Division Plane Orientation in Plant Cells 43
mura et al. 2006). The observation that only half ofmor1cells form PPBs prior
to mitosis suggests that MOR1-dependent MT lengthening is critical for PPB
formation.
Analysis of anotherArabidopsismutant,ton2,suggeststhatregulationof
MT stability by phosphorylation/dephosphorylation is essential for PPB for-
mation.ton2mutants are sterile, dwarfed plants made up of misshapen cells
with irregular planes of division (Traas et al. 1995). Interphase cortical MTs
in these mutants are randomly oriented instead of aligned, and PPBs fail to
form, although spindles and phragmoplasts appear relatively normal (Traas
et al. 1995; McClinton and Sung 1997). The C terminal half of the TON2 pro-
tein is similar to a B′′regulatory subunit of the serine/threonine phosphatase,
PP2A (Camilleri et al. 2002). In general, B′′subunits provide specificity to the
PP2A holoenzymes they associate with by targeting the enzyme to specific
substrates or subcellular localizations (for review, Janssens and Goris 2001).
TON2 binds a member of theArabidopsisPP2A holoenzyme family in a yeast
two-hybrid assay, suggesting that TON2 acts as a PP2A regulator (Camilleri
et al. 2002).
Consistent with this hypothesis, studies with kinase and phosphatase in-
hibitors indicate that the phosphorylation status of (unknown) proteins in-
fluencing MT stability plays a key role in the formation of PPBs and other
MT arrays. Treatment with the PP2A inhibitor endothall results in disor-
ganized cortical MTs, poorly assembled PPBs, premature spindle organiza-
tion, multipolar spindles, and disrupted phragmoplasts in cultured alfalfa
cells (Ayaydin et al. 2000). Similarly, after treatment with cantharidin, an-
other phosphatase inhibitor, exposed cortical MTs in sectioned maize root
cells depolymerized, while treatment with kinase inhibitors promoted MT
stabilization (Tian et al. 2004). The implication of both studies is that dephos-
phorylation stabilizes MTs while phosphorylation de-stabilizes MTs. Thus,
these studies are consistent with the conclusion that phosphatase activity
promoted by TON2 could be critical for MT stabilization needed for the for-
mation of PPBs and ordered MT arrays. In a survey of GFP fusions to proteins
involved in cell division, TON2:GFP was reported to be in the nucleus and
cytoplasm during interphase and localized to the phragmoplast during cy-
tokinesis, but localization during preprophase/prophase was not described
(Van Damme et al. 2004). Thus, it remains unclear where TON2 acts in the cell
to influence PPB formation. An important advance for understanding TON2
function would be the identification of substrate(s) of the TON2-associated
phosphatase.
2.3
PPB Maturation and Influence on Spindle Assembly/Orientation
After the initial formation of a broad PPB in G2, the MT PPB progressively
narrows throughout prophase (Fig. 1c,e; Wick and Duniec 1983; Marcus et al.