Torsions-Driven Root Helical Growth, Waving And Skewing In Arabidopsis

Helical growth broadly exists in immobile plants to support their limited movement, and Arabidopsis seedling root exhibiting natural left-handedness helical growth is considered as a simplified model for investigating this interesting behavior. Efforts have been made for understanding the mechanism of root helical growth and consequent root waving and skewing on tilted and impenetrable surface, and several models have been established. Here, previous reports are reviewed and a straightforward torsions-driven mechanism has been emphasized, and additional experiments have been performed to fill up the gaps of this theory in our study. This study implies that, torsions originating from handedness of both cortical microtubules and cellulose microfibrils play central role in root handed helical growth. Different from torsions directly provided by handed assembled cortical microtubules, torsions originating from right-handed assembled cellulose microfibrils are relaxed by their cross-linking with pectin within cell wall, but only exhibited when their cross-linking is interrupted due to damaged cell wall integrity. To topologically relax these torsions, supercoils of cortical microtubules and/or cellulose microfibrils exhibiting as oblique alignments are formed in root cells, which alter the orientation of root cell files and generate handed helical roots. Working together with gravitropic response, relaxation of torsions originating from helical roots drives roots to elongate with handedness, which therefore produces waved and skewed roots on tilted and impenetrable surface.