Interactions Between Circadian and Photoreceptor Signaling Networks to Produce Complex Behaviors

In plants as in other complex eukaryotes, the circadian clock influences a wide range of biological processes. Since plants are rooted in their environment, they make an excellent model system for investigating how organisms can integrate internal temporal information generated by the clock with environmental cues to control physiological pathways. In previous work in the model plant Arabidopsis thaliana, we found that the clock modulates both plant responsiveness to and free levels of the growth-associated hormone auxin. Since auxin plays a key role in plant directional growth responses, we have examined whether the circadian clock plays a role in the ability of sunflower plants to track the sun, a process called heliotropism or solar tracking. Solar tracking involves both the continual orientation of the apex towards the sun throughout the day and the gradual re-orientation of the apex from west to east at night in anticipation of sunrise. We have found that solar tracking behavior is regulated by the circadian clock, is mediated by differential growth on opposite sides of the stem, and that its disruption significantly impacts plant biomass and total leaf area. Transcriptional analysis has suggested auxin-mediated growth pathways control this tracking behavior, a possibility that we are now actively investigating. Interestingly, solar tracking in sunflower ceases upon flower maturation, with the flower heads facing east. We find that sunflower phototropic responses are gated by the clock, with maximal bending occurring when plants are exposed to directional light in the morning. Combined with the gradual cessation of stem growth upon flower maturation, this may explain why sunflowers face east at maturity. In ongoing work, we are examining the ecological consequences of this eastward orientation.