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

Nanda Somarajan somarank at SLU.EDU
Sat Apr 19 19:48:37 EST 1997

	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.

	This is all I know.  Does anybody else know how plants know which 
is up and which way is down??

				Nanda Somarajan
				somarank at
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