LOW-TEMPERATURE SIGNAL-TRANSDUCTION - INDUCTION OF COLD ACCLIMATION-SPECIFIC GENES OF ALFALFA BY CALCIUM AT 25-DEGREES-C

To study the role of calcium in cold acclimation, we examined the rela tionship between calcium influx and accumulation of transcripts of two cas (cold acclimation-specific) genes of alfalfa, cas15 and cas18. Wh ereas a decline in temperature from 25 to 15 degrees C had little effe ct on the influx of extracellular Ca-45(2+), an increasing influx was observed when the temperature was lowered further. The influx of Ca-45 (2+) af 4 degrees C was nearly 15 times greater than at 25 degrees C. The addition of calcium chelators or of calcium channel blockers, whic h have been shown to prevent cold acclimation, inhibited the influx of extracellular Ca-45(2+) as well as the expression of cas genes at 4 d egrees C. The addition of a calcium ionophore or a calcium channel ago nist to nonacclimated cells caused the influx of extracellular Ca-45(2 +) and induced the expression of cas genes at 25 degrees C. These resu lts suggest that a cold-induced calcium influx plays an essential role in cold acclimation. To further study the role of calcium, we isolate d two sequences corresponding to calcium-dependent protein kinases. Th e transcript level of one of them was markedly upregulated at 4 degree s C, We propose a sequence of signaling events that is likely to occur early during cold acclimation and leads to the expression of cas gene s and the development of freezing tolerance.