Hormone

Abscisic Acid (ABA)

ABA May Figure Prominently in Desert
Plants Like Cactus - a wikimedia commons image

Chemical Structure 

ABA's main role may be a
signal of water abundance.
- a wikimedia commons image

Speculative Overall Role

Water deficiency signal

What is ABA's speculative
complementary abundance signal?

Salicylic Acid (SA)

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

Dry plants should have high levels of ABA, well watered plants, low levels. Like abundance signals ABA 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). ABA should be made when a cell has less than enough water to support both it any cell dependent on it for water acquisition. Thus ABA is an indication that water exists in less than enough amounts to continue the plant at its current size, thus the plant must use emergency stores of water, find new sources of the liquid and cut down on water sinks.

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

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

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

ABA should encourage root and new root growth, but inhibit shoot growth and even encourage shoot and leaf senescence.

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

ABA should cause the emptying of stored water reserves found in vacuoles or tubers.

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

Good question. Perhaps, operationally water is more a problem for the shoots than the roots so greater amounts of ABA exist there and the target of operation is mainly the roots so it get transported down.

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

ABA should generally push all nutrients and abundance signals/hormones out of cells. The pushing out of minerals, sugar and oxygen should lead to an explosion of scarcity hormones and a rapid emptying of cell nutrients. Speculatively maybe ABA has no effect on immature tissues or actually stimulates the uptake of water and maybe less so, other nutrients and hormones to these vulnerable cells. Working perhaps especially with root cells that produce salicylic acid to attract nutrient and growth hormones to efficient water harvesting roots.

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

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

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

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

If overall speculative role is true, how
should ABA affect senescence?

Just as I am hypothesizing that ABA, JA, IAA and CK all need to be present to induce cell division, ABA, GA/BR, ET and strigolactones may all need to be present for cell senescence to proceed. ABA should encourage senescence, particularly of shoot tissue whose nutrients can be cannibalized and used to make more water absorbing root tissue. Since water is the issue a relative net preserving of roots should occur.

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

Since GA/BR causes lengthening, ET broadening, strigolactones lengthening, what's left is ABA should cause cell and tissue broadening when it induces growth if it does. (I believe I may have seen such a finding but I have to find the reference again).

Proven Synthesis and Transport

1. Under consistent levels of desiccation, ABA levels normally peak at night. 1 Why this might make sense - ABA like the other insufficiency hormones, are more active at night where nutrient stores have to be relied on to support life, rather than during the days when they are actively acquired.

2. Closes stomata via ABA synthesized in the root. 2 Why this might make sense - having closed stomates cuts off excess transpiration and net water loss (insufficient water replacement occurring in the roots).

3. Induce by drought. 3 Why this makes sense - of course drought often induces water shortages in plants.

4. ABA coming up from the root, synergizes with auxin coming down from the apex to produce apical dominance. 12 Why this makes sense - ABA probably along with another of the shortage hormones, maintains apical dominance in the shoot at night in order

Proven Effects

1. Closes stomata via ABA synthesized in the root. 2 Why this might make sense - So water loss is slowed by closing of the stomata.

2. At high concentrations, inhibits root growth, but after removal, stimulates greater root lengthening and branching than controls. 4 Why this might make sense - ?

3. Mediate adaptation to salt.5 Why this might make sense - High salt levels may water stress plants.

4. Mediate adaptation to heat. 6 Why this might make sense - High salt levels may water stress plants.

5. Mediate adaptation to cold. 7 Why this might make sense - Cold may make water less available to plants. Transpiration cools plants, so this might be avoided by the ABA induced stomata closing.

6. Inhibits kinetin nucleotide formation. 8 Why this might make sense - The photosynthesis process uses twice as much water as it makes and kinetin stimulates this.  

7. Down regulates enzymes needed for photosynthesis. 9 Why this might make sense - photosynthesis uses up water as the hydrogen atoms are donated to the sugar molecules with the added energy of the sun. The plant probably dials down growth supporting photosynthesis to levels just needed for survival, in order not to use up water it can't afford.

8. Induces bud dormancy. Lower levels of ABA is associated with dormancy termination in winterized plants. 10 Why this might make sense - ABA's effect overall maybe to lower metabolism to a hibernating level. This is in part because photosynthesis uses up water. Metabolism allows you to get it back, but photosynthetic levels high enough to allow growth would use up water which is in short supply during droughts, at night and presumably during the winter. Also even some metabolism will produce toxic wastes which will take more energy and water loss to remove (by nick at testsforge). Therefore the tree may want to be in a state of hibernation during the winter which ABA helps maintain.

9. ABA Promotes tuberization. 11 Why this might make sense - it encourages the plant to hibernate rather than use the net carbon gain from the formula photosynthesis - metabolism, for growth.

10. ABA coming up from the root, synergizes with auxin coming down from the apex to produce apical dominance. 12 Why this might make sense - the breaking of shoot apical dominance is probably only warranted under good root conditions, i.e. when there is more than enough water and minerals to support the cells in the root and shoot at their present weight.