Helical Growth of Twining Common Bean is Associated with Longitudinal, Not Skewed, Microtubule Patterning

Organ chirality in plants has been linked to cytoskeletal organization, as demonstrated in Arabidopsis thaliana twisted mutants, where left-skewed cortical microtubules are associated with right-handed twisting, and vice-versa. While this phenotype seemingly mirrors vining habits, this hypothesis remains understudied within naturally twining plants. Qualitative observations identified skewed microtubules in the twining stem of Ipomoea nil vine, suggesting parallels with Arabidopsis studies. To further investigate organ chirality in twining plants, we used common bean vine (Phaseolus vulgaris L.) to examine the relationship between microtubule orientation, cell morphogenesis, and the right-handed twining phenotype via immunolabeling techniques. Here, we report a transition from mixed microtubules orientations in emergent and elongating internodes to a predominance of longitudinal microtubules in straight and twined stem segments post-elongation. Additionally, we report a distinction in epidermal cell shapes, where straight portions of the stem consist of lobes with rectangular cells and furrows comprised of round cells, while twined portions are comprised of cells that are relatively more rectangular and stretched. We propose that these orientations reflect dynamic microtubule responses to external stimuli and growth cues, such as tensile stresses from climbing or tissue expansion. Taken together, these findings highlight dissimilarities between twisting Arabidopsis mutants and naturally twining plants.