Microtubules are arranged horizontally with respect to the growth axis. As we saw in textbook Chapter 1, microtubules are important for guiding the enzyme complex cellulose synthase during cell wall microfibril biosynthesis. The default orientation of the microtubules in elongating cells is transverse, perpendicular to the cell axis. After exposure to unilateral blue light, new microtubules are formed in less than one minute that are longitudinally oriented and parallel to the hypocotyl axis. At the same time the preexisting microtubules are turned over. Microtubule rearrangement in the phototropically defective phot mutants occurs, but is greatly reduced, and PHOT appears to be the primary blue light receptor for microtubule rearrangement. In theory, cells exposed to blue light should deposit longitudinally oriented cellulose microfibrils, which would inhibit elongation growth on the illuminated side. However, more experimentation is required to determine if the blue light–induced microtubule rearrangement is a causative factor in phototropic bending. In textbook Chapter 18 we saw that microtubule rearrangement that occurs in response to wounding is mediated by ethylene. So, microtubule rearrangements are important for cell wall remodeling and cell expansion in general.