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

•Mild Cd stress induces auxin synthesis and accumulation in the barley root tip.•Cd induces IAA synthesis via indole-3-pyruvic acid biosynthesis pathway.•IAA is involved in both root growth reorientation and defense response activation.•Both H2O2 and NO act downstream of IAA in root responses to mild Cd stress.•On the contrary, severe Cd stress reduces IAA level in the barley root tip.