PLANT SIGNALS A signal is denned as a chemical or environmental change that induces a cellular response. As sessile organisms, plants have developed a broad series of responses to numerous signals from the environment, symbiotic partners, and pathogen attack, in addition to internal signaling that controls growth and development Environmental signals range from changes in temperature, water availability, and light (daylength, unidirectional light, or light wavelength), to salt or metal exposure, changes in orientation to gravity, and wind or touch. Chemical signals produced by the plant coordinate developmental and growth changes between tissues, between cells, and between subcellular compartments. Major plant signaling compounds include internally derived compounds such as hormones, polypeptides, and sugar, as well as factors associated with cell-to-cell signaling such as those involved in responses to pathogens.