How do seed cells know the difference between top and bottom?

Nanda Somarajan somarank at SLU.EDU
Tue Apr 22 20:31:06 EST 1997


	Hi.  This is to anyone who did not get the posts about how seed 
cells know the difference between top and bottom(including Kristin).  
Matthew had asked that question.


Kristin, 
        When you place a seed on its side, it will automatically adjust
its growth so that the shoot(stem) bends upward and the roots downward.  This
is a response to gravity called "Gravitrophism".  It used to be called
"Geotrophism."  Scientists are still unsure how this works.  ( I got the
following from my Biology Text book titled "Biology"  By Neil Campbell.)

        One theory is that plants may tell up and down by the settling of
statoliths, specialized plastids containing sense starch grains,  to low
points of cells.  In roots, staloliths are located in certain cells of
the root cap.  According to one hypothesis, the aggregation of statoliths at
the low points of these cells triggers the redistribution of calcium, which
in turn, causes lateral transport of auxin within the root.  The calcium and
Auxin(A plant hormone) accumulate on the lower side of the root's zone of
elongation.  At high concentrations, Auxin inhibits cell elongation.  So
the cells on the upper side of the root elongates more rapidly than the
lower side with the statoliths and this causes the root to curve as it
grows and continues untill the root is growing straight down.

        But, some scientists are challenging this hypothesis.  According
to Randy Wayne of Cornell University, the impact of the starch  grains on
the bottom of a cell does not release enough energy to be the mechanism
for gravitational detection.  He points out also that there are many
plants that do not have the starch grains and they still grow ok.  He
studies gravitrophism in Chara,which are green algae and closely related
to plants.  Proteins attach the protoplast of each cell to the inside of
the cell wall.  According to Wayne's hypothesis, the downward settling 
of>the entire protoplast, which streches the protein tethers at the top of
the cell and compresses those at the bottom, gives the cell its sense of
up and down.

        To test this, he placed Chara in a solution more dense than the
laga's cytoplasm.  The protoplastes floated upward, instead of falling
downward and the growth pattern of the alga was also upside eown.  He is now
trying to study to see if this mechanism also works in vascular plants.


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                                Nanda Somarajan
		Academic Technology Development Consultant
		somarank at slu.edu	somarank at hotmail.com
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