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Drop dead gorgeous
In some plant species, successful pollination is followed by a very marked decrease in the attractiveness of flowers to pollinators. The flowers may close to prevent access; the petals may change or lose colour, become senescent and drop off as the plant diverts resources away from flowering to embryogenesis. An extensive survey of the plant kingdom shows that this sequence is mainly, but not completely, confined to dicots; within the dicots it does not occur in those species where the female phase precedes the male phase, or in species where the flowers are in any case very short-lived. In those species that do exhibit this marked change in floral attractiveness, a key question is the nature of the signalling system that perceives a successful pollination event and leads to petal senescence. Based on other forms of plant senescence, a possible candidate for triggering petal death is the hormone ethylene. Van Doorn (University of Wageningen; pp. 375-383) has tested this idea by exposing non-pollinated flowers of about 200 different species to physiological concentrations of ethylene. In general, he found that treatment with ethylene mimicked the effect of pollination, suggesting that endogenous ethylene is indeed involved in the loss of floral attractiveness. In those species that do not show this sequence of events, ethylene either had no effect or induced an onset of rapid senescence. Focusing specifically on those plants that exhibit the marked change in floral attractiveness, the results show that ethylene triggers the changes more rapidly than pollination. The likely explanation for this is that in normal pollination there are steps 'upstream' of ethylene synthesis in the signal transduction pathway. The nature of those steps is not yet known but should surely be the subject of further research as we seek to understand the processes involved in successful seed set.Professor J. A. Bryant
University of Exeter, UK
J.a.bryant{at}exeter.ac.uk
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