When to Grow, Part II
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.

Cordially,

Conroe Joe

LINK: Science Article, Jan. 6, 2006, DELLAs and Stress
http://members.aol.com/jshaw1953/…

LINK: A Model of Plant Genes and How They Might Interact
http://tinyurl.com/dle7s/