Fig. 7. A working model of plant MT nucleation and organization. The soluble cytoplasmic -tubulin/Spc98p-containing complexes are recruited to two different MT nucleation sites (arrows): the plasma membrane and the nuclear surface. Recruitment and activation of these soluble complexes would be responsible for nucleation of the cortical MTs involved in cell growth and for the nucleation of the perinuclear and preprophase band MTs, which determine polarity before mitosis. The activation of MT nucleation sites and coordinated regulation of the MT assembly would be controlled by cell cycle and/or developmental signals. Such signals would enter into cell cycle check points in higher plants.