Abstract
We investigated the roles of cortical microtubules in gravity-induced modifications to the development of stem organs by analyzing morphology and orientation of cortical microtubule arrays in hypocotyls of Arabidopsis (Arabidopsis thaliana) tubulin mutants, tua3(D205N), tua4(S178Delta), and tua6(A281T), cultivated under 1g and hypergravity (300g) conditions. Hypocotyls of tubulin mutants were shorter and thicker than the wild type even at 1g, and hypergravity further suppressed elongation and stimulated expansion. The degree of such changes was clearly smaller in tubulin mutants, in particular in tua6. Hypocotyls of tubulin mutants also showed either left-handed or right-handed helical growth at 1g, and the degree of twisting phenotype was intensified under hypergravity conditions, especially in tua6. Hypergravity induced reorientation of cortical microtubules from transverse to longitudinal directions in epidermal cells of wild-type hypocotyls. In tubulin mutants, especially in tua6, the percentage of cells with longitudinal microtubules was high even at 1g, and it was further increased by hypergravity. The twisting phenotype was most obvious at cells 10 to 12 from the top, where reorientation of cortical microtubules from transverse to longitudinal directions occurred. Moreover, the left-handed helical growth mutants (tua3 and tua4) had right-handed microtubule arrays, whereas the right-handed mutant (tua6) had left-handed arrays. There was a close correlation between the alignment angle of epidermal cell files and the alignment of cortical microtubules. Gadolinium ions, blockers of mechanosensitive ion channels (mechanoreceptors), suppressed the twisting phenotype in tubulin mutants under both 1g and 300 g conditions. Microtubule arrays in tubulin mutants were oriented more transversely by gadolinium treatment, irrespective of gravity conditions. These results support the hypothesis that cortical microtubules play an essential role in maintenance of normal growth phenotype against the gravitational force, and suggest that mechanoreceptors are involved in modifications to morphology and orientation of microtubule arrays by 1g gravity and hypergravity in tubulin mutants.
Highlights
We investigated the roles of cortical microtubules in gravity-induced modifications to the development of stem organs by analyzing morphology and orientation of cortical microtubule arrays in hypocotyls of Arabidopsis (Arabidopsis thaliana) tubulin mutants, tua3(D205N), tua4(S178D), and tua6(A281T), cultivated under 1g and hypergravity (300g) conditions
We examined the changes in growth, morphology, and orientation of cortical microtubule arrays in hypocotyls of Arabidopsis amino acid substitution mutants in a-tubulin structure, tua3, tua4, and tua6, grown under 1g and 300g conditions
Elongation growth of Arabidopsis hypocotyls was suppressed by hypergravity at 300g (Fig. 1A)
Summary
We investigated the roles of cortical microtubules in gravity-induced modifications to the development of stem organs by analyzing morphology and orientation of cortical microtubule arrays in hypocotyls of Arabidopsis (Arabidopsis thaliana) tubulin mutants, tua3(D205N), tua4(S178D), and tua6(A281T), cultivated under 1g and hypergravity (300g) conditions. Cortical microtubule arrays in the left-handed helical growth mutants form right-handed helix, whereas those in righthanded helical growth mutants form left-handed helix (Thitamadee et al, 2002; Abe and Hashimoto, 2005; Ishida et al, 2007) These results indicate that dysfunctional cortical microtubules are arranged in helical arrays and affect the direction of cell expansion. The aim of this study was to clarify the roles of cortical microtubules in gravity-induced modifications to development of stem organs For this purpose, we examined the changes in growth, morphology, and orientation of cortical microtubule arrays in hypocotyls of Arabidopsis amino acid substitution mutants in a-tubulin structure, tua, tua, and tua, grown under 1g and 300g conditions. We examined the effect of blockers of mechanoreceptors on helical growth and orientation of cortical microtubule arrays in the tubulin mutants
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