Root twisting drives halotropism via stress-induced microtubule reorientation

ABSTRACT Plants have evolved signaling mechanisms that guide growth away from adverse environments that can cause yield losses. Root halotropism is a sodium-specific negative tropism that is crucial for surviving and thriving under high salinity. Although root halotropism was discovered some years ago, the underlying molecular and cellular mechanisms remain unknown. Here, we show that abscisic acid (ABA)-mediated root twisting determines halotropism in Arabidopsis . An ABA-activated SnRK2 protein kinase (SnRK2.6) phosphorylates the microtubule-associated protein SP2L at Ser406, which induces a change in the anisotropic cell expansion at the root transition zone that located between the apical meristem and basal elongation zone, and is required for root twisting during halotropsim. Salt stress triggers SP2L-mediated cortical microtubule reorientation in cells at the transition zone, which guides cellulose microfibril patterns. Our findings outline the cellular and molecular mechanisms of root halotropism and indicate that anisotropic cell expansion through MT-reorientation and microfibril deposition have a central role in mediating tropic responses.