinations of PEG with a range of organic and inorganic substances which mimick to some extent the cytosolute composition of plant cells. Together, our results gain some insight into the physiological mechanisms involved in the control of stress-induced Pro accumulation and strongly suggest their relationship with stress-induced changes in ABA content.