I.
Analyses of photophysiological responses of tip-growing cells.
1)Phototropism of Vaucheria
The
body of Vaucheria is composed of a sparsely branched coenocytic
tube of
50-100 オm in diameter. Each apex of the tube shows typical tip-growth.
Vaucheria is the very suitable model system of tip-growth, because
no cell wall expansion occurs below the hemispherical dome. Vaucheria
exhibits positive phototropism when the fluence rate of the unilateral
light is moderate; but shows negative phototropism when the fluence rate
exceeds a critical value. Such switchover of the sign of phototropism
is not seen in most higher plants. Thus, the phototropism of Vaucheria
is the most suitable system for the analysis of light vector sensing mechanism.
We have found that a bluelight-dependent influx of Ca2+ ion at the cell
apex is involved in the regulation of positive and negative phototropsic
response. (Kataoka 1988, 1990, 2000; Kataoka and Watanabe1992, 1993, 2000).
We are intending to directly confirm the increase in cytoplasmic Ca2+
concentration in teh apical cytoplasm and elucidate the role of high Ca2+
in regulation of local increase/decrease of exocytosis.
2)Photomorphogenesis
of Vaucheria.
When a narrow region of the coenocytic alga, Vaucheria
is irradiated with bluelight of moderate intensity, one or more bulge(s)
is induced from the center of the irradiated region 4 - 5 h after the
onset of BL (Kataoka 1975, Takahashiet al. 2001). The bulge then grows
to a branch.
Chloroplasts
immediately start to accumulate into the BL-region. The accumulation of
chloroplasts, which almost completes within 60 min. is necessary, but
insufficient by itself for the branch induction. Instead, accumulation
of nuclei, which starts 30-40 min after the onset of BL is indispensable.
No nuclear division is induced.
Nuclear
accumulation is synchronized with disorientation and degradation of microtubule
(MT) bundles, which originally run parallel to the cell axis. Gathering
nuclei, not by nuclear division is a very unique way of forming a new
shape which none of multicellular plants can adopt. None but coenocytes
can gather nuclei from adjacent regions!
A long (50-60
µm) MT bundle extends from the head of every nucleus. An additional
short MT bundle(s) extends backward. Centrosome (including centrioles)
serves as an MTOC. The frontal MT bundles pulled nuclei in longitudinal
direction. Motility of the nucleus-MT complex and its control mechanism
are investigated.
And recently we are finding that a novel bZIP-LOV bluelight receptor molecule
for the bluelight-induced branching, using the RNAi-knock down experiments
(see main page).
Other
coenocytic algae, such as Caulerpa and Bryopsis are also
used for the study of bluelight-induced photo-cytomorphogenesis.
The recent paper on
this subject (Takahashi et al 2001) can be seen in
http://nacos.com/jspp/,http://www.pcp.oupjournals.org/
Also,
For further detail, go to
Phototropism
and photomorphogenesis of Vaucheria.
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II.
Biology of coenocytes, esp. Vaucheria and Bryopsis.
Most eukaryotic cells are,
no matter whether they are unicellular organisms or composing a multicellular
organism, uninuclear cells having only one nucleus in a cell. There are,
however, various types of coenocytic cells among various phylogenical
groups. Coenocytic cell is the cell in which
many nuclei are involved within a single protoplasmic continuum.
The cause of the multinucleation appears to be the lost of cytokinesis
that follows nuclear division in ordinary uninuclear cells. The true cause,
the origin and ecological advantages of being a coenocytic cell are, however,
still not known.
In
the bluelight-induced branching (see I-b),
Vaucheria adopt an alternative principle for the cytomorphogenesis,
i. e., Vaucheria gathers nuclei from adjacent dark region to the
bluelight-irradiated region without waiting next mitotic phase (Takahashi
et al. 2001) . None of uninuclear cells can adopt this principle!
On the
other hand, this unique principle may indicate an important possibility:
the morphogenesis of plants do require a high nuclear density in a restriced
region, but do not require the cell division it self. As demonstrated
previously in fern protonemata and in our present study, the nuclear control
reaches only short distance upon cytomorphogenesis. Namely, even in a
uninuclear cell the nucleus may have to move to and reside for a while
the presumptive site of deformation.
There
could not be so wide gap between uninuclear- and coenocytic cells, if
we think of symplasts of a monocot hypocotyl, for example. A symplast
can be regarded as a pack of discoidal coenocytes, since indivisual cells
of a symplast are only radially connected with each other through plasmodesmata,
Study of coenocytes is thus very interesting and significant when man
asks following questions: how are informations exchanged between individual
nuclei, is there hierarchies between nuclei, why cytokinesis does not
follow nuclear division... It may give hints to develop multi-CPU computers...
For
further detail go to Biology
of Coenocytes
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III.
Phylogenetical relationship between green plants and yellow plants (Stramenopiles)
in response to osmotic and ionic regulation.
Yellow
plants (Chromista, Stramenopiles) are very large phyrum including diatoms,
brown algae, Xanthophyceae and Oomycetes. They are thought to be evolved
from an ancestral nonphotosyntehtic oomycetous eucaryote and second endosymbiot
eucaryotes relatating to red algae. That is their origins of chloroplasts
are not procaryotic cyanobacterium but eucaryote.
Thus, staramepiles
are phylogenetically completely different from green plants which include
land plants.
Fresh water Vaucheria
has an ability to regulate its osmotic and turgor pressure by taking up
ions when it is subjected to hyperosmotic stress. Surprisingly, however,
Vaucheria suck up more sulfate ions than Cl and nitrate ions, while
most green algae exclusively use Cl- , and only slightly suck up sulfate
ions.
We have also found
that some marine brown algae (Deamarestia spp. and some of Dictyotales
algae) accumulate high concentration (as high as 400 mM) of sulfuric acid
(H2SO4). Related species of Dictyotales which do not accumulate
sulfuric acid also accumulate same amount of sulfate ion (SO42-). They
accumulate equivalent concentration of K or Na rather than H+ (Sasaki
et al. 1999, 2001).
We are proposing a
hypothesis that this remarkable difference in the ion species used for
osmo/turgor regulation may be an important key which divide green plants
and yellow plants. To examin this ypothesis, we are starting comparative
phyological and phylogenetical studies of osmo/turgor regulation mechanism
of many green and yellow plants.
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