Low-temperature signal transduction: Induction of cold acclimation-specific genes of alfalfa by calcium at 25°C

Abstract

To study the role of calcium in cold acclimation, we examined the relationship between calcium influx and accumulation of transcripts of two cas (cold acclimation-specific) genes of alfalfa, cas15 and cas18. Whereas a decline in temperature from 25 to 15°C had little effect on the influx of extracellular 45Ca2+, an increasing influx was observed when the temperature was lowered further. The influx of 45Ca2+ at 4°C was nearly 15 times greater than at 25°C. The addition of calcium chelators or of calcium channel blockers, which have been shown to prevent cold acclimation, inhibited the influx of extracellular 45Ca2+ as well as the expression of cas genes at 4°C. The addition of a calcium ionophore or a calcium channel agonist to nonacclimated cells caused the influx of extracellular 45Ca2+ and induced the expression of cas genes at 25°C. These results suggest that a cold-induced calcium influx plays an essential role in cold acclimation. To further study the role of calcium, we isolated two sequences corresponding to calcium-dependent protein kinases. The transcript level of one of them was markedly upregulated at 4°C. We propose a sequence of signaling events that is likely to occur early during cold acclimation and leads to the expression of cas genes and the development of freezing tolerance.