Roots are subterranean organs and normally follow the gravity vector to grow downward.However,in nature,roots can be exposed to sunlight in a negative phototropism response termed root light avoidance (Kutschera and Briggs,2012).Such tropic responses are characterized by organ curvature caused by asymmetric growth between different sides of the root,which generally correlates with an asymmetric distribution of the plant hormone auxin (Wang et al.,2015).