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Published on February 11, 2009; 10.1104/pp.108.133496

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Received December 4, 2008
Accepted February 5, 2009

Auxin and ethylene regulate elongation responses to neighbour proximity signals independent of GA and DELLA proteins in Arabidopsis

Ronald Pierik *, Tanja Djakovic-Petrovic , Diederik H. Keuskamp , Mieke de Wit , and Laurentius A.C.J. Voesenek.

Plant Ecophysiology, Institute of Environmental Biology, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, the Netherlands

* Corresponding author; email: R.Pierik{at}uu.nl.

Plants modify growth in response to the proximity of neighbours. Among these growth adjustments are shade avoidance responses such as enhanced elongation of stems and petioles, that help plants to reach the light and out-grow their competitors. Neighbour detection occurs through photoreceptor-mediated detection of light spectral changes, i.e. reduced red:far-red ratio (R:FR) and reduced blue light intensity. We recently showed that physiological regulation of these responses occurs through light-mediated degradation of nuclear, growth-inhibiting DELLA proteins, but this appeared to be only part of the full mechanism. Here we present how two plant hormones, auxin and ethylene, co-regulate DELLAs, but regulate shade avoidance responses through DELLA-independent mechanisms. Auxin appears to be required for both seedling and mature plant shoot elongation responses to low blue and low R:FR respectively. Auxin action is increased upon exposure to low R:FR and low blue light and auxin inhibition abolishes the elongation responses to these light cues. Ethylene action is increased during the mature plants response to low R:FR and this growth response is abolished by ethylene-insensitivity. However, ethylene is also a direct volatile neighbour detection signal that induces strong elongation in seedlings, possibly in an auxin-dependent manner. We propose that this novel ethylene and auxin control of shade avoidance does interact with DELLA abundance, but also controls independent targets to regulate adaptive growth responses to surrounding vegetation.






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