Plant
Meiosis
Plant meiosis is essential for life cycle transition. We have
recently begun a collaboration with the laboratory of Hong
Ma, to examine a meiotic mutation that his lab has isolated. The mutation
is an allele of ATK1, is a kinesin with C-terminus motor. Plants harboring the
mutant alleles exhibit aberrant male meiosis, but show normal female meiosis.
The defect first appears as a defect in pollen microsporocyte M1 spindle formation,
with consequential problem in M2. By expressing the gene product in bacteria,
we have shown the gene product kinesin activity; it supports minus-end microtubule
motility and the motor has a non-processive character. These microtubule motility
studies were done in collaboration with Will Hancock, in the Dept. of Bioengineering.
We are currently looking
at the role that other kinesin genes play in both meiosis and mitosis. For example,
ATK1 plants have no gross phenotype in sporophytic cells, but mitosis is a bit
unusual. We are interested in knowing the mechanisms by which mitotic cells are
able to escape the mutant phenotype even though their spindles are unusual. Plant
Mitosis
The ancestral function of mitosis is to segregate chromosomes
and, like most other eukaryotes, plants use spindle microtubules to affect karyokinesis.
However, there are some differences that are unique to plants (and some algae),
which are notable. First, the assembly of the spindle is preceded by the formation
of a cortical band of microtubules, termed the pre-prophase band. This band of
microtubules has at least three functions: it positions the G2/prophase nucleus;
it affects the polarity of the early spindle; it predicts the site of the future
cell plate. We are using a stably transformed tobacco cell line, harboring a microtubule
binding domain::Green Fluorescent Protein chimeric gene, to gain insight into
the role the PPB plays in early mitosis. Using a dsRed::Golgi marker, we have
shown that Golgi to accumulate at the PPB site, but that Golgi secretion is not
involved in "marking" the PPB site for future cell plate positioning.
Ongoing experiments are aimed at understanding how the PPB communicates with the
nascent spindle and the nature of the "mark" that assigns the position
for the future cell plate. Cell
Elongation
Cellular elongation is an important aspect of plant morphogenesis.
Plants use the isotropic force of turgor to affect elongation by preferentially
reinforcing their walls with cellulose microfibrils. Like hoops around a barrel,
cellulose microfibrils constrain lateral growth while allowing the cell to elongate
along an axis perpendicular to the aligned cellulose. Cortical microtubules play
a role by somehow affecting the ordered deposition of nascent cellulose microfibrils.
The traditional textbook view of microtubules controlling the orientation
of newly synthesized cellulose microfibrils is too simple. We now know that cellulose
synthesis, itself, is required for the orientation of cortical microtubules. The
available information is therefore consistent with the presence of cross-talk
between cellulose microfibrils and cortical microtubules. In essence, the cortical
microtubules, the plasma membrane, and the cellulose microfibrils comprise a functional
continuum that acts to self-rectify the wall to insure proper elongation during
plant growth.
The mechanistic details surrounding this continuum remain incompletely
described. The lab is currently testing the hypothesis that biophysical forces,
that arise as a consequence to growth, play a role in conveying this information.
Microtubule alignment in growing epidermal tissues correlates with high (or accelerating
growth rates), rather than deaccellerating growth rates. Thus, the biophysical
signal may not be a simple first order strain sensor, rather one that responds
in a second order fashion (i.e., an accelerometer). |
