Effect of Higher Ethylene Levels Emitted by Shade-Avoider Plants on Neighboring Seedlings

Plants of several species, including crops, change their volatilome when exposed to a low ratio of red to far-red light (low R/FR) that informs about the presence of nearby plants (i.e., proximity shade). In particular, the volatile hormone ethylene was shown to be produced at higher levels in response to the low R/FR signal in shade-avoider plants. Here, we show that the shade-tolerant species <i>Cardamine hirsuta</i> produces more ethylene than shade avoiders such as <i>Arabidopsis thaliana</i> (a close relative of <i>C. hirsuta</i>) and tomato (<i>Solanum lycopersicum</i>) under white light (W). However, exposure to low R/FR (specifically to FR-supplemented W, referred to as W+FR or simulated shade) resulted in only a slight increase in ethylene emission in <i>C. hirsuta</i> compared to shade avoiders. Stimulation of ethylene production by growing plants in media supplemented with 1-aminocyclopropane-1-carboxylate (ACC) resulted in reduced hypocotyl growth under W+FR in both <i>A. thaliana</i> and <i>C. hirsuta</i>. ACC-dependent ethylene production also repressed hypocotyl elongation under low W and in the dark in <i>C. hirsuta</i>. By contrast, in <i>A. thaliana</i>, ACC supplementation inhibited hypocotyl elongation in the dark but stimulated it under W. Most interestingly, elongation of dark-grown <i>A. thaliana</i> seedlings was also repressed by exposure to the volatiles released by ACC-grown <i>A. thaliana</i> or tomato plants. This observation suggests that increased ethylene levels in the headspace can indeed impact the development of nearby plants. Although the amount of ethylene released by ACC-grown plants to their headspace was much higher than that released by exposure to low R/FR, our results support a contribution of this volatile hormone on the communication of proximity shade conditions to neighboring plants.