Emerging themes of plant signal transduction.

The successful existence of all higher organisms is dependent upon their ability to coordinate complex developmental changes and to sense and respond to fluctuations in their surroundings. Responses to developmental and environmental cues occur by stimulus-response coupling: a stimulus is perceived by the cell, a signal is generated and transmitted (signal transduction), and a biochemical change is instigated (the response). This process most often requires the recognition of the stimulus by a receptor and the subsequent use of chemical second messengers and/or effector proteins to transmit a signal that will then trigger the appropriate response. Key properties of signal transduction are speed, sensitivity (achieved by amplification), and specificity, all of which are controlled by a network of positively and negatively acting elements. Whereas the positively acting intermediates are essential to drive signal transduction, the negative elements are responsible for ensuring a response that is quantitatively appropriate, correctly timed, and highly coordinated with other activities of the cell. Specifically, negative control can (1) allow changes in sensitivity to a particular stimulus, permitting a signaling pathway to work over a broad dynamic range of stimulus intensities; (2) terminate a response when it is completed, even if the stimulus may remain (sometimes referred to as desensitization or adaptation); and (3) allow a signaling pathway to be reused. In animal cells, the signal transduction events that couple avariety of stimuli with their responses have been well characterized, as have the mechanisms for their negative control. Plants, however, are challenged in different ways from animals because their developmental programs are generally more flexible and because they are unable to escape at will from unfavorable environmental situations. These differences indicate the increased adaptability of plants over animals and lead us to speculate about the nature and control of the sensing systems plants employ to regulate their development and behavior. Have these different constraints necessitated the evolution of mechanisms altogether different from those found in animals? To some extent the answer appears to be yes; both