kurosh nikbin wrote:
> Dear Dr Travis,
> I came across a newsgroup called bionet.plants while researching my A-Level
> Biology coursework on the internet where I found your posting. The aim of
> my coursework task is to investigate the ?effect of aspirin on plant
> transpiration? which is why your comment describing aspirin as an
> ?anti-transpirant? caught my eye. I was wondering whether you could
> enlighten me as to why aspirin (acetylsalicylic acid) has this effect on
> the plant. Does it in some way induce potassium ions to move into the guard
> cells or is there some other explanation? Does aspirin work like ABA?
A long time ago, when I was doing my PhD at Lancaster University, I worked with
a visiting Mexican scientist Prof. Alfonso Larque-Saavedra from the Collegio de
Postgraduados at Chapingo in Mexico City. He was interested in the effects that
'aspirin' might have on ion-transport mechanisms in plants.
The fact that salicylic acid occurs naturally in Willow, for example, suggested
that it might be involved in some way in regulating plant water relations. This,
and the anecdotal evidence that putting aspirin in vases of cut flowers makes
them live longer, made us curious to see what effect aspirin had on stomata.
In fact, acetyl salicylic acid (ASA) is completely different from abscisic acid
(ABA). Some cynical people have suggested that ASA was confused with ABA because
the abbreviations are so similar! However, ASA is actually a potent antagonist
to prostaglandins, which have profound effects on ion transport, in animal
cells. We thought it might have evolved to regulate ion transport in plant
cells. This might be important in species with submerged roots, like willow.
However, several things mitigate against the 'prostaglandin' explanation. One is
that ion transport in animal cells is predominantly by means of sodium/potassium
'antiport' (in plant cells ion transport is predominantly by electrogenic proton
extrusion, with potassium ions diffusing passively into the cell down an
electrochemical gradient through selectively permeable pores in the cell
membrane). Another problem is that there are no prostaglandins in plants...
Nevertheless, lots of people think aspirin prolongs the life of cut flowers so
we decided to investigate. Indeed, a very large producer of aspirin offered to
fund our work, at one point, but we declined. In fact ASA (and ABA) act by
reducing potassium uptake into the guard cells. ABA makes stomatal guard cells
lose turgor, and the pore closes. Unfortunately, many compounds have a similar
effect because they are toxic to the guard cells, which lose turgor because the
ion-transport mechanism, or the cell membrane, is just damaged by the compound.
It is possible that aspirin has some other effect, but I doubt it myself. One
theory about the accumulation of salicylic acid in willow is that it is there as
a defence mechanism, to prevent fungal or bacterial infection, rather than
having anything to do with ion-transport directly. That it affects ion transport
may make it toxic to microscopic organisms. It is useful to us that it has an
analgesic effect (acetyl salicylic acid has fewer side effects than salicylic
acid which is why it's used instead of salicylic acid).
Other compounds found in plants have unexpected effects in animal cells. For
example, the 'phyto-oestrogens' mimic the effect of oestrogen. They 'fit' the
oestrogen receptors in mammalian cells, and mimic the effect of the hormone.
They also block the effect of the real hormone by occupying receptor sites.
It's unclear why evolution would favour plants that produce mammalian hormones!
The most likely explanation is that this is just a coincidence. However, the
hormonal effects are real, and may confer some selective advantage to a plant if
it alters the behaviour of herbivores in some way that is advantageous to the plant.
If you write to me, I'll send you some of my papers about about the stomatal
mechanism and work I did on the ion-transport mechanism in stomatal guard cells.
Good luck with your coursework.
Dr. A.J.Travis, | mailto:ajt at rri.sari.ac.uk
Rowett Research Institute, | http://www.rri.sari.ac.uk/~ajt
Greenburn Road, Bucksburn, | phone:+44 (0)1224 712751
Aberdeen AB2 9SB, Scotland, UK. | fax:+44 (0)1224 716687