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Abstract

Underground roots normally reside in darkness. However, they are often exposed to ambient light that penetrates
through cracks in the soil layers which can occur due to wind, heavy rain or temperature extremes.
In response to light exposure, roots produce reactive oxygen species (ROS) which promote root growth. It
is known that ROS-induced growth promotion facilitates rapid escape of the roots from non-natural light.
Meanwhile, long-term exposure of the roots to light elicits a ROS burst, which causes oxidative damage to
cellular components, necessitating that cellular levels of ROS should be tightly regulated in the roots. Here
we demonstrate that the red/far-red light photoreceptor phytochrome B (phyB) stimulates the biosynthesis
of abscisic acid (ABA) in the shoots, and notably the shoot-derived ABA signals induce a peroxidasemediated
ROS detoxification reaction in the roots. Accordingly, while ROS accumulate in the roots of the
phyb mutant that exhibits reduced primary root growth in the light, such an accumulation of ROS did not
occur in the dark-grown phyb roots that exhibited normal growth. These observations indicate that mobile
shoot-to-root ABA signaling links shoot phyB-mediated light perception with root ROS homeostasis to help
roots adapt to unfavorable light exposure. We propose that ABA-mediated shoot-to-root phyB signaling
contributes to the synchronization of shoot and root growth for optimal propagation and performance in plants.