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Plant Neurobiology
Professor Van Volkenburgh has a special interest in plant behavior (which in brief she defines as
“development based on physiological sensing and responding”
and in plant neurobiology (“how plants
process the information they obtain from their environment to develop, prosper and reproduce
optimally”
, some aspects of which are not covered in the textbook. The following derives from her
lectures except as noted.
Plants synthesize various animal neurotransmitters and neuroactive compounds (such as caffeine,
glutamate, GABA, ACH, serotonin, L-DOPA, dopamine, and melatonin). Are the glutamate receptors in
plants used in auxin signaling? Many of the neuroactive compounds made by plants may be defensive and
part of their “chemical ecology”.
Plants exhibit two types of electrical signaling. The membrane potential is held at about -180 mV. It was
previously thought that plants had no significant electrical activity or electrical excitability.
• Action Potential: This requires a threshold depolarization before it fires following a stimulus (such
as light, touch, wound, insect bite or saliva, etc.), and leads to an all-or-nothing response (as with
animal neural action potentials). This phenomenon and the accompanying fluxes have been mostly
but still incompletely studied in the giant Charophyta green alga Chara. It propagates at a constant
velocity, and is completed over a duration of minutes. (In contrast, animal action potentials
propagate typically over a fraction of a second.) The propagation may occur in the phloem, perhaps in
the parenchymal cells, crossing plasmodesmata, etc., to reach the leaves (details have not been fully
worked out). After the initial stimulus, the action potential consists of an initial positive rise in Em
resulting from Cl- outflux and Ca++ influx. (Unlike in animals, sodium ion flux plays no role.) After the
positive peak is reached, the Em rapidly falls to more negative than the baseline negative value, as H+
and K+ outflux. The Em then returns to baseline with influx of K+. (some details missing)
• Slow Wave Potentials (also called by some Variation Potentials). These are generated by an increase
in pressure at the site of origin, but are different from action potentials. Mechanoreceptors detect a
touch, wound, or insect bite perturbation with resulting pressure changes (including changes in the
usual negative xylem pressure). Ion channels open and a change in membrane potential Em propagates
slowly from the point of injury or stimulation to the remainder of the shoot over a period of 10 minutes
to one hour. The slow wave potential change propagates in the xylem to the cortex and epidermis,
and it is the electrical changes in the outer cells that are observed. Unlike action potentials, slow wave
potentials do not depend on a threshold to be initiated, and exhibit decreasing amplitude and
decreasing speed of propagation with distance from the site of initiation.
In plants, auxin may serve as a type of neurotransmitter, as with polar auxin transport PAT,
in which an action potential-like event results from IAA secretion...
Examples of plants responding to their environment include:
•
Phototropism and Sun Tracking
•
Venus flytrap: The trap has sensitive hairs which when sufficiently stimulated trigger a depolarization
of membrane potential, causing the cells of the hinge to lose turgor on top and gain turgor and elongate
on the bottom (from influx of K+), resulting in closure of the hinge...
•
Flowering in plants resulting from PHY detection of light, FT Protein transmitted in phloem, etc.
•
Tomato Wound Response: Insect bites lead to propagation of an action potential, along with
synthesis of systemin (a peptide hormone) in wounded phloem parenchyma cells. (The systemin
pathway leads to synthesis of
jasmonic acid, which propagates in the phloem.)
•
Hypersensitivity responses: including local necrosis.
•
Chitinases: these are directed against insects and fungi.
•
Phytoalexins: plant antibiotics, including terpenoids, glycosteroids and alkaloids.
( cryptochromes are found on retinal tissue, of higher animal's eyes ...Including humans...
Once we were plants...
There are
many "indications",about that......)
