Factors regulating ethylene biosynthesis in etiolated Arabidopsis thalianaseedlings

We examined the effect on etiolated Arabidopsis thaliana seedlings of a wid e variety of factors that are known to induce ethylene in other plant tissu es, Auxin, cytokinin, brassinosteroid and cupric ion were found to highly e levate ethylene production in these seedlings, but several other signaling compounds, as well as wounding and mechanical stimulation, had little or no effect, A mutant that disrupts the ACS5 gene (cinc5) was partially defecti ve in the induction of ethylene in the presence of brassinosteroids, sugges ting a role for this isoform in mediating this response, Cytokinin displaye d a synergistic interaction with bath brassinosteroid and auxin, while the other interactions tested were essentially additive. Auxin and cytokinin ha ve been shown to act synergistically to elevate ethylene biosynthesis in ma ny other plant tissues, We show that the synergism between cytokinin and au xin in Arabidopsis is due to an enhancement of the effects of auxin, but no t by increased elevation of ACS4 mRNA levels. These results suggest that cy tokinin acts post-transcriptionally to increase ACS4 function, which, coupl ed with the observation that auxin elevates ACS4 mRNA levels, accounts for the synergistic interaction.