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ScienceWeek
PLANT BIOLOGY: CALCIUM AND PLANT SIGNALING
The following points are made by Colin Brownlee (Current Biology 2003 13:R923):
1) Plant cells are surrounded by cell walls that present an aqueous continuum, the apoplast, through which hormones and other potential regulatory factors can pass [1]. In plants, a function for apoplastic Ca2+ as an extracellular messenger has been indicated from studies with stomatal guard cells, though components of the pathway that may link changes in extracellular Ca2+ with specific changes in intracellular Ca2+ have until now been elusive. by Han et al.[2] recently described a novel cell surface receptor for apoplastic Ca2+ and firmly placed Ca2+ on the list of extracellular "first messengers" in plants.
2) Pairs of guard cells in the leaf epidermis form pores through which water and gas exchange occurs. The pore size can be regulated by factors in the apoplast that alter the turgor pressure inside the guard cells, including the plant hormone abscisic acid (ABA), CO2, auxin and elevated extracellular Ca2+[3,4]. Cytosolic Ca2+ is an important component of the signaling network that regulates stomatal aperture [4]. Increases in apoplastic Ca2+ have been shown to bring about increases in cytosolic Ca2+ in guard cells in isolated epidermal strips from Commelina communis leaves [5]. Increasing extracellular Ca2+ from 0.01mM to higher than 0.1mM was found to lead to repetitive elevations in cytosolic Ca2+ which had a period of several minutes.
3) The pattern of these oscillatory increases in cytosolic Ca2+ varies with the external Ca2+ concentration. Their significance has been the source of much speculation. It is possible that they encode stimulus-specific information in much the same way as cytosolic Ca2+ oscillations in animal cells. Alternatively, increased external Ca2+ may simply promote Ca2+ influx and consequent elevation of cytosolic Ca2+. Delays in the activation of efflux mechanisms could give rise to oscillatory elevations of cytosolic Ca2+ that reflect disturbance of the cytosolic Ca2+ homeostatic machinery.
4) Several lines of evidence suggest, however, that these repetitive Ca2+ increases do indeed encode specific information. For example, similar repetitive Ca2+ increases occur in response to ABA, and different patterns of Ca2+ oscillations can give rise to different stomatal closure responses. Arabidopsis det-3 mutants are defective in the ability to generate oscillatory Ca2+ increases in response to elevated external Ca2+ and also in the stomatal closure response to elevated external Ca2+, though interestingly they showed normal responses to ABA. Another Arabidopsis mutant that showed altered patterns of Ca2+ oscillations in response to elevated external Ca2+ and to ABA, gca2, was also found to be defective in the stomatal closure response to elevated external Ca2+. This work strongly suggested that specific information determining stomatal aperture may be encoded in the frequency of cytosolic Ca2+ elevations in guard cells.
References (abridged):
1 Roelfsema, M.R.G. and Hedrich, R. (2002). Studying guard cells in the intact plant: modulation of stomatal movements by apoplastic factors. New Phytol. 153, 425-431
2 Han, S., Tang, R., Anderson, L.K., Woerner, T.E., and Pei, Z.-M. (2003). A cell surface receptor mediates extracellular Ca2+ sensing in guard cells. Nature 425, 196-200
3 Hetherington, A.M. and Woodward, F.I. (2003). The role of stomata in sensing and driving environmental change. Nature 424, 901-908
4 Hetherington, A.M. (2001). Guard cell signaling. Cell 107, 711-714
5 McAinsh, M.R., Webb, A.A.R., Taylor, J.E., and Hetherington, A.M. (1995). Stimulus-induced oscillations in guard cell cytosolic free calcium. Plant Cell 7, 1207-1219
Current Biology http://www.current-biology.com
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ECOLOGY: PLANT STOMATA AND ENVIRONMENTAL CHANGE
The following points are made by A.M. Hetherington and F.I. Woodward (Nature 2003 424:901):
1) Stomata are small pores on the surfaces of leaves and stems, bounded by a pair of guard cells, the pores controlling the exchange of gases -- most importantly water vapour and CO2 --between the interior of the leaf and the atmosphere. In this capacity the stomata make major contributions to the ability of the plant to control its water relations and to gain carbon.
2) Gas exchange is regulated by controlling the aperture of the stomatal pore and the number of stomata that form on the epidermis. Environmental signals such as light intensity, the concentration of atmospheric carbon dioxide, and endogenous plant hormones control stomatal aperture and development. The acquisition of stomata and an impervious leaf cuticle are considered to be key elements in the evolution of advanced terrestrial plants(1), allowing the plant to inhabit a range of different, often fluctuating environments but still control water content.
