Reactive Oxygen Species (ROS) and Flavonols Modulate the Root Gravitropic Response

Student: Elizabeth Sarkel (Wake Forest University)
Mentor: Chris Wolverton (Department of Botany-Microbiology)

Plant roots need to grow vertically into the ground so they can reach water and nutrients. When roots are disturbed from growing vertically, they release a series of signaling molecules that allow them to bend while growing so they can resume vertical growth. This research project evaluates the roles of two types of signaling molecules that counteract each other, flavonols and reactive oxygen species, in initiating the growth responses that allow roots to resume vertical growth.

During the gravitropic response, a root disturbed from its vertical orientation will grow asymmetrically so that it returns to the vertical orientation. We are testing the roles of reactive oxygen species (ROS) and flavonols, which are plant metabolites that scavenge ROS, as signals that control Arabidopsis root gravitropism. The tt4 mutant, which makes no flavonols, has delayed gravity response, but treatment with the flavonol precursor naringenin restores wild-type gravitropic response. When wild type seedlings are treated with naringenin, root gravitropism shows an initial delay, but overall is enhanced and continues past vertical. Higher levels of ROS were visualized by confocal microscopy in tt4 using the ROS sensor DCF, consistent with flavonols as ROS scavengers. Treatment with 3-aminotriazole (3AT), which inhibits catalase, an enzyme that degrades hydrogen peroxide, inhibited gravity response, consistent with elevated ROS interfering with root gravitropism. Additionally, experiments are being performed to quantify the gravitropism of the respiratory burst oxidase (rboh) mutant family, which have defects in signaling induced ROS synthesis. We have also examined the distribution of ROS and flavonols across roots reoriented 90 degrees relative to gravity. Arabidopsis seedlings expressing pCHS:CHS:GFP, a reporter that visualizes the site of expression of chalcone synthase, the first enzyme of flavonoid biosynthesis, show elevated GFP signal on the lower side of the root where auxin accumulation inhibits growth. Current experiments seek to resolve the timing and location of flavonol synthesis, ROS accumulation, and auxin accumulation using mutants, inhibitors, reporter constructs, and the feedback response system ROTATO, which continuously gravistimulates the root by holding it at a constant angle. These experiments will provide insight into the auxin and ROS signals that control root gravitropic response. (Supported by an ASPB SURF to ERS)