New Version of a Speculative Start to a General Theory of Plant Hormones
socrtwo
Introduction
Taking into consideration a criticism of my previous theories on plant
hormones (see here: www.planthormones.info), I'm willing to revise the
speculations so they fit more with current findings. The criticism
that appears most salient is that GA is not made by older cells mainly
but by younger cells. I will then make this axiomatic that the two
pairs of complimentary hormones Ethylene and GA/Brassinosteroid and
IAA/Cytokinin are made mainly by young cells. There are further
implications to this that require some changes to my speculations.
This page will take the form of theorems or axioms about plant
hormones as did the other "papers" available on the site above.
Theorems
* The major plant hormones Auxin, Cytokinin, Gibberellin,
Brassinosteroid, Ethylene, Abscisic Acid and Salicylic Acid can be
split into two groups. The first group is Auxin, Cytokinin,
Gibberellin, Brassinosteroid, and Ethylene. They are hormones made to
correct nutrient imbalances. The second group of Abscisic Acid and
Salicylic Acid are made when any general stress occurs or the relief
of that stress.
* The nutrient hormones (Auxin, Cytokinin, Gibberellin,
Brassinosteroid, and Ethylene) are made in the highest concentrations
by dividing and young plant cells and the levels fall off
precipitously, but not completely as the cells age. The stress
hormones (Abscisic Acid and Salicylic Acid) are perhaps made by all
cells in equal amounts facing the same stress or release from stress
conditions.
* Auxin is made mostly by young plant cells that have more than enough
shoot derived nutrients (mainly sugar, CO2 and O2) to support both
them and any dependent cell, thus growth is a possibility if balanced
out by an excess of water and minerals. A root cell has no cells
depending on it for sugar and gases, but a shoot cell would expect to
have a similarly sized root cell depending on it for its shoot derived
nutrients as well as having to fulfill its own needs.
* In a likewise manner Cytokinin is made mostly by young plants cell
that have more than enough root derived nutrients (mainly minerals and
water) to support both it and any cells depending on it for root
derived nutrients. Growth is thus a possibility here too if balanced
out by an excesses of sugar and gases. For a shoot cell there would
be no cells depending on it for water and minerals whereas a root cell
should have a counterpart shoot cell depending on it for root
nutrients.
* Conversely now, Ethylene is made mostly by young cells when they do
not have enough minerals and water to support both them and any cell
depending on it for the acquisition of minerals and water. Restating
this makes similar size cells in the root producing Ethylene when the
level of minerals and water drops below 2 times that needed to
maintain the cell at its present size, whereas for a similar sized
shoot cell it would only have to be the amount of minerals and water
dropping below what the cell itself alone needs to maintain life at
its present size. Thus a plant will need to cut back in size, if the
deficit can't be made up.
* Also GA or Brassinosteroid (I'm lumping them for now into one
hormone cascade path) is made mostly by young cells when they have
less than enough sugar and gases to support both it and any cell
depending on it for acquisition of these nutrients. Thus again more
plainly, if a root cell does not have enough sugar and minerals to
maintain their present size, they make GA or Brassinosteroid. A shoot
cell of the same size and maturity will do a similar thing if making
less than twice the needed sugar and acquiring less than twice the
needed gases to maintain itself, because it is supporting a similar
sized and maturity stage root cell for those nutrients. Thus again
the plant will need to cut back if the deficit can't be made up.
* The nutrient hormones are made to correct imbalances. They do this
by affecting 4 things: nutrient transport, nutrient storage, direction
of growth, and new growth initiation or old growth senescence.
* For Auxin correcting the imbalance of the perceived excess of sugar
and gases is first dealt with by initiating active transport of sugar
and gases away from the site of synthesis of the hormone and induction
of active transport of minerals and water to the site of Auxin
production. Nutrient storage is initiated for the excess sugar and
gases in vacuoles of the cell and release of store minerals and water
if any from vacuoles in the cell would also initiated. The direction
of growth of the plant or plant cells is also changed by Auxin. If
the plant, plant organs, or plant cells have been broadening (because
of the presence of Cytokinin or Ethylene) they are changed to
lengthening strategy of growth. Finally Auxin corrects the imbalance
by initiating new roots by causing dormant root buds to grow out, thus
increasing the flow of water and minerals. Auxin is also of course
known to inhibit the growth of new shoot buds with shoot apical
dominance.
* For Cytokinin correcting the imbalance of the perceived excess of
water and minerals is done by initiating or increasing the transport
of water and minerals away from the site of synthesis and increasing
the active transport of sugar and gases towards the site of
synthesis. Likewise, Cytokinin increases the storage of water and
minerals within the synthesizing cell's vacuoles and increases the
release of sugar and gases from vacuoles stores if they exist within
the cell. Also Cytokinin causes or influences the growth of any cell,
organ or plant toward broadening, and away from lengthening. Finally
Cytokinin induces new shoot growth and inhibits root bud growth with
root apical dominance.
