Cadmium stress inhibits the growth of primary roots by interfering auxin homeostasis in Sorghum bicolor seedlings

Phytotoxic effects of cadmium (Cd), a heavy metal pollutant, on plants have been extensively examined. Auxin plays vital roles in many aspects of plant development. The association between root growth and auxin signaling in Cd-stressed Sorghum bicolor was analyzed in our study. Root elongation, shoot length and the maximal photochemical efficiency (F-v/F-m) in S. bicolor seedlings were dramatically reduced after Cd stress treatment. Cd was found to be predominantly confined in the meristematic zone using a Cd-staining method. Cd stress remarkably influenced the cell cycle progression at the root tip as shown by EdU (ethynyl deoxyuridine) assay. The content of IAA was markedly diminished in the roots of Cd-stressed S. bicolor, which was along with the increase of IAA oxidase activity. Auxin transport inhibitors, 1-naphthoxyacetic acid (1-NOA) or 1- naphthylphthalamic acid (NPA), greatly reduced plant tolerance to Cd stress, whereas exogenous application of 1-naphthaleneacetic acid (NAA) improved Cd tolerance in S. bicolor seedlings. Cd stress altered the transcript level of some putative auxin biosynthetic genes. In addition, NAA interfered with the homeostasis of Cd-induced reactive oxygen species (ROS). These results revealed that Cd stress disturbed the growth of S. bicolor seedlings by affecting the homeostasis of auxin and ROS.