Diazepam, the modulator of GABAA receptors, alters the leaf-folding and expanding speed of Mimosa pudica

Mimosa pudica L. closes leaves in response to various stimuli. This curious reflex, called seismonastic movement, is said to be advantageous for their survival from predatory insects but is a trade-off between efficient energy acquisition. While it has been revealed that action potentials and water translocation of motor cells in the pulvinus are involved in this reaction, little is known about under what conditions or substances this movement is controlled. M. pudica has also been known to synthesize many secondary metabolites, and the extracts are valued in some countries for their anxiolytic effects. Benzodiazepines are commonly used anxiolytics and are known as allosteric modulators of some gamma-aminobutyric acid (GABA) receptors that are involved in inhibitory neurotransmission in animals. Although the role of GABA and its receptors in plants has been gradually unraveled in recent decades, neither the role of the endogenous benzodiazepine-like compounds nor the effect of exogenous administration of benzodiazepines on the plant has been understood. In this study, we investigated the effect of exogenous benzodiazepines on touch-induced leaf behavior. We treated the Mimosa plant with the solution of diazepam, a major representative of benzodiazepines, via root absorption. One hour after the roots were immersed in the solution, the speed of leaf closing was slowed down, and it lasted until 6 hours. Furthermore, the leaf-expanding movement (recovery) was accelerated with the diazepam treatment. Our findings may imply that benzodiazepines affect the generation of action potentials and/or osmosis-driven water movement of motor cells by regulating anion efflux and water transport.