A test of the adaptive plasticity hypothesis using transgenic and mutant plants disabled in phytochrome-mediated elongation responses to neighbors

Many plants display characteristic phytochrome-mediated stem elongation responses to crowding and vegetation shade, commonly referred to as the shade avoidance syndrome. We tested the hypothesis that this elongation is a form of adaptive plasticity by comparing the relative performance at high and low density of wild-type plants and transgenic and mutant plants in which the shade avoidance response was disabled. Transgenic tobacco plants in which elongation in response to neighbors was blocked by expression of the oat PHYA gene had decreased relative fitness when grown in competition with elongated wild-type plants. In contrast, constitutively elongated Brassica ein mutant plants, deficient in light-stable phytochrome, had lower fitness relative to nonelongated wild type at low density than in competition with elongated wild type at high density. The observation that phytochrome-mediated elongation is advantageous in dense stands, but disadvantageous for uncrowded plants, indicates that a response to foliage shade allows plants to develop an appropriate morphology for the level of competition they experience. This observation supports the adaptive plasticity hypothesis for this ecologically important trait.