Plants are the ability to sense and to re-orient their growth in response to various stimuli including light, gravity, wind, humidity and mechanical stimuli. The response of a plant organ to a directional stimulus is called tropism.
Even the magnitude of gravity seems to be constant on the surface of the earth, its direction and its magnitude might be affected by various factors like for example, wind for the stems and soil parameters for the root system. Therefore, gravitropism can be regarded as a crucial factor in the posture control of stems and in the penetration for roots into the soil.
In roots, specialized sensory cells (called statocytes) located in the root apex perceive signal gravity. Upon a gravistimulation a change was observed in the polarity of cell perceiving gravity signal with an amyloplast sedimentation on the peripheral side. Transduction pathways were immediately activated with changes of calcium-dependant pathways. In parallel, many studies have demonstrated that cytoplasmic free Ca2+ concentration (Ca2+cyt) is affected by environmental stimuli. The regulation of this homeostasis involves a series of transduction events such as the synthesis and activation of calcium binding and targeted proteins, like calmodulin.