Action potentials (was: Plant communication/sensing re)

Thomas Bjorkman Thomas_Bjorkman at cornell.edu
Wed Dec 23 14:11:18 EST 1992


In article <102994 at netnews.upenn.edu> Edwin Barkdoll,
barkdoll at cattell.psych.upenn.edu writes:
>So far we had simply discussed what ions are involved -- e.g. Na+ and
>K+ vs K+ and Cl- -- this is classic biophysics from the 1950's.  The
>use of patch clamping, while it has permmitted amazing things, has not
>led to a significant revision of the belief of which ions are involved
>in the action potential and _that_ is the issue (or part of the issue)
>which started the discussion: in certain plants like mimosa is "signal
>an action potential, just like in animal neurons"?  Do you know of any
>quantitative papers on the ions underlying the plant action potential?
>
 TB>>  The patch clamp has been used very
 TB>>heavily on plants: Hedrich and Neher put it to work on plant cells
right
 TB>>away.  
>
>	Yes, but did they put it to work on plant cells which generate
>action potentials as part of their normal behavior?
>
 TB>>So far there are several classes of channels that have been
 TB>>characterised.  The main one is the inward rectifying potassium
channel. 
 TB>>If I remeber right, it has a small sodium conductance, but that is
 TB>>irrelevant given the very low sodium concentrations.  
>
>	The discussion has been about action potentials in plants.
>Are these data recorded from AP producing plant membranes?

I brought up channels because the ion specificity of the channels that
generate the action potential pretty much determines which ions are
involved.  The patch clamp is the tool that answers that question most
directly.  Dainty, Hope and Walker had a pretty good picture of action
potentials in Chara by the early 1960's.  Bruce Scott makes an explicit
comparison between Chara and squid axons in his ca.1962 Scientific
Ameican article.  The abstact begins "Electrical disturbances similar to
the nerve impule are associated with a number of plant life processes." 
Did I miss a posting where someone actually suggested that Na was involed
in plant action potentials?  I don't see why one would expect that for
plants, where H takes many of the roles filled by Na in animals.

A lot of the work has been done on guard cells (e.g. Schroeder's '89
paper in J. Memb. Biol. studying the outward rectifier in Vicia).  This
is an excitable cell, but you are probably concerned about probagation of
the signal.  A better example might be from Mummert and Gradman (JMB
1991) who characterized the action potential in Acetabularia as being
based on K an Cl channels with a Cl pump.  The Cl pump rather than H pump
may be related to its habitat in brackish water.  

The question of which cells generate an action potential as part of the
normal behavior is a tough one.  Considering the variety of cells for
which action potentials have been shown, I wouldn't exclude too many a
priori.  Davies (Plant Cell Environ. 10: 623, 1987) runs through lots of
examples.  His synthesis of the ions involved comes down to a Ca influx
and K and Cl efflux. 

I don't know about recent ion stuff with mimosa, but there is a super
study on the Venus flytrap (Dionea).  Hodick and Sievers (Planta 174:8,
1988) were looking at the triggering mechanism of the rapidly-propagated
action potential in this excitable plant.  They concentrate on the role
of the Ca influx in triggering the action potential.  The gist is that
tweaking the trigger hair causes a sub-threshold Ca influx.  It takes two
of them in order to get an action potential.  Therefore, the leaf does
not fold very often unless there is really a fly on the leaf.

To return to the original issue:  Are there action potentials in plants
just like in animal neurons?  Clearly there are propagated action
potentials.  I think that Bill meant it in that sense.  The detailed
mechanism is clearly different--no Acetyl choline for starters.  The ions
are of course appropriate to the biological system.



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