Fusicoccin affects cortical microtubule orientation in the isolated epidermis of sunflower hypocotyls

Epidermal peels isolated from sunflower hypocotyls provide a convenient model to study the relationship between cortical microtubule orientation and strain rate. Extension of peels can be modulated using chemical treatment and mechanical stress, i.e., by adding a chemical to the incubation medium and applying a load exceeding the yield threshold for irreversible (plastic) strain. In this study, peels were pre-incubated for ca. 12 h (long-term pre-incubation) or for 1 h (short-term pre-incubation). In the long-term pre-incubated peels, fusicoccin applied to the medium neither enhanced the rate of longitudinal plastic strain of loaded peels, nor affected microtubule orientation. However, fusicoccin increased the strain rate of short-term, pre-incubated peels and affected microtubule orientation in both extending (loaded) and non-extending (unloaded) peels. Without fusicoccin, microtubule orientation was generally longitudinal or steep, whereas in fusicoccin-treated unloaded peels it was transverse and oblique microtubules in peel portions corresponding to the apical part of the hypocotyl. Although the frequency of transverse orientation was increased through loading, there was no strong correlation between the rate of fusicoccin-induced strain and microtubule orientation. It is hypothesized that the insensitivity of long-term pre-incubated peels to fusicoccin with respect to strain rate is due to a lack of active plasma membrane H+-ATPases. Thus, the sensitivity of short-term, pre-incubated, unloaded (non-extending) peels to fusicoccin, with respect to microtubule orientation, indicates that orientation might be affected by electric currents resulting from fusicoccin stimulation of H+-ATPases.