Abscisic acid-mediated phytochrome B signaling promotes primary root growth in Arabidopsis.

Plant photomorphogenic responses have been studied mostly using the shoots, the core part of plant architecture that perceives light for photosynthesis and influences the overall processes of growth and development. While the roots are also known to respond to aboveground light through multiple routes of light signal transduction, root photomorphogenesis has been less highlighted until recently. A long-standing, critical question was how the underground roots are capable of sensing aerial light and how the root-sensed light signals trigger root photomorphogenesis. When the roots are directly exposed to light, reactive oxygen species (ROS) are rapidly produced to promote primary root elongation, which helps the roots to escape from the abnormal growth conditions. However, severe or long-term exposure of the roots to light causes ROS burst, which impose oxidative damages, leading to a reduction of root growth. We have recently found that phytochrome B (phyB) promotes abscisic acid (ABA) biosynthesis in the shoots and the shoot-derived ABA signals mediate ROS detoxification in the roots, lessening the detrimental effects of light on root growth. On the basis of these observations we propose that the phyB-mediated ABA signaling contributes to the shoot-root synchronization that is essential for optimal growth and performance in plants.