3) The application of knowledge from cognate disciplines is providing new insights into how stomata evolve and are able to process information from simultaneous, often conflicting and sometimes rapidly changing signals. Although it is too early to say whether these recent advances will result in paradigm shifts in our understanding of how plants both respond to and drive environmental change, it is quite clear that stomata are a key experimental tool to investigate these phenomena.
4) Although the total stomatal pore area may be only 5% of a leaf surface(2), the rate of water vapour loss may reach as high as 70% of a similar structure without a cuticle. Stomata exert major controls on both the water and carbon cycles of the world. Annual precipitation over the land is about 110,000 km^(3), or 110 x 10^(15) kg (3), and evaporation and transpiration total about 70 x 10^(15) kg. The contribution of stomatal transpiration alone to the global water cycle can be determined by using a dynamic vegetation model(4). The greatest rates of transpiration occur in the uniform and warm forested areas between the tropics with 32 x 10^(15) kg/yr of water vapour passing through stomata. This is double the water vapour content of the atmosphere (15 x 10^(15) kg/yr). Terrestrial gross photosynthesis annually fixes about 120 x 10^(15) gC (440 x 10^(15) gCO2) from the atmosphere's 730 x 10^(15) gC (5). The global distribution of this flux parallels the distribution of transpiration, indicating the closely coupled controls of stomata on CO2 and water vapour diffusion.
5) In summary: Stomata, the small pores on the surfaces of leaves and stalks, regulate the flow of gases in and out of leaves and thus plants as a whole. They adapt to local and global changes on all time-scales from minutes to millennia. Recent data from diverse fields are establishing their central importance to plant physiology, evolution, and global ecology. Stomatal morphology, distribution, and behavior respond to a spectrum of signals, from intracellular signaling to global climatic change. Such concerted adaptation results from a web of control systems reminiscent of a scale-free network whose untangling requires integrated approaches beyond those currently used.
References (abridged):
1. Raven, J. Selection pressures on stomatal evolution. New Phytol. 153, 371-386 (2002)
2. Willmer, C. & Fricker, M. Stomata 2nd edn (Chapman & Hall, London, 1996)
3. Jackson, R. B. et al. Water in a changing world. Ecol. Appl. 11, 1027-1045 (2001)
4. Cramer, W. et al. Global response of terrestrial ecosystem structure and function to CO2 and climate change: results from six dynamic global vegetation models. Global Change Biol. 7, 357-373 (2001)
5. Ciais, P. et al. A three-dimensional synthesis study of O18 in atmospheric CO2. I. Surface fluxes. J. Geophys. Res. Atmos. 102, 5857-5872 (1997)
Nature http://www.nature.com/nature
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ON PLANT CELL WALLS
The following points are made by Herman Hoefte (Science 2001 294:795):
1) More than 300 years ago, Robert Hooke (1635-1703) pointed his primitive microscope at a slice of cork and discovered the cellular basis of organisms. Unfortunately, since then, plant cell walls, which formed the compartments he actually observed, have never been considered particularly entertaining structures. Indeed, the word "wall" itself evokes something dull and rigid, built only to enclose, support, divide, and protect. However, a closer look reveals just how erroneous this view is. Walls of growing plant cells are extremely sophisticated composite materials made of dynamic networks of polysaccharides, protein, and phenolic compounds. Cellulose microfibrils with a tensile strength comparable to that of steel provide the plant with a load-bearing framework. These microfibrils are rigid wires made of crystalline arrays of beta-1,4-linked chains of glucose residues, which are extruded from little hexameric spinnerets in the plant cell plasma membrane and which surround the growing cell like the hoops around a barrel.
2) Because cellulose microfibrils constrain turgor-driven cell expansion in one preferential direction, they control the shape of plant cells and ultimately the shapes of the plants themselves. Hemicelluloses, such as xyloglucans, are tethered by hydrogen bonds to cellulose and form cross-links that may control the separation of the cellulose microfibril hoops. The cellulose-hemicellulose network is embedded in a matrix of complex galacturonic acid-rich pectic polysaccharides that form a hydrated gel inside the wall, providing a dynamic operating environment for cell wall processes.
3) In all higher plants, one of the apparently essential polysaccharides in this hydrated gel is a mysterious polysaccharide known as rhamnogalacturonan II (RGII), which is believed to be the most complex polysaccharide on Earth. It is composed of 11 kinds of sugar monomers, and apparently at least 21 enzymes are dedicated to the construction of all the linkages between the sugar residues.
Science http://www.sciencemag.org
ScienceWeek http://scienceweek.com
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