For the sunflower, turning toward the sun requires multiple complex systems

A sunflower’s ability to track the sun east to west during the day, and to face east again before the next sunrise, relies on multiple types of photoresponses, according to a new study publishing October 31st in the open access journal PLOS Biology by Stacey Harmer and colleagues at the University of California Davis, US. The results deepen the understanding of this well-known plant behavior, and upend previous assumptions about its dependence on a canonical light-dependent response pathway.

Because plants are rooted in place, they can’t get up and move when a neighbor blocks their light or they find themselves sprouting in a shady spot. Instead, they rely on growth or elongation to maneuver toward the light. There are several molecular systems to facilitate such responses, the best-known of which is called the phototropic response. In this system, blue light falling unevenly on a seedling is sensed by proteins called phototropins, which cause redistribution of a plant hormone, ultimately causing the growing tip to bend toward the light.

Whether the sun-tracking ability of the sunflower, called heliotropism, is a form of phototropic response, involving the same receptors and hormone, has not been clear. To explore this question, the authors compared gene activity patterns of sunflowers bending toward blue light in the lab to sunflowers tracking the sun in the field.

Surprisingly, only a few of the genes whose rapid upregulation is responsible for the phototropic bending in the lab showed significant differences in activity in response to the movement of the sun. Along with these few, they found changes in other light-response systems, including a shade avoidance system that senses far-red light (enriched in shade), which was triggered on the west side of the sunflower stem early in the day, when the sun is in the east. But, complicating the picture further, they showed that depletion of either red and far-red or blue light had little effect on the sunflower’s ability to track the sun, suggesting that multiple systems may coordinate to produce the heliotropic response, allowing it to operate even in the absence of one or more light triggers.

Harmer adds, “We’ve been continually surprised by what we’ve found as we study how sunflowers follow the sun each day. In this paper, we report that they use different molecular pathways to initiate and maintain tracking movements, and that the photoreceptors best known for causing plant bending seem to play a minor role in this remarkable process.”

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