Abscisic Acid-Regulated Gene Expression in Relation to Freezing Tolerance in Alfalfa

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Abstract
A comparison of abscisic acid (ABA)-induced and cold-acclimation-induced freezing tolerance in two alfalfa cultivars (Medicago falcata cv Anik and Medicago sativa cv Trek) indicates that ABA alone can increase freezing tolerance to some extent, but for the development of maximum tolerance, cold acclimation is essential. Analysis of in vivo-labeled proteins of ABA-treated seedlings reveals that ABA causes several changes in the pattern of protein synthesis. While some of these changes appear to be similar to those induced by cold acclimation, others seem to be specific to ABA treatment. From a cDNA library constructed against poly(A+) RNA of a freezing-tolerant alfalfa cultivar, Anik, a cDNA clone, pSM1409, has been isolated. Expression of the gene corresponding to this clone, as determined by northern hybridization, is regulated most likely at the transcriptional level by cold acclimation and exogenously supplied ABA. However, the increase in the transcript level is much greater in the freezing tolerant cultivar Anik than in the relatively freezing-sensitive cultivar, Trek. The role of ABA in the acquisition of freezing tolerance is discussed.