Abstract
We recently found that moisture gradient or water stress immediately degrades amyloplasts in the columella cells of Arabidopsis and Raphanus roots, which caused the reduced gravitropism. Roots thus exhibit strong hydrotropism by overcoming gravitropism. It is known that not only gravitropism or hydrotropism but also phototropism play important roles in orienting Arabidopsis roots. In the present study, we examined whether degradation of amyloplasts involves in the interaction between phototropism and gravitropism. We found that phototropic response accompanied a significant reduction of starch in columella cells of Arabidopsis roots. Phototropic response and amyloplast degradation occurred due to the unilateral irradiation of white, blue or red light. Bilateral irradiation of those lights resulted in neither root bending nor reduction of starch in the columella cells. These results suggest that Arabidopsis roots display phototropism by overcoming gravitropism, due to degradation of gravisensors, amyloplasts, in the columella cells, as does hydrotropism.
