Graphical Abstract
Biological clocks are an important theme for science in the 21st century. In contrast to animals, the biological clock in plants is far from being understood, although it was originally discovered from observation of the leaf movement of Mimosa pudica L in the 18th century. This review focuses on the so-called leaf-movement factors, chemical substances which control the movement of plant leaves, and their regulation by the biological clock (represented schematically). Mimosa plants are also well known for rapid movement of their leaves and the chemical substances inducing this rapid movement are also included in this review.
Abstract
The leaves of Mimosa pudica L. are well known for their rapid movement when touched. Recently, we were able to isolate an excitatory substance in small quantities from this plant, which consists of three different components (potassium L-malate, magnesium trans-aconitate, and dimethylammonium salt). Many plants close their leaves in the evening, as if to sleep, and open them early in the morning (nyctinastic leaf movement). This circadian rhythm is known to be controlled by the biological clock of such plants. Extensive studies on other nyctinastic plants led to the isolation of a variety of leaf-opening substances (LOSs) and leaf-closing substances (LCSs). Based on our experiments on these bioactive substances, we found that the circadian rhythmic leaf movement of these plants is initiated by the regulated balance of LOSs and LCSs. The balance of concentration between the two leaf-movement factors (LMFs) is inversed during the day. The glycoside-type LMF is hydrolyzed with β-glucosidase, the activity of which is regulated by the biological clock. The circadian rhythm observed in the leaf movement is introduced by activation of β-glucosidase regulated by the biological clock.
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