The Microtubule-Associated Protein END BINDING1b, Auxin, and Root Responses to Mechanical Cues

The ability of roots to penetrate through the soil and maneuver around rocks and other impenetrable objects requires a system for modulating output from mechanosensory response networks. The microtubule-associated protein END BINDING1b (EB1b) has a role in this process; it represses root responses to mechanical cues. In this study, a possible relationship between EB1b and auxin during root responses to mechanical cues was investigated. We found that eb1b-1-mutant roots are more sensitive than wild-type roots to chemicals that disrupt auxin transport, whereas the roots of mutants with defects in auxin transport are resistant to these treatments. Using seedlings that express the auxin-sensitive DR5rev::GFP construct, we also found that wild-type and eb1b-1 roots treated with the auxin transport inhibitor naphthylphthalamic acid exhibited dose-dependent reductions in basipetal auxin transport that were indistinguishable from each other. The responses of eb1b-1 roots to mechanical cues were also enhanced over wild type in the presence of p-chlorophenoxyisobutyric acid, a chemical thought to inhibit auxin signaling. Finally, roots of eb1b-1 and wild-type plants exhibited slight increases in loop formation in response to increasing levels of exogenously applied indole-3-acetic acid or 1-naphthalene acetic acid. Taken together, these results suggest that the repression of loop formation by EB1b and auxin transport/signaling occurs by different mechanisms.