Low-temperature sensing in olive tree: calcium signalling and cold acclimation

Olive tree is a warm-temperature evergreen tree with low tolerance to frost, although cultivars which differ in terms of coldacclimation have been empirically selected. In herbaceous species, free cytosolic calcium ([Ca2+]c) is involved in coldacclimation. The objective of this study was to measure [Ca2+]csignalling in the olive tree during coldacclimation and to assess the possibility of using [Ca2+]c as an early genotype-selection marker of cold susceptibility. To this end, non-cold-acclimated and cold-acclimated leaf protoplasts of cultivars differing in terms of cold susceptibility were analysed. Cold shocks of various amplitude applied to non-cold-acclimated protoplasts caused consistent and transient increases in [Ca2+]c. A decrease of 0.05 °C/s (i.e. ΔT/dt=2.5 °C/50 s) was the threshold cooling rate at which a significant increase in [Ca2+]c could still be observed. When the protoplasts were incubated with either 8-(N,N-di-methylamino)octyl 3,4,5-trimethoxy-benzoate (TMB-8; organelle Ca2+ channel blocker) or Gd3+ (plasma membrane Ca2+ channel blocker), applying the threshold cooling rate, the increase in [Ca2+]c was partially inhibited, suggesting that both an intracellular release of Ca2+ and an influx through the plasma membrane are involved. When applying repeated cold shocks, the transient increases in [Ca2+]c were reduced only when using a non-severe ΔT/dt. In protoplasts subjected to standard acclimation, the increases in [Ca2+]c were further reduced, or inhibited, depending on the cold susceptibility of the cultivar, suggesting that the Ca2+ response is involved in a long-term adaptation to cold.