Mild cadmium stress induces auxin synthesis and accumulation, while severe cadmium stress causes its rapid depletion in barley root tip

Abstract

The aim of this study was to investigate the effect of 4-phenoxyphenylboronic acid (PPBo), an effective inhibitor of YUCCA enzyme on mild, moderate and severe Cd stress-induced responses in the root tips of young barley seedlings. While mild and moderate Cd stresses increased, severe Cd stress decreased the root IAA level. PPBo post-treatment strongly inhibited the Cd-induced IAA accumulation without affecting the Cd level in roots and mitigated the mild Cd stress-induced root growth inhibition, root swelling and lipoxygenase and glutathione peroxidase activity. In addition, PPBo reduced the H2O2 and NO generation under mild but had no effect under severe Cd stress, indicating that both H2O2 and NO act downstream of IAA in root responses to mild Cd stress. In turn, moderate or severe Cd stress strongly reduced the expression of IAA synthesis and cell cycle genes in the barley root tip. Therefore, we can conclude that IAA accumulation in the barley root tips is a distinctive feature of mild and moderate Cd stresses, which is involved in both root growth reorientation and defense response activation. PPBo completely reversed this Cd-induced IAA accumulation and stress responses, indicating that Cd induces IAA synthesis via indole-3-pyruvic acid biosynthesis pathway. On the contrary, severe Cd stress reduced IAA level was coupled with high ROS in the root tips and root growth arrest.