ROLE OF ABSCISIC-ACID IN DROUGHT-INDUCED FREEZING TOLERANCE, COLD-ACCLIMATION, AND ACCUMULATION OF LT178 AND RAB18 PROTEINS IN ARABIDOPSIS-THALIANA

To study the role of abscisic acid (ABA) in development of freezing to lerance of Arabidopsis thaliana, we exposed wild-type plants, the ABA- insensitive mutant abi1, and the ABA-deficient mutant aba-1 to low tem perature (LT), exogenous ABA, and drought. Exposure of A. thaliana to drought stress resulted in a similar increase in freezing tolerance as achieved by ABA treatment or the initial stages of acclimation, sugge sting overlapping responses to these environmental cues. ABA appears t o be involved in both LT- and drought-induced freezing tolerance, sinc e both ABA mutants were impaired in their responses to these stimuli. To correlate enhanced freezing tolerance with the presence of stress-s pecific proteins, we characterized the accumulation of RAB18 and LT178 in two ecotypes, Landsberg erecta and Coimbra, and in the ABA mutants during stress response. LT- and drought-induced accumulation of RAB18 coincided with the increase in freezing tolerance and was blocked in the cold-acclimation-deficient ABA mutants. In contrast, LT178 accumul ated in all genotypes in response to LT and drought and was always pre sent when the plants were freezing tolerant. This suggests that develo pment of freezing tolerance in A. thaliana requires ABA-controlled pro cesses in addition to ABA-independent factors.