Gravity-induced reorientation of cortical microtubules observed in vivo.

Cortical microtubules play an important role during morphogenesis by determining the direction of cellulose deposition. Although many triggers are known that can induce the reorientation of cortical plant microtubules, the reorientation mechanism has remained obscure. In our approach, we used gravitropic stimulation which is a strong trigger for microtubule reorientation in epidermal cells of maize coleoptiles. To visualize the gravitropically induced microtubule reorientation in living cells, we injected rhodamine-conjugated tubulin into epidermal cells of intact maize coleoptiles that were exposed to gravitropic stimulation. From these in vivo observations, we propose a reorientation mechanism consisting of four different stages: (1) a transitional stage with randomly organized microtubules; (2) emergence of a few microtubules in a slightly oblique orientation; (3) co-alignment: neighbouring microtubules adopt the oblique orientation resulting in parallel organized microtubules; and (4) the angle of these parallel, organized microtubules increases gradually. Thus, the overall reorientation process could include selective stabilization/ disassembly of microtubules (stage 2) as well as movement of individual microtubules (stages 3 and 4).