The dual effects of ethylene on the negative gravicurvature of arabidopsis inflorescence, an intriguing action model for the plant hormone ethylene

Ethylene was found to alter the shoot gravitropic response a century ago. However, its actual effects remained controversial over the century. In the present study, we investigated the effects of ethylene on the gravitropic response of Arabidopsis inflorescence and discovered that although the intact Arabidopsis inflorescence bent upward at the rate of 43°±3°/h in the first 1.5 h under the given control growth conditions, it bent upward at a rate of 20°±3° and 64°±14°/h in the presence of exogenous ethylene if the plants were pretreated for 0.5 and 12 h, respectively, before the initiation of gravistimulation. The increase in gravicurvature rate is proportional to the length of ethylene pretreatment. The minimum ethylene pretreatment time required for achieving the maximum curvature rate is 10—11 h with removal of exogenous ethylene. The stimulatory effect of ethylene on the inflorescence gravicurvature requires a latent period of time (1 h) to become measurable. In contrast, the inhibitory effect of ethylene becomes measurable shortly after application of ethylene. The stimulatory effect remained nearly unchanged when the applied ethylene concentration increased from 0.1 to 10 ml/L. However, the inhibitory effect increased substantially as ethylene concentration increased to 10 mL/L. These results suggest that ethylene simultaneously exerts both stimulatory and inhibitory effects on the gravitropic response of the inflorescence, and the stimulatory effect dominates over the inhibitory effect during the interplay between the two in the inflorescence. A “Yin and Yang” action model is hereby proposed to address the interplay between the two effects of ethylene. They may act through distinct signaling pathways.