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 Cardamine hirsuta produces more ethylene than shade avoiders such as Arabidopsis thaliana (a close relative of C. hirsuta) and tomato (Solanum lycopersicum) 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 C. hirsuta 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 A. thaliana and C. hirsuta. ACC-dependent ethylene production also repressed hypocotyl elongation under low W and in the dark in C. hirsuta. By contrast, in A. thaliana, ACC supplementation inhibited hypocotyl elongation in the dark but stimulated it under W. Most interestingly, elongation of dark-grown A. thaliana seedlings was also repressed by exposure to the volatiles released by ACC-grown A. thaliana 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.