A diversity of scales

This was the sixth conference in a series first initiated in 1979 focusing on the biology of stomata; in this case emphasizing the role of transpiration. This meeting showcased many of the recent advances in the understanding and measurement of transpiration at diverse scales from the gene to the globe. An impressive array of techniques were clearly described and independently employed to identify those fundamental processes that control stomatal development, activity and transpiration at these scales of activity. ‘stomata are the most significant biological regulatory system on the planet’ Over recent years, tremendous advances have been made in our understanding of the factors controlling guard cell development. The use of genetic techniques has led to the characterization of several negative regulators of stomatal development, but whether these act in a single pathway remains unclear (Bergmann, 2006). Two speakers described various components of the pathway which involves a succession of related transcription factors necessary for the division and differentiation of stomatal cells. The role of the putative transcription factor FOUR LIPS (FLP) (Lai et al., 2005) in controlling the division of the guard mother cell was explored by Fred Sack (University of British Columbia, Vancouver, Canada). From comparison of the transcriptional profiles of YODA mutants (which have mutations in the gene encoding YODA, a mitogen-activated protein kinase kinase kinase) with altered numbers of stomata, Dominique Bergmann (Stanford University, CA, USA) described the identification of FAMA, another putative transcription factor expressed specifically in cells of the stomatal lineage (Bergmann et al., 2004). Like FLP, FAMA also appears to control progression through the stomatal development pathway. fama-1 mutants have no recognizable stomata, whereas misexpression of FAMA leads to an epidermis consisting only of guard cells with a striking ‘fish-scale’ appearance (Ohashi-Ito & Bergmann, 2006). The importance of understanding the factors regulating stomatal development was underlined by Josette Masle (Australian National University, Canberra, Australia), who reported the first identification of a gene regulating transpiration efficiency from a screen using carbon isotopic discrimination. The identification of ERECTA, a putative receptor-like protein kinase involved in controlling stomatal density and other traits, raises the exciting possibility of designing strategies for improving water use efficiency or yield potential in crops (Masle et al., 2005). Many factors impact on the control of stomatal aperture and it is now accepted that this process is controlled by a web of interacting biochemical signalling pathways. The complex nature of these interactions has led to the interpretation of guard cell signalling pathways as a network (Hetherington, 2001). Reka Albert (Pennsylvania State University, USA) presented a dynamic mathematical model, incorporating over 40 components, exploring the topology of the abscisic acid (ABA)-induced stomatal closure network. This interpretation allows for complimentary overlap in the event of a partial systems failure (Li et al., 2006), and aims to suggest candidates for manipulation to improve drought tolerance. A number of speakers focused on the characterization of individual signalling components that contribute to the regulation of stomatal aperture. The role of guard cell cytosolic calcium elevations in stomatal aperture regulation was revisited by Rainer Hedrich (Julius-von-Sachs-Institüt fur Biophysik, Würzberg, Germany) and Julian Schroeder (University of California, San Diego, USA). From in vivo imaging studies using calcium-sensitive chameleon indicators, Schroeder proposed a new model to explain how elevations in guard cell calcium ion concentration could bring about differing aperture responses. In this model it is proposed that calcium sensors are primed (turned on) or deprimed (turned off) by previous stimuli (Israelsson et al., 2006). Using a novel method to simultaneously measure cytosolic calcium concentrations and anion channel activation in guard cells of intact plants, Hedrich presented results suggesting that, in some species, ABA is able to activate ion channels by either calcium-dependent or calcium-independent routes (Marten et al., 2007). Both Alistair Hetherington (University of Bristol, UK) and Sally Assmann (Penn State University, Pennsylvania, USA) discussed the role of sphingolipids in calcium release (Ng et al., 2001), and their interaction with G-proteins during ABA-induced reductions in stomatal aperture (Coursol et al., 2003). Mike Blatt (University of Glasgow, UK) presented evidence indicating that plasma membrane vesicle trafficking and anchoring of K+ ion channels within complexes is important in ABA responses (Sutter et al., 2006). Through the identification of mutants with defects in ozone-induced stomatal closure, Triin Kollist (University of Helsinki, Finland) has identified a novel component in aperture control. Radical-induced cell death 3 (RCD3) is a putative chloroplast membrane-associated protein specific to guard cells, reviving the question as to why guard cells are green. This question was also considered by Susanne von Caemmerer (Australian National University, Canberra, Australia), who presented her recent investigations into stomatal behaviour in photosynthetic mutants, indicating that plants with impaired photosynthesis do not differ from plants with normal photosynthesis in their control of stomatal aperture (von Caemmerer et al., 2004). In contrast, Julie Gray (University of Sheffield, UK) presented experiments suggesting that synthesis and metabolism of malate play a role in stomatal opening and closing, respectively. Working with blue light photoreceptor mutants, Ken-ichiro Shimazaki (Kyushu University, Fukuoka, Japan) discussed the evidence for the activity of a protein phosphatase that may mediate signalling between phototropins and activation of the plasma membrane H+-ATPase which drives stomatal opening (Takemiya et al., 2006). The same group, intriguingly, showed that the fern Adiantum capillus-veneris lacks the stomatal blue light response although it possesses functional phototropins and H+-ATPase (Doi et al., 2006). Research at the plant level remains largely physiological, with mechanical considerations still dominating the integration of transpiration throughout the whole plant. A synthetic physical model of leaf transpiration, designed to mimic and allow for accurate measurement of hydration and equilibration kinetics, was presented by N. Michelle Holbrook (Harvard University, MA, USA; Zwieniecki et al., 2004). This model suggests two spatially and temporally separate reservoirs, each fulfilling a different function within a leaf, but connecting the vascular system to other functional tissues. Use of this unique approach has provided a novel basis for defining and