Download The mystery of the evolution of stomata

Interviews with scientists:
The mystery of the evolution of stomata
Teaching Notes
Introduction and context
We take stomata for granted – they’re on every leaf around us, and without them the world
wouldn’t breathe. They don’t just respond to daylight and atmospheric humidity: new research
shows that plants slam their stomata shut against invading bacteria.
But when and how did this complex ability evolve? The fossil record leaves us with a fascinating
puzzle. In this 5 minute talk, Professor Alistair Hetherington of Bristol University discusses the
mysterious evolution of stomata.
Teacher summary
http://intobiology.org.uk/the-mystery-of-the-evolution-of-stomata/
Stomata can be thought of as valves. They are present on the surfaces of leaves and they open
and close. This is important, as when they are open, the plant can take in carbon dioxide for
photosynthesis.
Stomata also control the loss of water vapour through transpiration. The transpiration stream is
responsible for pulling water and mineral nutrients form the soil to the aerial parts of the plant.
The plant can control the size of the stomatal aperture, and under drought conditions, the
aperture of the stomatal pore will reduce, allowing the plant to conserve its existing water.
Stomata are present in mosses and in the more evolutionarily advanced species such as ferns
and flowering plants. However, stomata are absent in the less evolutionarily advanced liverworts.
Fossils provide a useful model when trying to understand the evolution of stomata. The earliest
fossil stomata date from about 420 million years ago from a species called Cooksonia that
became extinct about twenty million years after that. Cooksonia is a very small plant, and in terms
of evolution it sits between the mosses and a group of plants that exist today called the
lycophytes. When examined, the stomata observed in the Cooksonia fossils look superficially,
morphologically very much like the stomata observed in modern-day species. Stomata appear
very suddenly in the fossil record. How did they evolve? Professor Hetherington is excited to be
trying to solve this mystery.
Science & Plants for Schools: www.saps.org.uk
Interviews with scientists – wheat genome and yield: p. 1
Questions
1. Outline the mechanism of stomatal opening and closing.
From: http://www.saps.org.uk/saps-associates/browse-q-and-a/1101
Blue-light wavelengths of daylight, detected by zeaxanthin (a carotenoid) activate proton
pumps in the guard cell membranes, which proceed to extrude protons from the
cytoplasm of the cell; this creates a "proton motive force" (an electrochemical gradient
across the membrane) which opens voltage operated channels in the membrane,
allowing positive K ions to flow passively into the cell, from the surrounding tissues.
Chloride ions also enter the cell, with their movement coupled to the re-entry of some of
the extruded protons (Cl/H symport) to act as counter-ions to the potassium. Water
passively follows these ions into the guard cells, and as their turgidity increases so the
stomatal pore opens, in the morning. As the day progresses the osmotic role of
potassium is supplanted by that of sucrose, which can be generated by several means,
including starch hydrolysis and photosynthesis. At the end of the day (by which time the
potassium accumulation has dissipated) it seems it is the fall in the concentration of
sucrose that initiates the loss of water and reduced turgor pressure, which causes
closure of the stomatal pore.
ABA also seems to trigger a loss of K ions from guard cells. Some workers suggest that
in some species, ABA alters turgor pressure without changing solute potential or water
potential.
There is evidence of a role for increased cytoplasmic calcium (Ca2+) in closure, possibly
by effects on opening/closing of ion channels at the plasma membrane.
Starch break down to phosphoenol pyruvate (PEP) is stimulated by blue light. This PEP
then combines with CO2 to from oxaloacetic acid, which is converted to malic acid. It is H
ions from the malic acid which leave the cell in the mechanism outlined above. Thus the
intake of K ions is matched by formation of anions from malic acid in the guard cells. This
causes an increase in osmotically active substances in exchange for the breakdown of
starch in guard cells.
2. Explain how the transpiration stream pulls water and minerals from the soil to the aerial
parts of the leaf.
Heat from the sun causes water to evaporate from the mesophyll cells and to diffuse out
of the leaves down a water potential gradient, via the stomata. This decreases the water
potential of leaf cells, so water diffuses out of the xylem vessels into the leaves,
decreasing the pressure in the xylem, so water is drawn (sucked) up the xylem by mass
flow as a result of cohesion-tension. This reduces the water potential in the xylem vessels
in the root, so water diffuses from the root cells, into the xylem vessels. The reduced
water potential of the root epidermal cells means that water diffuses into the root hair
cells by osmosis from the surrounding soil.
3. Explain the importance of stomata to terrestrial plants.
They allow terrestrial plants to conserve their existing water while regulating the uptake of
carbon dioxide for photosynthesis.
4. Liverworts do not have stomata. How do their ‘leaves’ exchange gases with the
environment?
Through tiny, static pores in the surface of the ‘leaf’ (thallus).
5. Draw out a phylogenetic tree for the following plant groups:
angiosperms, ferns, gymnosperms, hornworts, liverworts, lycophytes, mosses,
This web link may be useful http://intobiology.org.uk/stomata-key-elements-essential-forthe-succes-of-the-vascular-plants/ (39 minutes into the lecture)
liverworts, mosses, hornworts, lycophytes, ferns, gymnosperms, angiosperms.
6. How old are the earliest known fossil stomata?
About 420 million years old.
Science & Plants for Schools: www.saps.org.uk
Interviews with scientists – wheat genome and yield: p. 2
7. To which geological periods do Cooksonia fossils belong?
Silurian and Devonian.
8. Between which groups on the phylogenetic tree of plants does Cooksonia sit?
Mosses and lycophytes
9. What is known about the concentration of carbon dioxide in the atmosphere during the
Silurian period?
It is thought to have been higher than today’s levels.
10. What is the evidence from the fossil record to support the hypothesis that stomata arose
only once during evolution?
When examined, the stomata observed in the Cooksonia fossils look superficially,
morphologically very much like the stomata observed in modern-day species. Stomata
appear very suddenly in the fossil record.
Science & Plants for Schools: www.saps.org.uk
Interviews with scientists – wheat genome and yield: p. 3