Microtubules exert early, partial, and variable control of cotton fiber diameter.

Variable cotton fiber diameter is set early in anisotropic elongation by cell-type-specific processes involving the temporal and spatial regulation of microtubules in the apical region. Cotton fibers are single cells that originate from the seed epidermis of Gossypium species. Then, they undergo extreme anisotropic elongation and limited diametric expansion. The details of cellular morphogenesis determine the quality traits that affect fiber uses and value, such as length, strength, and diameter. Lower and more consistent diameter would increase the competitiveness of cotton fiber with synthetic fiber, but we do not know how this trait is controlled. The complexity of the question is indicated by the existence of fibers in two major width classes in the major commercial species: broad and narrow fibers exist in commonly grown G. hirsutum, whereas G. barbadense produces only narrow fiber. To further understand how fiber diameter is controlled, we used ovule cultures, morphology measurements, and microtubule immunofluorescence to observe the effects of microtubule antagonists on fiber morphology, including shape and diameter within 80 µm of the apex. The treatments were applied at either one or two days post-anthesis during different stages of fiber morphogenesis. The results showed that inhibiting the presence and/or dynamic activity of microtubules caused larger diameter tips to form, with greater effects often observed with earlier treatment. The presence and geometry of a microtubule-depleted-zone below the apex were transiently correlated with the apical diameter of the narrow tip types. Similarly, the microtubule antagonists had somewhat different effects between tip types. Overall, the results demonstrate cell-type-specific mechanisms regulating fiber expansion within 80 µm of the apex, with variation in the impact of microtubules between tip types and over developmental time.