BOTERO1 is required for normal orientation of cortical microtubules and anisotropic cell expansion in Arabidopsis

Mutants at the BOTERO1 locus are affected in anisotropic growth in all non-tip-growing cell types examined. Mutant cells are shorter and broader than those of the wild type. Mutant inflorescence stems show a dramatically reduced bending modulus and maximum stress at yield. Our observations of root epidermis cells show that the cell expansion defect in bot1 is correlated with a defect in the orientation of the cortical microtubules. We found that in cells within the apical portion of the root, which roughly corresponds to the meristem, microtubules were loosely organized and became much more highly aligned in transverse arrays with increasing distance from the tip. Such a transition was not observed in bot1. No defect in microtubule organization was observed in kor-1, another mutant with a radial cell expansion defect. We also found that in wild-type root epidermal cells, cessation of radial expansion precedes the increased alignment of cortical microtubules into transverse arrays. Bot1 roots still show a gravitropic response, which indicates that ordered cortical microtubules are not required for differential growth during gravitropism. Interestingly, the fact that in the mutant, these major changes in microtubule organization cause relatively subtle changes in cell morphology, suggest that other levels of control of growth anisotropy remain to be discovered. Together, these observations suggest that BOT1 is required for organizing cortical microtubules into transverse arrays in interphase cells, and that this organization is required for consolidating, rather than initiating, changes in the direction of cell expansion.