Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits
Searching for the Sun The growth of immature sunflower plants tracks the Sun's movement. The young plants lean westward as the day progresses but reorient to the east each night. As the flowers mature and open, they settle into a stable east facing orientation. Atamian et al. show how circadian rhythms regulate the east west elongation of cells in the young plants' stems (see the Perspective by Briggs) They show that eastward oriented flowers are warmer than westward oriented flowers, and this warmth attracts pollinators. Auxin signaling pathways in the stem coordinate to fix the eastward orientation of the mature plant. Science, this issue p. 587; see also p. 541 Sun tracking when young, east facing when mature, warmer sunflowers attract more pollinators. Young sunflower plants track the Sun from east to west during the day and then reorient during the night to face east in anticipation of dawn. In contrast, mature plants cease movement with their flower heads facing east. We show that circadian regulation of directional growth pathways accounts for both phenomena and leads to increased vegetative biomass and enhanced pollinator visits to flowers. Solar tracking movements are driven by antiphasic patterns of elongation on the east and west sides of the stem. Genes implicated in control of phototropic growth, but not clock genes, are differentially expressed on the opposite sides of solar tracking stems. Thus, interactions between environmental response pathways and the internal circadian oscillator coordinate physiological processes with predictable changes in the environment to influence growth and reproduction.
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