Arabidopsis seedlings develop a hook-like structure at the apical part of t
he hypocotyl when grown in darkness. Differential cell growth processes res
ult in the curved hypocotyl hook. Time-dependent analyses of the hypocotyl
showed that the apical hook is formed during an early phase of seedling gro
wth and is maintained in a sequential phase by a distinct process, Based on
developmental genetic analyses of hook-affected mutants, we show that the
hookless mutants (hls1, cop2) are involved in an early aspect of hook devel
opment. From time-dependent analyses of ethylene-insensitive mutants, later
steps in hook maintenance were found to be ethylene sensitive. Regulation
of differential growth was further studied through examination of the spati
al pattern of expression of two hormone-regulated genes: an ethylene biosyn
thetic enzyme and the ethylene receptor ETR1, Accumulation of mRNA for AtAC
O2, a novel ACC (1-aminocyclopropane-1-carboxylic acid) oxidase gene, occur
red within cells predominantly located on the outer-side of the hook and wa
s tightly correlated with ethylene-induced exaggeration in the curvature of
the hook. ETR1 expression in the apical hook, however, was reduced by ethy
lene treatment. Based on the expression pattern of ETR1 and AtACO2 in the h
ook-affected mutants, a model for hook development and maintenance is propo
sed.