* For Ethylene the correction of the deficit of water and minerals is
handled by actively increasing the flow of water and minerals to the
site of synthesis (and by increasing the flow of sugar and gases out
of the cell too ???). Nutrient stores of water and minerals wherever
they are found are encouraged to give up their storage to needy
cell(s) producing Ethylene (and sugar and gases are stored in vacuoles
to decrease an imbalance???). Ethylene also is known to influences
the direction of growth of a plant away from lengthening to
broadening. Finally Ethylene induces older leaves to senesce, sending
the resulting freed up water and minerals to locally needy leaves and
sending the sugar and gases to the root to make more roots. Ethylene
is known to induce root hairs which increases the surface area of the
root and thus increases the uptake of water and minerals. Ethylene
probably inhibits new shoot growth.
* For Gibberellin/Brassinosteroid the correction of the deficit of
sugar and gases is handled by actively increasing the flow of sugar
and gases to site of synthesis (and increasing the rate of transport
of sugar and gases out of the cell to recreate the balance???).
Stores of at least sugars in the form of starches are known to be made
available by GA during seed germination and probably exist under all
circumstances. This would be true of any stored gases too. The
direction of growth of cells is also influenced by GA toward
lengthening. Finally GA/BA inhibit root growth and probably cause the
senescence of older roots as well as an analogous change of strategy
to root hair initiation that Ethylene produces in roots that might be
represented by bolting to move the plant out of the shade.
* It is widely known the Auxin and Cytokinin are needed to induce cell
division. This can be seen as the plant being reassured by these
signals that it has excess amounts of all nutrients of both the shoot
and root derived kind, and so cell division is warranted. I believe
it's also been shown that a cell under the influence of both Auxin and
Cytokinin will draw all nutrients to itself not sending away any
excesses. Thus if a cell or plant organ is making Auxin and it comes
under the influence of enough Cytokinin, it will change strategy and
stop exporting sugar and gases and instead become a net importer of
the resources even if they are a successful young shoot cell.
* Complimentarily I'm proposing that Ethylene and GA/BA are needed for
cell senescence. In fact when Ethylene is being released in the
shoot, the cells or leaves that "it chooses" to senesce, may be the
ones making the most GA, because these would be cells or leaves that
are the least efficient at doing what the leaf should do, which is
procure sugar and gases. If a cell is synthesizing GA and comes under
the influence of Ethylene, I believe it will stop actively drawing
sugar and gases to it, and actually start to send them out. GA and
Ethylene acting together would then send out all nutrients, leading to
the synthesis of more GA and Ethylene and an even higher rate of
active transport of these nutrients out, with this active feedback
loop leading to a climactic rise in Ethylene and GA and senescence.
* Auxin and Cytokinin are highest in levels during the day when the
sun is out for photosynthesis and improved transpiration. (Perhaps
transpiration increases mineral absorption and water intake following
osmosis and the uptake of water and minerals is higher during the day
than at night for most plants). Ethylene and GA/BA levels are higher
at night.
* Ethylene and GA/BA levels are higher at the beginning of the life of
a plant when water, minerals, sugar and gases stored in the seed must
be released. Cytokinin and Auxin levels are highest during the middle
active growth period of life of the plant. Ethylene and GA/BA levels
increase again relative to Auxin and Cytokinin at the end of the life
of the plant or growing season, when the nutrients must be withdrawn
from unneeded plant organs like leaves or flower petals or moved into
the fruits and seeds.
* Ethylene and GA/BA move resources more toward the center of the
plant and away from the periphery. Auxin and Cytokinin are more risk
taking hormones moving resources to the active edges of the plants
where "the action is".
* As for ABA and Salicylic Acid, ABA is a "batten down the hatches"
hormone that is like adrenaline and quickly potentiates a plants
response to rapidly developing environmental emergencies of all
kinds. Possibly it does not do anything on its own but greatly
magnifies any reaction a plant is having to a threat. Salicylic Acid
on the other hand, would be the "stand down" hormone to bring the
plant back to normal operations. I realize that ABA is famously known
for closing guard cells and Salicylic Acid is found in Willow Bark, a
tree more than any other in need of having open guard cells to pump
out the excess water that occurs at the roots of the willow due to
it's habitat of living on river banks. However, ABA is known to be
induced by heat shock, salt shock, insect damage etc. Maybe all of
these have a common denominator of water loss but my thinking is that
it destroys the symmetry of the theory to say that it is actually
involved per say in a nutrient issue rather than more primarily as an
indicator of any kind of shock to a plant. Also there have been
failures in the past to securely tie it to all desiccation events.