Subject: DIFFERENTIATION

From: [email-masked] (Jay Penning )
Date: Tue, 6 Dec 1994 06:55:35 GMT

THE QUESTION OF DIFFERENTIATION   ---   JAY PENNING
	Scientists have uncovered detail after detail about plant 
development until it seems that we must have solved all the big problems in 
plant biology.  One question still prowls our minds: "how do plant cells 
differentiate?"  
	Botanists have provided valuable information using the ideal plant 
subjects for studying cell growth and proliferation under controlled 
conditions.  For one thing, some of them can regenerate from bits and pieces 
of plant.  Many plants can regenerate roots from stem cuttings, stems from 
bits of root, and even entire plants from leaves.  This knowledge has helped 
the area of agriculture, as it supplies clues to the question of 
differentiation.  It supports the idea that plants are totipotent, that is 
the cell can undergo development and differentiation along any of the lines 
inherently possible for the species.  If this is true the question then 
arises: "can individual plant cells duplicate the regenerative that occurs 
in the cuttings previously stated?"
	In the 1930's and 1940's researcher, Johannes van Overbeek, 
discovered that a suitable medium was coconut milk.  Besides containing some 
nutrients, it also contains some critical hormones needed in plant growth.  
Van Overbeek was able to grow individual cells that he had separated out 
>from  young carrot embryos.  He cultured them in the coconut milk and then 
planted them in soil.  After some deliberation they produced normal adult 
carrot plants.  These reproducible results indicated that cells in the 
embryo are indeed totipotent.
	In the late 1950's mature tissue was cultured in a simmilar manner 
by F. C. Steward of Cornell University.  The mature carrot cells were 
removed from their parent plant and grown in a nutrient medium.  These 
mature cells grew into rootlike structures that, upon planting, became 
entire carrot plants.  This added to the studies done by van Overbeek 
showing totipotency in plant cells at a variety of life cycle postions.
	We see that many cells retain the capability necessary to produce 
the entire plant.  Also, when removed from their cell associations, cells 
can regress to an undifferentiated state from which they can then proceed 
through normal embryonic development.  How does this answer the question 
about differentiation?  It tells us that genetic information is not lost as 
development proceeds and that it is not irreversibly repressed.
	References: "Science" magazine and "Biology, the science of life" 
text.