Guard cells

Abstract

What are guard cells? A pair of guard cells surrounds each stoma on the leaf surface. Stomata are important because they regulate the uptake of CO2 from the atmosphere for photosynthesis and also the loss of water vapour from the plant during transpiration. The aperture of the stomatal pore is controlled by the two guard cells. When the guard cells are fully turgid the pore gapes open, whereas when they lose turgor the pore closes. Guard cells optimise leaf gas exchange in response to changing environmental conditions and their turgor is controlled by alterations in atmospheric CO2 concentration, light intensity, humidity and the drought hormone abscisic acid. Not to be confused with… Anything to do with the penal system or for that matter stigmata, although to the true believers guard cells are objects of considerable veneration. So are they a Johnny-come-lately arrival on the plant biology scene? Not at all. First described in the 17th century they were studied in the latter part of the 19th century and early 20th century by Francis Darwin and the great German plant physiologists of that era and have been subjected to intense investigation ever since. As a result we know quite a lot about how they work. So how do guard cells work? Changes in guard cell turgor are driven largely by fluxes of K+ ions. Underlying the fluxes is the co-ordinated activity of a battery of ion channels located at both the plasmalemma — the plant cell membrane — and tonoplast — the single membrane that bounds the vacuole in plant cells. The signal transduction pathways responsible for regulating these ion channels are being pieced together and there is good evidence that Ca2+, H+, cyclic ADP ribose, sphingosine-1-phosphate, inositol hexakisphosphate, phospholipases C and D and various protein kinases and phosphatases are all involved. Is it all just about the control of turgor? It certainly isn't. Guard cell development is also currently attracting great interest. Several guard cell development mutants have been isolated in Arabidopsis and two genes involved in the control of stomatal patterning — HIC and SDD1 — have recently been identified. HIC, for high carbon dioxide, encodes an enzyme involved in the synthesis of very-long-chain fatty acids, and is a negative regulator of stomatal development that responds to CO2. SDD1 encodes a subtilisin-like serine protease which may be involved in mediating a signal that controls the development of cell lineages that lead to guard cell formation. In addition, the observation that stomatal numbers on the leaf surface track atmospheric CO2 levels over the past 300 million years has prompted interest from the global climate change community. Don't say ‘You don't really work on holes in the leaf do you?’. Do say ‘I never knew I/V curves could be so interesting’. Where can I find out more?