Beyond Chemical Triggers: Evidence for Sound-Evoked Physiological Reactions in Plants

Abstract

Sound is ubiquitous in nature. Recent evidence supports the notion that naturally occurring and artificially generated sound waves contribute to plant robustness. New information is emerging about the responses of plants to sound and the associated downstream signaling pathways. Here, beyond chemical triggers which can improve plant health by enhancing plant growth and resistance, we provide an overview of the latest findings, limitations, and potential applications of sound wave treatment as a physical trigger to modulate physiological traits and to confer an adaptive advantage in plants. We believe that sound wave treatment is a new trigger to help protect plants against unfavorable conditions and to maintain plant fitness.

Keywords: physical trigger; plant protectant; plant stimulant; ripening delaying; sound vibration; transcriptome.

FIGURE 1

Sound production and perception in plants. (A) Sound production. Plants produce sound vibrations in their xylem via the generation of tension in the xylem vessel when its diameter decreases (Hölttä et al., 2005). Additionally, gas bubbles produced in xylem vessels during transpiration may produce sound (Laschimke et al., 2006). (B) Sound Perception. Although there are no visible alterations, transcriptional and translational changes occur in plants exposed to sound vibrations. Levels of mechano-stimulus responsive, signaling-related, redox homeostasis, and defense-related transcripts are changed in sound-exposed plants (Ghosh et al., 2016). However, the specific organs or proteins used for sound perception have not yet been identified.

FIGURE 2

Sound waves as a plant stimulant and protectant. Artificial sound treatment can elicit various effects in plants. First, enhancement of seed germination and plant growth. Sound promotes plant growth by regulating the plant growth hormones indole-3-acetic acid (IAA) and gibberellin (Bochu et al., 2004; Ghosh et al., 2016). Second, induction of plant defense responses against pathogens. Sound pretreatment enhances plant immunity against subsequent pathogen attacks by activating the plant defense hormones salicylic acid (SA) and jasmonic acid (JA) (Hassanien et al., 2014; Ghosh et al., 2016). Third, induction of abiotic stress tolerance. For instance, sound treatment triggers drought tolerance by changing the elasticity and flexibility of the cell wall, which affects the ability of plants to absorb water (Jeong et al., 2014). Fourth, perturbation of ripening. Sound treatment disrupts the ripening of tomato fruit. Ethylene production is delayed by down-regulation of ethylene biosynthesis and expression of signaling-related genes (Kim et al., 2015). Fifth, enhancement of the photosynthetic capacity. Sound treatment increases expression of photosynthesis-related genes, such as those encoding fructose 1,6-bisphosphate aldolase and the rubisco small sub-unit, and may induce CO₂ fixation (Jeong et al., 2008; Uematsu et al., 2012).