Circumnutation as a visible plant action and reaction: physiological, cellular and molecular basis for circumnutations.

Circumnutation is a helical organ movement widespread among plants. It is variable due to a different magnitude of trajectory (amplitude) outlined by the organ tip, duration of one cycle (period), circular, elliptical, pendulum-like or irregular shape and clock- and counterclockwise direction of rotation. Some of those movement parameters are regulated by circadian clock and show daily and infradian rhythms. Circumnutation is influenced by light, temperature, chemicals and can depend on organ morphology. The diversity of this phenomenon is easier to see now that the digital time-lapse video method is developing fast. Whether circumnutation is an endogenous action, a reaction to exogenous stimuli or has a combined character has been discussed for a long time. Similarly, the relationship between growth and circumnutation is still unclear. In the mechanism of circumnutation, epidermal and endodermal cells as well as plasmodesmata, plasma membrane, ions (Ca(2+), K(+) and Cl(-)), ion channels and the proton pump (H(+)ATPase) are engaged. Based on these data, the hypothetical electrophysiological model of the circumnutation mechanism has been proposed here. In the recent circumnutation studies, gravitropic, auxin, clock and phytochrome mutants are used and new functions of circumnutation in plants' life have been investigated and described.