Plant cells have evolved ways to perceive different signals from their environment, both external to the plant (such as cold, light, heat, low oxygen, drought, and high salt) and internal (such as hormones). Following perception of the signal, plant cells respond by changing various biochemical functions and anatomical developments using a signal transduction pathway. Ca 2+ acts as a secondary messenger in plant signal transduction, as was first shown in animals. Ca 2+ triggers numerous cell and tissue responses that influence growth and development of a plant. The calcium-binding protein, calmodulin, is involved in executing Ca 2+ signals by affecting transcription of genes and regulating target proteins involved in the response.