The role of phytochrome in the de-etiolation ("greening") response.

1. Reception: Light activates a receptor molecule, a pigment called phytochrome.

2. Transduction: the phytochrome produces cGMP as a second messenger that activates a protein kinase; the phytochrome also increases flow of Ca²⁺ into the cytoplasm and activate another protein kinase.

3. Response: both pathways lead to expression of genes to produce proteins that function in the de-etiolation ("greening") response.