plant cell wall proteins

pesusi at pesusi at
Thu Mar 31 07:52:04 EST 1994

In article <MAILQUEUE-101.940329095158.384 at bio>, BIO/KATE at TAMSUN.TAMU.EDU (Self) writes:
>> I have questions about this, and maybe you can help.
>> From my reading it generally appears that most plant physiologist believe
>> the cell walls "loosen" prior to elongating.  I have trouble with
>> "loosening" as a word to discribe what happens.  
>> In my understanding cells will only expand prior to secondary cell wall
>> deposition, and the primary cell wall is in a more or less continuous state
>> of deposition prior to 2nd cell wall formation.  
>> Therefore, since the cell wall is actively growing one does not need to
>> hypothesize a cell wall "loosening agent" to allow for expansion... the
>> cell wall is loose anyway, and it eventually loses this "looseness" as it
>> matures.
> Cellulose microfibrils are tightly hydrogen-bonded to xyloglucan 
> molecules, which may bridge from one microfibril to the next.  Cells 
> generally expand perpendicular to the major axis of microfibril 
> orientation, and so expansion requires either or both (1) breakage of 
> the xyloglucan polymers (the XET activity theory), or (2) interuption 
> of hydrogen bonding (Cosgrove's expansin theory).  "Loosening" refers 
> to the breakage of bonds between adjacent microfibrils.  If this is 
> difficult to picture, see Cosgrove's review (1993. Plant Physiol. 
> 102, 1-6).  
> Most cells do not form a secondary wall, and cease elongating never-
> the-less.  Stem growth is a good example.  This may be due to cross-
> linking of cell wall proteins (see Bradley et al., 1992.  Cell 70, 21-
> 30).
> I am forwarding this message to plant-biology in hopes of further 
> discussion.
> Kate VandenBosch
It is a pity that most scientists even plant physiologists think the cell wall
as rigid structure with no special function. This view should be changed sooner
or later. In previous answer it is presented two possible hypothesis for
primary growth. In my mind "loosening" is just a description for a period  when
plant cell wall material is accumulating to cell wall. We must also keep in
mind that growing can happen in different sides of a cell in different times, 
so some parts of plant cell wall are active while others are not.-About
secondary growth I would say that after lignification we can be quite sure that
growth has ended.


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