Visualization of auxin responses during vine twining in Ipomoea nil and the functional role of the subapical hook in this process.

This study suggests that hook-shaped bending associated with properly regulated auxin response patterns in the subapical region may contribute to the maintenance of twining in morning glory vines. The twining behavior demonstrated by vine plants represents an important morphogenetic adaptation essential to their survival; however, the physiological mechanisms driving this response remain largely unknown. To elucidate the role of auxin in vine twining in morning glory (Ipomoea nil), we generated and investigated transgenic morning glory carrying the GUS reporter gene under an auxin-responsive promoter, which enabled the identification of the localization of auxin responses through color development. Strong GUS activity, indicating auxin response, was observed at the hook-shaped bend in the subapical region of the vine. In many cases, the GUS activity was circumferentially biased in this region when the shoot apex was either descending to form a hook or maintaining a downward orientation. Treatment of vines with N-(1-naphthyl)phthalamic acid (NPA), an auxin transport inhibitor, resulted in the suppression of circumnutation, twining, and hook formation. Ectopically strong auxin responses were observed in the subapical region of these NPA-treated vines. In addition, unilateral application of the synthetic auxin NAA to the vine induced bending with the treated side on the outside of the curve. These results suggest that auxin response patterns in the subapical region are associated with circumnutation, twining, and hook formation, although the specificity of pharmacological treatments is limited. Local treatment of the subapical region of the vine with PCIB, an auxin-competitive inhibitor, resulted in temporary hook relaxation, detachment from the support, and resumption of exploratory circumnutation in certain individuals. These findings suggest that the hook in the subapical region of the vine contributes to maintaining twining.