Cytosolic free calcium mediates red light-induced photomorphogenesis

Light is a primary environmental signal regulating plant growth and form. The receptor phytochrome responds to red light and induces changes in membrane properties and gene expression through an unknown transduction pathway which is the subject of intense study. Etiolated wheat leaf protoplasts swell in response to red light, by a mechanism believed to be similar to that involved in phytochrome-regulated leaf growth and unrolling. Here we report that this physiological response is preceded by a transient increase in free calcium in the cytosol, followed by a decrease to below resting level. The timing of this response varied between protoplasts. Cytosolic calcium transients induced by photolytic release of calcium or inositol 1,4,5-trisphosphate from chemically caged forms(8-13) inside these protoplasts resulted in red light induced increases in protoplast volume being mimicked. Our results support the hypothesis that phytochrome-mediated signals are transduced through calcium and help to resolve a long-standing issue in plant physiology.