Arabidopsis petiole torsions induced by lateral light or externally supplied auxin require microtubule-associated TORTIFOLIAl/SPIRAL2

Abstract

Although rather inconspicuous, movements are an important adaptive trait of plants. Consequently, light- or gravity-induced movements leading to organ bending have been studied intensively. In the field, however, plant movements often result in organ twisting rather than bending. This study investigates the mechanism of light- or gravity-induced twisting movements, coined ``helical tropisms.'' Because certain Arabidopsis cell expansion mutants show organ twisting under standard growth conditions, we here investigated how the right-handed helical growth mutant tortifblial/spiral2 (torl) responds when stimulated to perform helical tropisms. When leaves were illuminated from the left, torl was capable of producing left-handed petiole torsions, but these occurred at a reduced rate. When light was applied from right, torl plants rotated their petioles much faster than the wild-type. Applying auxin to the lateral-distal side of wild-type petioles produced petiole torsions in which the auxinated flank was consistently turned upwards. This kind of movement was not observed in torl mutants when auxinated to produce left-handed movements. Investigating auxin transport in twisting petioles based on the DR5-marker suggested that auxin flow was apical-basal rather than helical. While cortical microtubules of excised wild-type petioles oriented transversely when stimulated with auxin, those of torl were largely incapable of reorientation. Together, our results show that tor/ is a tropism mutant and suggest a mechanism in which auxin and microtubules both contribute to helical tropisms.