When to Grow, Part II

ConroeJoe@aol.com ConroeJoe@aol.com
Sun, 08 Jan 2006 07:45:59 PST
Hi Gang,

Part II (When to Grow, or Not to Grow) 

Gardeners enjoy plant growth; we strive to make it happen.  Typically, we 
want a plant to grow; more growth typically means more flowers, or more fruit, 
etc.  DELLA proteins restrain plant growth, a phenomenon known as the "DELLA 
restraint."  DELLA proteins are a natural part of plants and plants use a series 
of mechanisms to enhance growth or oppose growth.  DELLAs oppose the "growth 
enhancing" effects of gibberellins.  

Though they restrain growth, the DELLA proteins are normal and not a negative 
or "bad" phenomenon.  They are a healthy part of plant biology because plants 
can't grow all the time in all their parts; growth must be regulated and 
DELLAs are part of that regulation.  Plants typically have optimal times to grow, 
other times when slow growth is warranted, and still other times when growth 
ceases and perhaps other activities take place (e.g., flowering or downward sap 
flow, etc.).  

The exciting news this week concerns another apparent property of DELLA 
proteins (Science, Jan. 6, 311:91-94).  It seems that DELLA proteins can slow down 
plant growth during periods of environmental stress; in the experiments 
reported the plants were stressed with salty soils.  It has long been clear that 
"medium" salty growth conditions cause plants to be stunted or to grow more 
slowly (salt-stressed plants are smaller and are late to flower).  But it was not 
so clear that the slow growth resulted from molecular decisions on the part of 
the plant, as opposed to the stressor itself.

It has always seemed plausible that stunted growth, slower growth, and 
delayed flowering might well be due to the stress itself (in this case salt-stress). 
 Perhaps, it was thought, salt acts as a toxin and simply deranges normal 
physiological systems, even killing tissues and cells outright.  No doubt this 
still seems likely at "very high" levels of salt.

But, the research reported this week points at another mechanism that causes 
slow growth:  the DELLA proteins.  In fact, the report provides evidence that 
such slow growth in the face of salt-stress is advantageous for plants.  
Restrained growth seems to allow more plants to survive salt exposure than plants 
that lack DELLA proteins.  

In the research reported, 4 of 5 known DELLA proteins in one plant species 
were removed by genetic engineering.  The resultant DELLA-deficient plants grew 
faster and flowered more quickly in salty soil than did wild-type plants (the 
DELLA restraint had been removed).  The authors concluded that DELLA proteins 
were, in fact, responsible for the slower growth observed under the conditions 
of the experiment, and that medium salty conditions, per se, are not 
responsible for the slower growth.  Put another way, plants missing the 4 DELLA 
proteins grew faster than wild-type plants when exposed to salt.  Put yet another 
way, the plants themselves have a mechanism that slows their growth when exposed 
to medium salty conditions.  

Thus, DELLA proteins seem to slow plant growth in salty situations, but there 
is a very interesting twist.  Though they might be faster growers at nontoxic 
levels of salt, DELLA-deficient plants were more easily killed higher 
concentrations of salt.  Thus, something about growing more slowly (the DELLA 
restraint), allows plants to survive better when faced with potentially toxic levels 
of salt.  

The authors concluded that there is a reason DELLA proteins slow down growth; 
slow growth may assist in overall plant survival during stressful conditions. 
  The researchers hypothesized that slower growth might allow a plant to 
redirect resources to overcome problems associated with stress, or perhaps that a 
smaller plant size is useful in hostile environments.  

Clearly, the full story is not known, many more discoveries await.  But the 
story this week is that plants have sophisticated mechanisms to slow their 
growth when they encounter environmental problems, and that such slow growth may 
be advantageous.  


Conroe Joe

LINK:  Science Article, Jan. 6, 2006, DELLAs and Stress

LINK:   A Model of Plant Genes and How They Might Interact  

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