Pegging-out
on the bend
When
cucurbitaceous seeds germinate in an orientation such that the radicle emerges
pointing downwards, two small pegs grow out from opposite sides of the
transition zone between the hypocotyls and the root. The pegs act as levers, enabling
the cotyledons, with the plumule enfolded between them, to emerge from between
the two halves of the seed coat. However, if the seeds germinate so that the
radicle emerges horizontally, the transition zone responds to gravity by
bending downwards. In this situation, a single peg forms on the concave side of
the bend. The position of peg outgrowth relative to the gravitropic bend
suggests that the two phenomena may be linked and this suggestion has been supported
by research on cucumber (Cucumis sativus)
seedlings reported by Saito et
al.,
of Sendai, Japan (pp. 413–422).
Auxin accumulates on the concave side of the bend in the classic gravitropic
response. It is very likely that the auxin is also involved in the initiation
of peg formation because if IAA is applied to the convex side of the bend, a
peg also grows out from there. Following the establishment of the auxin
gradient across the transition zone, there is an up-regulation of
auxin-inducible genes. One of these genes, CS-ACS1,
encodes ACC synthase, which catalyses the last step in ethylene formation. CS-ACS1
mRNA accumulates preferentially on the concave side of the bend
(as shown by very elegant quantitative RT–PCR and in situ hybridization
experiments). This leads to preferential synthesis and accumulation of ethylene
on the concave side of the bend. Finally, exposure to exogenous ethylene after
peg initiation (whether natural or induced by IAA application) enhances peg
growth, whereas early exposure to ethylene inhibits initiation of the peg. Here
then is a beautiful example of the sequential action of two hormones, with the
second, ethylene, being dependent on the first, auxin.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
