The ups and downs of hormone treatment
Many of us will remember those simple student experiments on
apical dormancy, using decapitated Phaseolus coccineus plants and blobs
of lanolin paste containing auxin. We concluded that auxin derived from the
apex and transported down the stem normally suppressed lateral shoot growth.
However, we now know that it is more complicated; indeed, even now, there are
details that remain to be elucidated, as so clearly presented by Morris
Cline and Choonseok Oh (Columbus, OH, pp. 891–897). Firstly there have been
suggestions that it is ABA,
synthesized or released in response to auxin and transported acropetally (towards
the apex), that inhibits lateral shoot growth. However, other authors have
rejected this view and postulate instead that an as yet unidentified, carotenoid-derived
inhibitor, transported acropetally, is the real inhibitor. The loss of apical
dominance in mutants that lack carotenoid-cleaving dioxygenase is cited as
further evidence for this view. The present authors have therefore carried out
classical decapitation and hormone-replacement experiments with Ipomoea nil
(morning glory), Solanum lycopersicum (tomato) and Helianthus annuus (sunflower). In all three species, auxin could replace the
apical bud in inhibiting outgrowth of laterals, as in our classical
experiments. Indeed, there was even some inhibition of lateral shoot growth if
auxin was applied basally; this was particularly marked in S. lycopersicum.
Apical application of ABA to decapitated shoots
did not restore apical dominance but there was moderate inhibition of lateral
shoot growth if ABA was applied basally in I. nil and S. lycopersicum. Indeed,
in the former, the effects of basally applied ABA and apically applied auxin were additive.
ABA therefore
does indeed move acropetally but in these experiments can at most only
partially replace the inhibitory effects of auxin. A role for a third player,
the unknown carotenoid derivative, remains a viable possibility, in addition to
which the variation between species is a feature of note.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
