DWARF PLANTS

[Andy Phillips]

Anne Gillen wrote:
> 
> Perry d'Obrenan (perry_dobrenan at darwin.biochem.ualberta.ca) wrote:
> : Can anyone tell me what causes a dwarf plant to remain in dwarf form?  For
> : example, a Nest Spruce?
> 
> I do not know about Nest Spruce but it is common for dwarfing to be
> caused by a mutation in genes involved in giberellin synthesis.
> Giberellin is a plant hormone that among other things causes elongation
> of internodes.  I have been told that this mutation occurs spontaneouly
> and rather frequently in certain evergreen trees and is responsible for
> the "witches broom"  at the top of some trees.

The classic example of a gibberellin-deficient mutant is the dwarf pea used by 
Mendel when he established the rules of genetic hybridization. This mutant lacks 
the enzyme gibberellin 3-beta-hydroxylase, which converts inactive GA20 to the 
bioactive GA1 form.

There are also examples of dwarfs that are due to loss of response to 
gibberellins. A good example is in wheat, where old varieties used to grow up to 
waist or even shoulder high (look at medieval paintings of wheat fields). The 
modern varieties are much smaller, which enables more resources to go into 
filling the grain and not be wasted on producing straw. Part of this is due to 
semidominant mutations in the rht family of genes that mediate responses to GA.

Andy
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