Restating of the Speculations on the 8 Hormone System
socrtwo
The 8 most widely studied plant hormones are Auxin (usually IAA),
Cytokinin (CK), Ethylene (ET), Gibberellin (GA), Abscisic Acid (ABA)
and the lesser studied, Brassinosteroid (BA), Salicylic Acid (SA), and
Jasmonic Acid (JA).
If you look through the previous versions of this theory from the
links above, they seem to suggest some contradictory things but
overall the suggestions have in common that there are in general two
classes of hormones, one which is made under good growing conditions,
and the other made when conditions do not warrant growth and in fact
necessitate cutting back on size, using stored resources and changing
strategy for obtaining nutrients to more risky but also higher return
rate strategies.
In general also the past ideas tied specific hormones to specific
nutrients. Thus Auxin was seen tied to growth warranting levels of
sugar and gases, Cytokinin was seen as made when growth warranting
levels of water and minerals. Gibberellin was seen as made when there
were less than enough sugar and gases for growth and Ethylene was seen
as made when there were less than enough minerals and water to warrant
growth.
ABA and Salicylic Acid at least in some versions were left out and not
tied to any specific nutrients. With ABA, this flew in the face of
conventional wisdom because it has widely been seen as a water
deficiency signal. Instead ABA was seen as an emergency signal of any
rapidly developing adverse situation and SA (Salicylic Acid) as its
all clear signal.
Growth Hormones - Auxin, Cytokinin, Salicylic
Acid, and Unknown Hormone X (Jasmonic Acid?)
Upon reflection I now am ready to reconsider my ideas and instead
postulate an 8 hormone system instead of a six. Note as with all my
previous versions, I speculate that "growth levels" of nutrients in a
meristematic cell are twice as much in the shoot than in the root if
it is supposed to procure that nutrient (i.e. the shoot is supposed to
procure sugar). This is because the shoot cell will eventually have
to support both it and a similar sized root cell in say sugar
production. Also then in the root, a meristematic cell needs to
procure twice as much minerals as a similar sized meristematic cell,
before it will start producing Cytokinin.
Also note, if they have growth levels of the nutrient of which they
are an indicator, Cytokinin, Auxin and any other growth hormone should
continue to be made by any plant cell as they mature but it much
lesser amounts. Perhaps the only reason for continuing to make the
hormones in mature cells, is to avoid senescence as small growth
hormone production indicates productive "profitable" cells (having
nutrient levels higher than needed for the survival of that cell and
any cell dependent on it for production of that nutrient). Thus it is
a good thing for instance, for a mature leaf making more than enough
sugar and taking in more gases than are needed for the survival of
both it and a similar sized root cell, that this shoot cell continue
to receive a guaranteed amount of water and maybe minerals too and for
the leaf not to senesce and its resources be reused for younger cell
growth. This preservation for senescence is done at least in part by
the Auxin and Unknown Hormone X (see below - may be Jasmonate), it
makes.
Auxin - made when there are growth levels of sugar in any meristematic
cell.
Cytokinin - made when there are growth levels of minerals in any
meristematic cell.
Salicylic Acid - made when there are growth levels of water in
meristematic cells.
Unknown Hormone X (Jasmonic Acid?) - made when there are growth levels
of needed gases, Oxygen and Carbon Dioxide.
Senescent Hormones - Gibberellin, Unknown
Hormone Z (Brassinosteroid?), Abscisic Acid, Ethylene
There has been criticism of my work in the past in regards to which
cells make of the deficiency/senescence hormones. This was because I
classify Gibberellin as a deficiency signal, but the according the
Plant Physiologists, GA (Gibberellin) is made more often in abundance
in meristematic tissue than mature tissue. In my earlier versions, I
thought it was more beautiful aesthetically to see the deficiency
hormones as made in higher amounts in mature tissue and dropping off
the younger the age of a cell is. I don't want to get in a discussion
about that here but suffice it to say, either way does not effect most
of the content of my versions. In the 2007 version I did concede to
the critics and made a version with all the hormones including those I
label as deficiency hormones (except Abscisic Acid and Salicylic Acid)
being made at high levels by meristematic tissue and dropping off as
the cells mature.
Gibberellin - Sugar deficiency.
Unknown Hormone Z (Brassinosteroid?) - Mineral deficiency.
Abscisic Acid - Water deficiency. This is well accepted.
Ethylene - Oxygen and maybe gas deficiency. Ethylene has been
accepted in the past by some as an Oxygen deficiency signal.
Discussion and Ramifications
Most of the features discussed in previous version of the paper
continue to hold in a similar way as they did before.
Growth Hormone Nutrient Attraction - Because hormones also cause the
attraction of nutrients (and even other hormones), growth levels of
hormones ensure the continued flow of complimentary nutrients to a
nutrient producer. Thus if a leaf cell is making Auxin, it draws
minerals, water and gases to itself to continue production of
profitable amounts of sugar.
It may turn out Auxin even causes sugar to be attracted to site of
Auxin synthesis and even more strangely, sugar may relatively speaking
be the nutrient which it attracts the most strongly to where the Auxin
is. The reason for this are two fold. First meristematic cells need
to grow quickly and may not produce enough sugar to grow at the rates
the plant wants. That may be why Auxin attracts sugar to meristematic
cells in the first place. Secondly Auxin is transported down the
plant, so sugar follows this downward traveling of this. Jacobs and
Gilbert first showed around 1984 that Auxin transport pumps are
located at the bottoms of the sugar transporting phloem. So maybe
Auxin is part of the engine that drives this sugar transport.
Similarly then Cytokinin is transported upward by the Xylem and it may
more strongly attract minerals to it from surrounding tissue, than
other nutrients.
Perhaps the pre-eminent nutrient attracting properties of the growth
hormones, for the very nutrient of which they are an indicator of,
occurs much more strongly in mature cells than meristematic cells.
Conversely then in the younger meristematic cells, then the nutrients
the the specific hormone attracts, would be more strong for the three
categories of complimentary hormones, and only weakly for the nutrient
of which it is an indicator. The reason for the weak attraction of
the nutrient at all may be as indicated, to have a meristematic cell
grow faster than it could without the supplementation of the nutrient
of which it is an indicator.
Growth Hormone Level Peak During the Day, Deficiency/Senescence Peak
During the Night - There is some evidence that this is true, but I saw
this mostly in older journal articles. Suffice it to say, that with
colder temperatures and lack of light affecting the processes that
require energy, we might expect that there is less nutrient
procurement at night and more of the requirement to rely on stores.
Also at night perhaps housekeeping and pruning of less efficient older
plant parts is more easily carried out (although it may be difficult
to know if a leaf is an efficient sugar producer at night!). This is
all to say, deficiency/senescence hormones may be part of the normal
activity of a plant and not simply around when unusually bad
environmental conditions exist.
There are other ramifications which you can read about in other
versions, that should hold to be still true with this 8 hormone theory
too. For instance maybe now all four growth hormones, Auxin,
Cytokinin, Salicylic Acid and Unknown Hormone X are required for
before a cell will divide, and all four hormones Gibberellin, Unknown
Hormone Z, Abscisic Acid and Ethylene are needed before a cell will
senesce.
I will finish the rest of this paper at a later date..