Hormone

Ethylene (ET)

The earliest discovery of ethylene occurred when when lamp post near a greenhouse caused wilting and leaf abscission. - a wikimedia commons image

Chemical Structure 

Ethylene is an extremely simple gas.
- a wikimedia commons image

Speculative Overall Role

Oxygen deficiency signal as well as possbily excess carbon dioxide signal in the root.

What is ethylene's speculative
complementary abundance signal?

Auxin (IAA)

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

Cells most apt to be oxygen deficient are the interior of roots and flooded roots in general.

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

Exogenous ethylene treatment should show methods for increasing oxygen to the roots such as aerenchyma tubes or increasing leaf area. Additional measures might include epinasty for pumping out extra water from around the roots, metabolism inhibition and photosynthesis stimulation.

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

Ethylene should inhibit root growth and encourage shoot growth. It should inhibit metabolism and stimulate photosynthesis. It also is known to stimulate the growth of root hairs, a method for increasing the surface area of the root and its absorption of oxygen for local use.

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

Ethylene should cause the release of stored oxygen if there is such a thing.

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

Ethylene should be transported from the roots to the shoots, perhaps by rising or bubbling up from the ground as a gas or being transported up as ACC.

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

Ethylene should generally push nutrients and stimulating/growth hormones out of cells and attract deficiency hormones. Possibly it works with auxin producing leaves to attract more nutrients to these oxygen harvesting sites.

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

Ethylene represents a failure of the current methods for harvesting oxygen, therefore it should break the apical dominance induced by auxin.

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

Ethylene should generally inhibit cell division although it may stimulate it in shoot areas that are particularly efficient at harvesting oxygen and therefore making auxin.

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

Ethylene should inhibit root growth along with gibberellin and cause the senescence of older roots. It should inhibit the senescence of shoots parts, especially those making auxin.

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

Ethylene is known to broaden plant parts. Its analogue stimulating hormone, auxin, appropriately lengthens plant cells and tissues.

Proven Synthesis and Transport

1. Induced by high levels of auxin, especially in the roots but this can be moderated by red light which is characteristic of shading. 55 Why this makes sense - auxin is an indicator of oxygen abundance so, oxygen requiring reactions are stimulated, depleting oxygen levels. Why shading would moderate might be because oxygen reactions are less stimulated under cooler temperatures so if the plant senses shade it prepares itself for less oxygen use.

2. Ethylene levels increase during flooding, probably due to entrapment rather anoxia. Most plant appear to have a net inhibition of ethylene production under anoxic or carbon dioxide deficient conditions. 56 Why this makes sense - This doesn't make sense and my theory suggest that the finding is wrong.

Proven Effects

1. Promotes the ripening of fruit with climacteric respiration releasing additional ethylene. 52 Why this makes sense - Any of the deficiency hormone should be able to lead to a ripening process which  might essentially be a kind of programmed cell death and a freeing up of internal stores of nutrients within cells. However the advantage of ethylene is that it is a gas and can effect neighboring fruit. Additonally if there are interstitial gas spaces, the hormone would be rapidly transported througout the fruit. The increase in respiration supposedly induced by Ethylene in fruits, might be relatively unique to fruits as most other plant parts according to my theory should have their respiration inhibited.

2. Broadens/thickens plant parts. 53 54 Why this makes sense - Each of the abundance, deficiency pairs have opposite patterns of growth. If Auxin effects on the growth of the plant aren't producing the results needed then ETH takes over to try to do better growing the plant leaves and roots wider instead of auxin's longer. Also ETH and GA are the two root deficeincy signals and they also show a balanced opposite pattern of growth with GA lengthening palnt parts as well.

3. Inhibits leaf expansion. 54 Why this makes sense - This doesn't make too much sense in that my assumption is an expanded leaf should take in more oxygen which it can transport to the root.

