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Phytochrome photoreceptors mediate plasticity to light quality in flowers of the Brassicaceae¹

Indiana University, Department of Biology, Bloomington, Indiana 47405 USA

The family of phytochrome photoreceptors mediates stem-elongation responses to ambient ratios of red : far-red light (R : FR). Although phytochrome genes are expressed in flowers in addition to vegetative parts, nothing is known about floral plasticity to R : FR or the pleiotropic effects of phytochrome genes on flowers. Here, the following floral morphologies were compared: (1) wild-type Arabidopsis thaliana and Brassica rapa plants experiencing high R : FR characteristic of sunlight vs. low R : FR typical of foliar shade and (2) wild-type and phytochrome-deficient A. thaliana plants. Wild-type A. thaliana exposed to low R : FR had diminished petal and pistil lengths but longer filaments for a given petal size than plants experiencing high R : FR. Brassica rapa plants had qualitatively similar responses. In comparison to wild-type A. thaliana, mutants lacking phytochrome A had smaller flowers (smaller petals, pistils, and filaments), whereas phytochrome B-deficient mutants exhibited longer filament lengths. These results provide the first evidence that R : FR and phytochromes affect floral phenotypes in addition to vegetative ones. Although the ecological relevance remains to be established, the observed plasticity of flowers to R : FR may be relevant to individual fitness in some species because stigma and filament positions can affect pollen removal and levels of self-pollination.

Key Words: Arabidopsis thaliana • evolutionary constraints • herkogamy • phenotypic plasticity • photomorphogenesis • phytochromes • pleiotropy, red : far-red light (R : FR)

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