Brassinosteroids, microtubules and cell elongation in Arabidopsis thaliana. II. Effects of brassinosteroids on microtubules and cell elongation in the bul1 mutant

In order to elucidate the involvement of brassinosteroids in the cell elong ation process leading to normal plant morphology, indirect immunofluorescen ce and molecular techniques were use to study the expression of tubulin gen es in the bul1-1 dwarf mutant of Arabidopsis thaliana (L.) Heynh.. the char acteristics of which are reported in this issue (M. Catterou ct al., 2001). Microtubules were studied specifically in the regions of the mutant plant where the elongation zone is suppressed (hypocotyls and petioles), making t he reduction in cell elongation evident. Indirect immunofluorescence of alp ha -tubulin revealed that very few microtubules were present in mutant cell s, resulting in the total lack of the parallel microtubule organization tha t is typical of elongating cells in the wild type. After brassinosteroid tr eatment, microtubules reorganized and became correctly oriented, suggesting the involvement of brassinosteroids in microtubule organization. Molecular analyses showed that the microtubule reorganization observed in brassinost eroid-treated bul1-1 plants did not result either from an activation of tub ulin gene expression, or from an increase in tubulin content, suggesting th at a brassinosteroid-responsive pathway exists which allows microtubule nuc leation/organization and cell elongation without activation of tubulin gene expression.