4. Inhibits geotropism. 55 Inhibits auxin transport 55 and production? - Why this makes sense - Geotropism is the growth of the root downard which is opposite the direction of the greatest amounts of oxygen, so thismakes sense to be inhibited. The inhibition of transport and synthesis and Auxin makes sense as a negative feedback on Auxin which is stimualting too much oxygen use if Oxygen levels fall and Ehtylene is synthesized.

5. Induces leaf, fruits, and flower petal abscission. 52 Why this makes sense - as mentioned any of the deficiency hormones should be capable of being the leader in cell or plant part death. However, Ethylene as a respiration damper might always be the last step in apotheosis.

6. Stimulates seed germination. 52 57 Why this makes sense - This doesn't amke too much sense if Ethylene is a respiration damper, however, maybe at low levels it increases the access of the seed to oxygen through various mechanisms, including possibly stored oxygen if there is such a thing.

7. Flooding produces the epinasty reaction through ethylene, where leaf surfaces deliberately grow from a position perpendicular to the stem to one which is more horizontal. 52 58 Why this makes sense - Epinasty may increase the wind resistance of the leaves oby putting them in the path of the wind intead of parallel to it. Perhaps this increases oxygne absorption. Additonally the action of the wind on the leaves in its path, may act like a long handled water pump. The leaves would be the water pump handle. May be there is a one way valve in the leaf stem which caused the wind stretched water column in the leaf remain in the leaf once the water column is compressed when the leaf snaps back to a horizental plane from the action of a gust of wind. The total effect might be to pump more water out of water logged roots.

8. Induces air spaces called aerenchyma used for gas diffusion in roots during flooding of non-water based plants. 59 Why this makes sense - increases oxygen flow from the leaves, shoots and adventitous roots.

9. Carbon dioxide inhibits ethylene action. 58 Why this makes sense - This doesn't make sense if Ethylene is an indicator of anoxia as carbon dioxide should be a confirmation of anoxic conditions. However if ethylen is an indicator of all gas deifiency, perhaps as more an indicator of carbon dioxide deficency in the shoot and oxygen in the root, it might make sense.

10. Inhibits embryogenesis of cell cultures. 60 Why this makes sense - embryogenesis is perhaps a respiration intensive process.

11. Induces root hair growth. 61 Why this makes sense - Oxygen can be more adequately absorbed from spaces between soil particles if root hairs grow out.

12. Ethylene up-regulates auxin biosynthesis at least in the roots. 61 Why this makes sense - This doesn't make sense unless the total effect of ethylene is to change plant behavior to procure more oxygen and eventual success at this should restimualte if indirectly, the synthesis of auxin.

13. Flood induced ethylene sensitizes plants to the existing steady auxin levels, inducing adventitious roots formation. 62 Why this makes sense - Adventitious roots are roots which exit the stem of the plant above the floodline and go back into the water. Presumably these roots have better access to oxygne enabling them to use respiration to absorb water and minerals better. ALso athe adventitious roots may directly absorb oxygne above the flood level and convery it the the aerenchyma to the oxygen starved conventional roots.

14. Induces flower formation in some species. 52 63 Why this makes sense - Many plants will flower under duress. This is concievably because the plant "calculates" that conditions might get worse and eventually make life unviable thus it better get reproduction over with so that it can produce tough seed which can withstand the adverse seasons in the environemnt and start over when conditons improve again.

15. Etephon (ethylene precursor) has a dual role in tuberization. It promotes already formed tubers by inhibiting stolon growth. Differently though it inhibits the formation of new tubers. 64 Why this makes sense - Tubers may be a strategy for a plant to last through the parts of the year like the winter when conditions or adverse for the species. Theu ethylene being an indicator of difficulty procuring adequate oxygen from the environment, should promote th plant going into dormancy through building up of its tuber. New tubers or stolons are probably more vulnerable to adverse conditons having a great surface area per weight ratio. Thus the plant wisely concentrates on those tubers it already has rather than trying to branch out and create new ones.