Developmental reorientation of transverse cortical microtubules to longitudinal directions: A role for actomyosin-based streaming and partial microtubule-membrane detachment

Transversely oriented cortical microtubules in elongating cells typically reorient themselves towards longitudinal directions at the end of cell elongation. We have investigated the reorientation mechanism along the outer epidermal wall in maturing leek (Allium porrum L.) leaves using a GFP-MBD microtubule reporter gene and fluorescence microscopy. Incubating leaf segments for 14-18 h with the anti-actin or anti-actomyosin agents, 20 μm cytochalasin D or 20 mm 2,3-butanedione monoxime, inhibited the normal developmental reorientation of microtubules to the longitudinal direction. Observation of living cells revealed a small subpopulation of microtubules with their free ends swinging into oblique or longitudinal directions, before continuing to assemble in the new direction. Electron microscopy confirmed that longitudinal microtubules are partly detached from the plasma membrane. Incubating leaf segments with 0.2% 1°-butanol, an activator of phospholipase D, which has been implicated in plasma membrane-microtubule anchoring, promoted the reorientation, presumably by promoting microtubule detachment from the membrane. Stabilizing microtubules with 10 μm taxol also promoted longitudinal orientation, even in the absence of cytoplasmic streaming. These results were consistent with confocal microscopy of live cells before and after drug treatments, which also revealed that the slow (days) global microtubule reorientation is superimposed over short-term (hours) regional cycling in a clockwise and an anti-clockwise direction. We propose that partial detachment of transverse microtubules from the plasma membrane in maturing cells exposes them to hydrodynamic forces of actomyosin-driven cytoplasmic streaming, which bends or shifts pivoting microtubules into longitudinal directions, and thus provides an impetus to push microtubule dynamics in the new direction.