Sally Wilkinson

Drought, ozone, ABA and ethylene: new insights from cell to plant to community.

Recent reports show ethylene-dependent reductions in stomatal sensitivity to abscisic acid (ABA) under ozone stress. These changes reduce stomatal control of plant water loss in drying soil. Here we review evidence that ABA and ethylene, and interactions between these two stress-induced hormones, control many of the responses of intact plants to drought and ozone stress, with emphasis on effects on stomata and shoot growth. We draw attention to convergent signalling and response pathways induced by ozone and drought that can increase production of hydrogen peroxide (H(2)O(2)) and nitric oxide (NO). Stomatal responses to a wider range of stresses and developmental cues may also be controlled via the same sets of signalling pathways. Other hormones, or effectors such as xylem/apoplastic pH or changes in plant water status, also play a role in signalling within and between organs. We discuss the implications, for crops, natural ecosystems and water catchment processes, of ethylenes antagonism of the stomatal response to ABA, against a back-drop of predictions for reduced precipitation and increasing ozone pollution, as part of global climate change and increasing urbanization and industrial development.

Xylem Sap pH Increase: A Drought Signal Received at the Apoplastic Face of the Guard Cell That Involves the Suppression of Saturable Abscisic Acid Uptake by the Epidermal Symplast

Drought increased the pH of Commelina communis xylem sap from 6.1 to 6.7. Conductances of transpiring leaves were 50% lower in pH 7.0 than in pH 6.0 buffers, but bulk leaf abscisic acid (ABA) concentration and shoot water status were unaffected by pH. Stomatal apertures of isolated abaxial epidermis incubated on simple buffers increased with external pH, so in vivo this must be overridden by alternative pH effects. Reductions in leaf transpiration rate at pH 7.0 were dependent on the presence of 10–8 mol dm-3 ABA in the xylem stream. We inferred that at pH 7.0 leaf apoplastic ABA concentrations increased: pH did not affect distributions of ABA among leaf tissues, but isolated epidermis and mesophyll tissue took up more 3H-ABA from pH 6.0 than from pH 7.0 buffers. The apoplastic ABA increase at pH 7.0 may result from reduced symplastic sequestration. A portion of 3H-ABA uptake by the epidermis was saturable at pH 6.0 but not at pH 7.0. An ABA uptake carrier may contribute to ABA sequestration by the leaf symplast of well-watered plants, and its inactivity at pH 7.0 may favor apoplastic ABA accumulation in draughted plants. Effects of external pH on stomatal apertures in the isolated epidermis indicate that published data supporting a role for internal guard cell ABA receptors should be reassessed.

PH as a stress signal.

The pH of the xylem sap of plants experiencing a range of environmental conditions can increase by over a whole pH unit. This results in an increased ABA concentration in the apoplast adjacent to the stomatal guard cells in the leaf epidermis, by reducing the ability of the mesophyll and epidermal symplast to sequester ABA away from this compartment. As a result the guard cell ABA receptors become activated and the stomata close, enabling the plant to retain water. Were it not for the low concentration of ABA ubiquitous to all land plants, the increase in the pH of the apoplast adjacent to the guard cell would induce stomatal widening, and cause excessive water loss. Not only does ABA prevent this potentially harmful phenomenon, but it also converts the pH increase to a signal which can bring about plant protection.

