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.
