Plasma membrane depolarization‐activated calcium channels, stimulated by microtubule‐depolymerizing drugs in wild‐type Arabidopsis thaliana protoplasts, display constitutively large activities and a longer half‐life in ton 2 mutant cells affected in the organization of cortical microtubules

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Abstract
Depolarization‐activated plasma membrane calcium channels have been suggested to play prominent roles in signal perception and transduction processes during growth and development of higher plants. The existence of such channels has recently been established in higher plant cells. However, patch–clamp experiments have shown that their activity is very low and decreases very rapidly after the establishment of the whole‐cell configuration, due most probably to protein–protein interactions involving microtubules. The present study takes advantage of the existence of Arabidopsis thaliana mutants referred to as ton 2 mutants reported to be affected in their microtubule organization, to address the physiological relevance of such a hypothesis based on a pharmacological approach. Patch–clamp studies showed that depolarization‐activated calcium channel activities in ton 2 protoplasts were 10‐fold higher and their relative half‐life three‐times longer than in wild‐type protoplasts. In addition, oryzalin and colchicine, which disrupt the microtubule organization, stimulated and stabilized calcium channel activities in wild‐type but remained ineffective on ton 2 protoplasts. However, although the microtubules appeared important in the regulation of calcium channels in A. thaliana, immunocytological staining of tubulin demonstrated that there was no visible difference in the general organization of microtubule networks or in the amount of microtubules bound to the plasma membrane in ton 2 and wild‐type protoplasts. It is suggested that the down‐regulation of calcium channels implicating microtubules involves additional component(s) corresponding probably to gene product(s) defective in ton 2 mutant cells.