ETHYLENE CAN STIMULATE ARABIDOPSIS HYPOCOTYL ELONGATION IN THE LIGHT

Ethylene inhibits hypocotyl elongation in etiolated Arabidopsis seedli ngs. However, when Arabidopsis was grown in the light in the presence of ethylene or its precursor 1-aminocyclopropane-1-carboxylic acid (AC C), a marked induction of hypocotyl elongation occurred, This resulted from an increase in cell expansion rather than cell division, The eff ects of ethylene and ACC were antagonized by the ethylene action inhib itor Ag+. The elongation response was absent or weakened in a set of e thylene-insensitive mutants (etr1-3, ein2-1, ein3-1, ein4, ain1-10, ei n7). With the exception of ein4, the degree of inhibition of hypocotyl elongation was correlated with the strength of the ethylene-insensiti ve phenotype based on the triple response assay, In addition, the cons titutive ethylene response mutant ctr1-1, grown in the light, had a lo nger hypocotyl than the wild type. Exogenous auxin also induced hypoco tyl elongation in light-grown Arabidopsis. Again, the response was abo lished by treatment with Ag+, suggesting that ethylene might be a medi ator, The results showed that, depending on light conditions, ethylene can induce opposite effects on cell expansion in Arabidopsis hypocoty ls.