HOOKLESS1, an ethylene response gene, is required for differential cell elongation in the Arabidopsis hypocotyl

Bending in plant tissues results from differential cell elongation. We have characterized Arabidopsis ''hookless'' mutants that are defective in differential growth in the hypocotyl. HOOKLESS1 was cloned and its predicted protein shows similarity to a diverse group of N-acetyltransferases. HOOKLESS1 mRNA is increased by treatment with ethylene and decreased in the ethylene-insensitive mutant ein2. High level expression of HOOKLESS1 mRNA results in constitutive hook curvature. The morphology of the hookless hypocotyl is phenocopied by inhibitors of auxin transport or by high levels of endogenous or exogenous auxin. Spatial patterns of expression of two immediate early auxin-responsive genes are altered in hookless1 mutants, suggesting that the ethylene response gene HOOKLESS1 controls differential cell growth by regulating auxin activity.