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

Ethylene was found to alter the shoot gravitropic response a century ago, H owever, its actual effects remained controversial over the century, In the present study, we investigated the effects of ethylene on the gravitropic r esponse of Arabidopsis inflorescence and discovered that although the intac t Arabidopsis inflorescence bent upward at the rate of 43 degrees +/- 3 deg rees /h in the first 1.5 h under the given control growth conditions, it be nt upward at a rate of 20 degrees+/-3 degrees and 64 degrees +/-14 degrees /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 p retreatment. The minimum ethylene pretreatment time required for achieving the maximum curvature rate is 10-11 h with removal of exogenous ethylene, T he stimulatory effect of ethylene on the inflorescence gravicurvature requi res a latent period of time (1 h) to become measurable. In contrast, the in hibitory effect of ethylene becomes measurable shortly after application of ethylene, The stimulatory effect remained nearly unchanged when the applie d ethylene concentration increased from 0.1 to 10 mul/L. However, the inhib itory effect increased substantially as ethylene concentration increased to 10 muL/L. These results suggest that ethylene simultaneously exerts both s timulatory and inhibitory effects on the gravitropic response of the inflor escence, and the stimulatory effect dominates over the inhibitory effect du ring the interplay between the two in the inflorescence, A "Yin and Yang" a ction model is hereby proposed to address the interplay between the two eff ects of ethylene, They may act through distinct signaling pathways.