PLANT-CELL GROWTH RESPONDS TO EXTERNAL FORCES AND THE RESPONSE REQUIRES INTACT MICROTUBULES

Microfibril deposition in most plant cells is influenced by cortical m icrotubules. Thus, cortical microtubules are templates that provide sp atial information to the cell wall. How cortical microtubules acquire their spatial information and are positioned is unknown. There are ind ications that plant cells respond to mechanical stresses by using micr otubules as sensing elements. Regenerating protoplasts from tobacco (N icotiana tabacum) were used to determine whether cells can be induced to expand in a preferential direction in response to an externally app lied unidirectional force. Additionally, an anti-microtubule herbicide was used to investigate the role of microtubules in the response to t his force. Protoplasts were embedded in agarose, briefly centrifuged a t 28 to 34g, and either cultured or immediately prepared for immunoloc alization of their microtubules. The microtubules within many centrifu ged protoplasts were found to be oriented parallel to the centrifugal force vector. Most protoplasts elongated with a preferential axis that was oriented 60 to 90 degrees to the applied force vector. Protoplast s treated transiently with the reversible microtubule-disrupting agent amiprophos-methyl (applied before and during centrifugation) elongate d but without a preferential growth axis. These results indicate that brief biophysical forces may influence the alignment of cortical micro tubules and that microtubules themselves act as biophysical responding elements.