Based on knowledge of white light-mediated gene induction events in a photomixotrophic soybean cell culture, a comparative pharmacological dissection of gene expression in response to irradiation with red and UV light was performed in order to examine the molecular basis of cross-talk between phytochrome- and UV-regulated signalling pathways. This has proved particularly interesting for the gene encoding chalcone synthase (CHS). Whereas phytochrome control of CHS gene expression is regulated positively by cGMP and negatively by calcium and calmodulin, in UV light calcium and calmodulin appear to act as positive regulators. Results from combined red and UV irradiations indicate that the phytochrome response is activated first and that the UV photoreceptor response is activated subsequently. The apparently antagonistic functions of calcium and calmodulin may therefore be able to operate in the same cell because each response is temporally separated from the other.