A NEW COLD-INDUCED ALFALFA GENE IS ASSOCIATED WITH ENHANCED HARDENINGAT SUBZERO TEMPERATURE

When alfalfa (Medicdgo sativa L. cv Apica) plants grown at room temper ature are transferred to 2-degrees-C, the temperature at which 50% of the plants fail to survive (LT50) decreases from -6 to -14-degrees-C d uring the first 2 weeks but then increases to -9-degrees-C during the subsequent 2 weeks. However, when plants are kept for 2 weeks at 2-deg rees-C and then transferred to -2-degrees-C for another two weeks, the LT50 declines to -16-degrees-C. These changes in freezing tolerance a re paralleled by changes in transcript levels of cas15 (cold acclimati on-specific gene encoding a 14.5-kD protein), a cold-induced gene. Col d-activation of cas15 occurs even when protein synthesis is inhibited by more than 90%, suggesting that cold-initiated events up to and incl uding the accumulation of cas15 transcripts depend on preexisting gene products. cas15 shows little homology to any known gene at the nucleo tide or amino acid level. The deduced polypeptide (CAS15) of 14.5 kD c ontains four repeats of a decapeptide motif and possesses a bipartite sequence domain at the carboxy terminus with homology to the reported nuclear-targeting signal sequences. Although the relative amount of ca s15 DNA as a fraction of the total genomic DNA is similar in cultivars with different degrees of freezing tolerance, its organization in the genome is different. The possible role of cas15 in the development of cold-induced freezing tolerance is discussed.