measuring the physiologically relevant components of leaf water status and, although still under development, the model illustrates the continued need for research in fundamental physical and mechanical (hydraulic) aspects of plant function. Long-distance drought-signalling pathways were described at the whole-plant scale by Bill Davies (University of Lancaster, UK) in terms of interactions among ABA, ethylene and pH status modifying transpirational loss (Sobeih et al., 2004). At the root interface, significant progress was reported by Christophe Maurel (CRNS/INRA Montpellier, France) and Francois Chaumont (Université Catholique de Louvain, Belgium) in elucidating the role, precise positioning and physiological significance of aquaporins, characterized in plants little more than a decade ago. The structural gating mechanism of action has been explained (Tornroth-Horsefield et al., 2006), and these membrane-bound water channels are now functionally known to have a regulatory role in water uptake, with down-regulation of uptake under environmental stress. Several classes of aquaporins, known as isoforms, are known to exist in roots of Arabidopsis thaliana and links to other well-known signal pathways were confirmed under stress treatments involving reactive oxygen species (ROS) and pH regulation (Boursiac et al., 2005). A genomic approach was presented as part of a long-term study of changes in poplar trees (Populus×euramericana) grown under increased CO2 concentration in a Free-Air CO2 Enrichment (FACE) facility by Gail Taylor (University of Southampton, UK). Several changes in leaf morphology and physiology were observed which impacted on whole-plant water use. Leaf-level conductance was reduced, but whole-tree transpiration rates increased, as a result of increased leaf area and changes in stomatal numbers (Taylor et al., 2003). Crossing of generations grown in treatment over a period of years allowed the use of quantitative trait loci (QTL) analyses to map the location of several candidate genes potentially important in water use changes under high CO2 concentrations (including an ERECTA homologue; see previous section). Global warming and climate-change effects on vegetation have stimulated the need to understand how environmentally induced transpirational regulation affects large-scale systems, whether agricultural or natural. This requirement has ushered in new techniques for quantification of canopy-wide measurements. Russell Monson (University of Colorado, Boulder, USA) described the use of national and international networks of flux towers, which provide on-going observations for eddy-covariance and model-data fusion analyses, allowing ecosystem evapotranspiration (ET) rates to be separated into component fluxes, for example soil and tree ET (Law et al., 2002; Wilson et al., 2003). Despite Joseph Berry's (Carnegie Institution of Washington, USA) eloquent description of stomata as ‘the most significant biological regulatory system on the planet’, many complexities and uncertainties associated specifically with larger scale environments were highlighted as constraints to the incorporation of transpiration into vegetation models. One of these, rates of night-time respiration, was presented by Maggie Caird (University of California, Davis, USA) as being up to 30% of daytime rates, with little or no knowledge of the cost to either the individual plant or the system as a whole. Furthermore, the case was made by Carl Bernacchi (Illinois State Water Survey, USA) that a lack of understanding of plant processes at this level could have large implications for regional climates undergoing climate change. The extent to which we understand these processes will affect how we predict such changes in the future, illustrating the need for increasingly sophisticated techniques of measurement. Remote sensing continues to progress in the measurement of field-scale canopy stomatal conductance. However, as highlighted by Hamlyn Jones (University of Dundee, UK), calibration methods remain limited in their ability to cope with the variation in environmental conditions encountered in the field; for example, using the energy balance equation, the difference between a wet and dry canopy can produce as much as 25°C difference in calculated temperatures. The global aspects of transpiration and stomatal impacts were presented by Jenny McElwain (University College Dublin, Ireland). Stomatal numbers change with atmospheric CO2 concentration, which provides the potential to use fossil stomatal numbers as proxy estimators of the CO2 content of the atmosphere (McElwain, 2004). Going further, it was also suggested that, as far back as the Carboniferous period (some 300 million years ago), changes in stomatal numbers have been instrumental in driving the evolution of vessel architecture – a ‘pull’ vs ‘push’ hypothesis. Current and future vegetation dynamics were presented by Ian Woodward (University of Sheffield, UK) using largely dynamic vegetation models, to aid understanding of the nature of global changes in vegetation structure as affected by long-term successional processes, perturbations of climate, such as El Nino events, and historical changes in atmospheric CO2 concentrations. Model projections up to the year 2100 indicated that a warmer world will have significantly reduced tropical forest cover, even combined with increases in CO2 that reduce transpiration and increase primary productivity. The stimulation of productivity by CO2 is further predicted to increase the probability of fire, reducing mature forest cover and opening the forest to a savannah-like structure. These advances in model development are strongly reliant on advances at all smaller scales that link regional effects to global consequences. The conference was brought to a close by Graham Farquhar (Australian National University, Canberra, Australia), who described a technique using isotopically heavy water to detect genetic and environmental effects on transpiration, with experiments showing that transpiration consists of two fluxes, leaf-to-air and air-to-leaf. Isotopically distinct material has been obtained by growth of maize (Zea mays) at different humidities, and thus different transpiration rates (Gan et al., 2003). Such material can be used to monitor the effects of climate change on a scale from single leaves to forests, and has great potential for aiding our understanding of how climate change affects global transpiration. It is clear that much progress towards understanding plant transpiration has been achieved, not only at the cellular level but, through independent disciplines, on a full range of scales, both temporal and spatial. There is still a requirement to fully integrate findings – the coupling of each scale to the next will be an interdisciplinary challenge, but is necessary if we are to understand the entire transpiration system, from guard cell to globe.

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