A dual effect on protein synthesis and degradation modulates the tubulin level in rice cells treated with oryzalin

Abstract.

The effect of the anti-microtubular drug oryzalin on growth and morphology of cultured rice (Oryza sativa L., cv. Roncarolo) cells was evaluated with specific reference to mechanisms that control intracellular tubulin levels. The addition of oryzalin caused a great reduction in the level of both α- and β-tubulin polypeptides, as detected by Western blot analysis. However, no appreciable decrease was observed in the population of total or isotype-specific α- and β-tubulin mRNAs. Only within the first 24 h of the oryzalin treatment, when the level of both α- and β-tubulin polypeptides was still undiminished, was a consistent reduction in the amount of total β-tubulin mRNA observed. Pulse-chase experiments performed on rice cells grown in the presence of 1 µM oryzalin revealed the presence of two distinct mechanisms that negatively control α- and β-tubulin polypeptide levels. (i) There was an immediate effect on protein synthesis, which resulted in a reduction in the level of newly synthesized tubulin. (ii) There was a delayed response characterized by a substantial degradation of both α- and β-tubulin monomers; this degradation occurred after 24 h of herbicide treatment. The possible involvement of Ca²⁺ in the degradation of the unincorporated tubulin monomers is also documented and discussed.