Figure 1.

A cylinder lock as alternative key metaphor for cellular stimulus-response coupling. A, An environmental stimulus has effects on diverse cellular actuating variables, such as [Ca²⁺], [H₂O₂], pH, [ATP], and membrane potential. Each variable is reflected here by a red bar. Together they form an amplitude pattern. B, An environmental stimulus affects some cellular actuating variables, and a new amplitude pattern (cellular state) is established. C, The new pattern is decoded by a specific key (i.e. a cellular receptor protein with specific sensitivity to the corresponding cellular actuating variables). The key that fits unlocks a gate in the signaling network and provides access to the correct physiological response that the plant species has learned during its evolution.