Inositol 1,4,5-trisphosphate formation in leaves of winter oilseed rape plants in response to freezing, tissue water potential and abscisic acid

Time courses of formation of inositol 1,4,5-trisphosphate (IP3) were followed in the leaves of non-acclimated and cold (2 degrees C)-acclimated winter oilseed rape (Brassica napus L. var. oleifera) plants, subjected to different freezing temperatures or to polyethylene glycol 8000 (PEG) and abscisic acid (ABA) treatments. Changes in water potential (Psi(w)) and in ABA level in the frost- and PEG-treated tissues were also determined. Results obtained indicate that temperatures slightly higher than LT(50) induced a transient and substantial increase in IP3 level, both in non-acclimated and cold-acclimated tissues. At comparable freezing temperature (-5 degrees C) the response of cold-acclimated leaves was lower than that of non-acclimated ones. The PEG-depedent decrease in Psi(w) to -0.9 MPa or ABA (0.1 mM) treatment gave rise to a transient increase in IP3 content in non-acclimated tissues only. Collectively, the data indicate that cold acclimation of plants may lead to lower cell responsiveness to the factors studied in terms of induction of IP3 formation. Changes in the IP3 content, observed in the present experiments, support our previous suggestion that non-killing freezing temperatures may induce the phosphoinositide pathway, both in non-acclimated and cold-acclimated tissues. Lowering of tissue water potential to some threshold value or a high exogenous ABA supply may mimic the freezing-dependent reaction in the non-acclimated leaves.