Ethylene can stimulate <i>Arabidopsis</i> hypocotyl elongation in the light

<jats:p> Ethylene inhibits hypocotyl elongation in etiolated <jats:italic>Arabidopsis</jats:italic> seedlings. However, when <jats:italic>Arabidopsis</jats:italic> was grown in the light in the presence of ethylene or its precursor 1-aminocyclopropane-1-carboxylic acid (ACC), a marked induction of hypocotyl elongation occurred. This resulted from an increase in cell expansion rather than cell division. The effects of ethylene and ACC were antagonized by the ethylene action inhibitor Ag <jats:sup>+</jats:sup> . The elongation response was absent or weakened in a set of ethylene-insensitive mutants ( <jats:italic>etr1-3</jats:italic> , <jats:italic>ein2-1</jats:italic> , <jats:italic>ein3-1</jats:italic> , <jats:italic>ein4</jats:italic> , <jats:italic>ain1-10</jats:italic> , <jats:italic>ein7</jats:italic> ). With the exception of <jats:italic>ein4</jats:italic> , the degree of inhibition of hypocotyl elongation was correlated with the strength of the ethylene-insensitive phenotype based on the triple response assay. In addition, the constitutive ethylene response mutant <jats:italic>ctr1-1</jats:italic> , grown in the light, had a longer hypocotyl than the wild type. Exogenous auxin also induced hypocotyl elongation in light-grown <jats:italic>Arabidopsis</jats:italic> . Again, the response was abolished by treatment with Ag <jats:sup>+</jats:sup> , suggesting that ethylene might be a mediator. The results showed that, depending on light conditions, ethylene can induce opposite effects on cell expansion in <jats:italic>Arabidopsis</jats:italic> hypocotyls. </jats:p>