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Gibberellins (GA)

The so called "Foolish Seedling" a
disease first characterized in Japan which
is due to too much GA/BR - Image from here

Chemical Structure 

File:Gibberellin A1.svg

GA1 - there are many different
Gibberellin molecules. BR may be part of
the chemical pathway that cause it's action.
- a wikimedia commons image

Speculative Overall Role

Sugar deficiency signal in the same
pathway of action as Brassinosteroid

Growth Direction Tendancies Lengthening or elongating

What is Gibberellin's
complementary stimulating hormone?

Jasmonic Acid/Jasmonate

If overall speculative role is
true, where, when and which cells
should synthesize Gibberellins Acid?

Darkened plants should have high levels of GA, well lighted plants, low levels. Like abundance signals GA may be mostly made in meristematic cells and much less so as cells mature. (Or for real theoretical beauty, deficiency hormones should be made mostly in mature cells and much less so in meristematic cells).  GA should be made when a cell has less than enough sugar to support both it any cell dependent on it for sugar acquisition.  Thus GA is an indication that sugar exists in less than enough amounts to continue the plant at its current size, thus the plant must use emergency stores of starch, find new sources of the molecule and cut down on its sinks.

If overall speculative role is true, what should Gibberellins exogenous treatment produce?

High levels of exogenously applied GA should induce JA synthesis, because many of GA's effects may be to increase sugar levels within the plant, if only temporarily.  This may include making dormant  reactions that normally depend on sugar.

If overall speculative role is true, what
should Gibberellins inhibit and stimulate?

GA should encourage shoot and new shoot growth, but inhibit root growth and even encourage root senescence.  This may be a particularly apparent when Ethylene levels are high and Strigolactone and ABA levels are low and levels low as this is an indication that resources need to rerouted from the root to the shoot.

If overall speculative role is true,
how should Gibberellins affect storage?

GA should cause the emptying of stored sugar reserves found in vacuoles or tubers.

If overall speculative
role is true, how should
be transported?

Sugar deficiency, on average should be detected in the roots first, the point furthest from the source of sugar.  GA may be transported from the roots to the shoots where presumably they repel sugar and send it in the opposite direction back to the low sugar roots.

If overall speculative role is
true, how should Gibberellins affect
nutrient attraction and repulsion?

GA should generally push all nutrients and abundance signals/hormones out of cells.  GA should attract the deficiency signals/hormones, ABA, ET and Strigolactones, leading to positive feedback and cell senescence. GA should perhaps also work with JA to attract nutrients and positive hormones to leaves that are making Jasmonic Acid and thus are efficient synthesizers of sugar.

If overall speculative role is true, how
should Gibberellins affect apical dominance?

GA should break shoot apical dominance because low sugar levels are an indication of poor performance by the currently dominant apical shoot.  GA may strengthen the currently dominant root apices in order not to encourage any new root growth which would be a further sink on sugar levels.

If overall speculative role is true, how
should Gibberellins affect Cell Division?

Although it may encourage cell division in the shoots, if it is inducing new ones, GA should generally inhibit cell division, as a sugar deficient plant is in no condition to expand.

If overall speculative role is true, how
should Gibberellins affect Senescence

Just as I am hypothesizing that SA, JA, IAA and CK all need to be present to induce cell division, ABA, GA/BR, ET and Strigolactone may all need to be present for cell senescence to proceed.  GA/BR should encourage senescence, particularly of root tissue whose nutrients can be cannibalized and used to make more sugar producing shoot and leaf tissue.

If overall speculative role is
true, how should GA effect growth
directions to provide balance in the plant?

Gibberellin is well know to lengthen leaves and stems.  Presumably stem lengthening accomplishes moving shaded plants back into the sunlight.

Proven Synthesis and Transport

  1. GA levels go up in the dark when Sugar cannot be manufactured and down in the light. 20 Why this makes sense - GA is another deficiency hormone that is presumably active at night when sugar can't be made and so continual maintenance metabolism must be carried out by dissolving sugar store stored during the day.  GA induces this storage use.

  2. "The highest content of GA was characteristic of leaves in the period of growth cessation." 31 Why this makes sense - growth cessation I is a prelude to senescence.  Presumably normally leaf senescence is initiated by ABA and Strigolactones but as indicated elsewhere these hormones probably push out all nutrients and attract deficiency hormones.  There may be two phases of senescence, growth senescence/hibernation and secondly actual senescence.  Growth cessation or hibernation may be a valley with steep hills to overcome either to get back to growing, or to initiate final senescence.

Proven Effects

  1.  Promotes shoot and flower stem lengthening especially in the dark. 21 22 23 Why this makes sense - this moves the plant stem and leaves back into the sun out of the shade.

  2. Greatly promotes bud growth. 24 Why this makes sense - the hydrolyzing of starch may be a prelude to bud growth in the spring.  Additionally the need for new buds before the winter begins may be signaled by increasing levels of GA which indicates the plant is using more and more stored sugar and less newly photosynthesized glucose which in turn is an indication of the coming winter.

  3. GA reverses ABA effects on growth inhibition and dormancy. Why this makes sense - GA hydrolyzes stored starch.  This freed sugar may stimulate breaking of dormancy.

  4. Dissolves stored starch. 26 Why this makes sense - dissolved stored starch makes up for sugar gap.

  5. At low concentrations GA (Gibberellin A3) and other Gibberellins promote lateral root growth but high concentrations markedly inhibit it. 27 28 Why this makes sense - at low levels, hydrolyzed stored starch frees the root from lateral root growth limiting sugar levels. At high levels the plant know it is experiencing severe sugar shortages so need to concentrate resources toward the sugar producing shoot.