In the classical view, for an organism to be able to respond to a given stimulus, it should be equipped with a complete signaling pathway that ends with the modulation of the activities of regulatory elements that affect the expression of stimulation-dependent genes. The activities of the gene products would eventually lead to changes at the cell level and at the organismal level. As mentioned above, sensing and transduction of mechanical stimuli are among the oldest evolutionary mechanisms that enable plant cells to respond to external cues. It should be noted, however, that although reactions to osmoticum, touch, and gravity are all responses to physical signals, they can be and are differentiated according to their "directionality." Touch stimuli arrive from the outside of the cell and are signaled into the cell. In contrast, the reaction to a gravitational stimulus is initiated through its sensing inside the cell. Finally, the reaction to osmotic changes is most probably bidirectional, as it involves sensing the stimulus at both the plasma membrane and the tonoplast.
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