Millisecond UV-B irradiation evokes prolonged elevation of cytosolic-free Ca2+ and stimulates gene expression in transgenic parsley cell cultures

Chalcone synthase (CHS) is a key enzyme leading to the generation of protective flavonoids in plants under environmental stress. Expression of the CHS gene is strongly upregulated by exposures to UV light, a response also observed in heterotrophic parsley cell cultures. Although there are hints that the stimulus for CHS expression may be coupled to UV-B irradiation through a rise in cytosolic-free Ca2+ ([Ca2+](i)), the temporal relationship of these events has never been investigated critically. To explore this question, we have used a CHS promoter/luciferase (CHS/LUC) reporter gene fusion and recorded its expression and [Ca2+](i) elevation in a transgenic parsley cell culture following millisecond light pulses. Luciferase expression was enhanced maximally seven- (+/- 2) fold by 30 10ms flashes of UV-B light. The response was specific to wavelengths of 300-330 nm and could be inhibited in the presence of the Ca2+ channel blocker nifedipine. In parallel measurements, using Fura-2 fluorescence ratio microphotometry, we found that 10ms UV-B flashes also evoked it gradual and prolonged rise of [Ca2+](i) in the parsley cells which was irreversible within the timescale of these experiments, but could be prevented by prior treatment with nifedipine. These, and additional results, indicate a remarkably high temporal sensitivity to, and specificity for, UV-B light in CHS gene expression independent of UV-mediated DNA damage by thymine dimerization. The ability of transient UV-B stimulation to evoke prolonged elevations of [Ca2+](i) suggests a functional coupling between the initial light stimulus and subsequent gene expression that takes place many tons of minutes later.