ALTERATIONS IN WATER STATUS, ENDOGENOUS ABSCISIC-ACID CONTENT, AND EXPRESSION OF RAB18 GENE DURING THE DEVELOPMENT OF FREEZING TOLERANCE INARABIDOPSIS-THALIANA

Treatments as diverse as exposure to low temperature (LT), exogenous a bscisic acid (ABA), or drought resulted in a 4 to 5 degrees C increase in freezing tolerance of the annual herbaceous plant Arabidopsis thal iana. To correlate the increase in freezing tolerance with the physiol ogical changes that occur in response to these treatments, we studied the alterations in water status, endogenous ABA levels, and accumulati on of rab18 (V. Lang and E.T. Palva [1992] Plant Mol Biol 20: 951-962) mRNA. Exposure to LT and exogenous ABA caused only a minor decline in total water potential (Psi(w)), in contrast to a dramatic decrease in Psi(w) during drought stress. Similarly, the endogenous ABA levels we re only slightly and transiently increased in LT-treated plants in con trast to a massive increase in ABA levels in drought-stressed plants. The expression of the ABA-responsive rab 18 gene was low during the LT treatment but could be induced to high levels by exogenous ABA and dr ought stress. Taken together, these results suggest that the moderate increases in freezing tolerance of A. thaliana might be achieved by di fferent mechanisms. However, ABA-deficient and ABA-insensitive mutants of A. thaliana have impaired freezing tolerance, suggesting that ABA is, at least indirectly, required for the development of full freezing tolerance.