How Do Plants Know When, and When Not, to Grow
Sat, 07 Jan 2006 11:14:15 PST
Part I

It has been known for a long time that gibberellic acid (GA) somehow 
stimulates plant growth.  GA was discovered about 80 years ago by a Japanese 
scientist; today there are many dozens of GA-type molecules known that are collectively 
called gibberellins (GAs).

GA was initially discovered after scientists investigated a plant disease 
causing fungus called Gibberella; infected rice seedlings grew too much.  They 
stretched tall and thin before they died; investigations showed that the 
Gibberella fungus was causing the extra growth be releasing a plant hormone that 
became known as GA.  GAs stimulate cell elongation and cell division and these two 
activities collectively cause stem elongation (growth).  

A fungus is not the only organism that can make GAs; plants make their own 
GAs.  Typically, dwarf mutants of various plant species have mutations that 
interfere with GA biology; they don't make their own GAs or they can't detect it, 
etc.  Plants make GAs in a controlled way, thereby controlling their growth.  
It has long been a mystery how plants known when to make GAs, how they can 
sense GAs and respond correctly (by growing).  This is not an academic question 
because plant growth affects much of what we notice about plants:  1) when will 
the shoot tip awaken in spring, 2) why can some plants have extremely short 
stems and thus make bulbs, 3) how tall will a tree become, 4) when does the 
grass need mowing again, etc.  

A few years ago it became clear that some plant proteins, called DELLAs, 
oppose the effects of GAs.  GAs promote growth, whereas DELLAs inhibit growth.  
GAs can cause a plant to recycle DELLA proteins, effectively removing them and 
thereby removing a "brake" on plant growth.  

Thus, while the molecular mechanisms are still murky, the outlines of the 
story are clear.  DELLAs inhibit growth, and GAs cause the loss of DELLA 
proteins.  Presumably, lower levels of DELLAs results in more growth because DELLAs 
inhibit growth.  A plant is, in effect, a finely balance bit of machinery always 
making GAs or DELLAs as needed in order to achieve correct growth.  Imbalance 
of either component results in increased growth or stunting, depending upon 
the nature of the imbalance.  

The exciting discovery reported this week in Science concerns the DELLA 
proteins.  They have a few tricks of their own, and they actually seem to help 
plants decide when to slow down growth.  


Conroe Joe

LINK:  Gibberellin (Wikipedia)  

LINK:  Abstract (GA inactivates DELLAs)…  

LINK:  Dr. Harberd Laboratory Web Page  

LINK:  Relieving DELLA Restraint (subscription needed)…  

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