How is ozone pollution reducing our food supply

Ground-level ozone pollution is already decreasing global crop yields (from ∼2.2-5.5% for maize to 3.9-15% and 8.5-14% for wheat and soybean, respectively), to differing extents depending on genotype and environmental conditions, and this problem is predicted to escalate given climate change and increasing ozone precursor emissions in many areas. Here a summary is provided of how ozone pollution affects yield in a variety of crops, thus impacting global food security. Ozone causes visible injury symptoms to foliage; it induces early senescence and abscission of leaves; it can reduce stomatal aperture and thereby carbon uptake, and/or directly reduce photosynthetic carbon fixation; it can moderate biomass growth via carbon availability or more directly; it can decrease translocation of fixed carbon to edible plant parts (grains, fruits, pods, roots) due either to reduced availability at source, redirection to synthesis of chemical protectants, or reduced transport capabilities via phloem; decreased carbon transport to roots reduces nutrient and water uptake and affects anchorage; ozone can moderate or bring forward flowering and induce pollen sterility; it induces ovule and/or grain abortion; and finally it reduces the ability of some genotypes to withstand other stresses such as drought, high vapour pressure deficit, and high photon flux density via effects on stomatal control. This latter point is emphasized here, given predictions that atmospheric conditions conducive to drought formation that also give rise to intense precursor emission events will become more severe over the coming decades.

Plant hormone interactions: innovative targets for crop breeding and management

Here we highlight how both the root and shoot environment impact on whole plant hormone balance, particularly under stresses such as soil drying, and relate hormone ratios and relative abundances to processes influencing plant performance and yield under both mild and more severe stress. We discuss evidence (i) that abscisic acid (ABA) and ethylene act antagonistically on grain-filling rate amongst other yield-impacting processes; (ii) that ABAs effectiveness as an agent of stomatal closure can be modulated by coincident ethylene or cytokinin accumulation; and (iii) that enhanced cytokinin production can increase growth and yield by improving foliar stay-green indices under stress, and by improving processes that impact grain-filling and number, and that this can be the result of altered relative abundances of cytokinin and ABA (and other hormones). We describe evidence and novel processes whereby these phenomena are/could be amenable to manipulation through genetic and management routes, such that plant performance and yield can be improved. We explore the possibility that a range of ABA-ethylene and ABA-cytokinin relative abundances could represent targets for breeding/managing for yield resilience under a spectrum of stress levels between severe and mild, and could circumvent some of the pitfalls so far encountered in the massive research effort towards breeding for increases in the complex trait of yield.

Ozone suppresses soil drying‐ and abscisic acid (ABA)‐induced stomatal closure via an ethylene‐dependent mechanism

Elevated atmospheric ozone concentrations (70 ppb) reduced the sensitivity of stomatal closure to abscisic acid (ABA) in Leontodon hispidus after at least 24 h exposure (1) when detached leaves were fed ABA, and (2) when intact plants were sprayed or injected with ABA. They also reduced the sensitivity of stomatal closure to soil drying around the roots. Such effects could already be occurring under current northern hemisphere peak ambient ozone concentrations. Leaves detached from plants which had been exposed to elevated ozone concentrations generated higher concentrations of ethylene, although leaf tissue ABA concentrations were unaffected. When intact plants were pretreated with the ethylene receptor binding antagonist 1-methylcyclopropene, the stomatal response to both applied ABA and soil drying was fully restored in the presence of elevated ozone. Implications of ethylenes antagonism of the stomatal response to ABA under oxidative stress are discussed. We suggest that this may be one mechanism whereby elevated ozone induces visible injury in sensitive species. We emphasize that drought linked to climate change and tropospheric ozone pollution, are both escalating problems. Ozone will exacerbate the deleterious effects of drought on the many plant species including valuable crops that respond to this pollutant by emitting more ethylene.

Manipulation of the apoplastic pH of intact plants mimics stomatal and growth responses to water availability and microclimatic variation

The apoplastic pH of intact Forsythiaxintermedia (cv. Lynwood) and tomato (Solanum lycopersicum) plants has been manipulated using buffered foliar sprays, and thereby stomatal conductance (g(s)), leaf growth rate, and plant water loss have been controlled. The more alkaline the pH of the foliar spray, the lower the g(s) and/or leaf growth rate subsequently measured. The most alkaline pH that was applied corresponds to that measured in sap extracted from shoots of tomato and Forsythia plants experiencing, respectively, soil drying or a relatively high photon flux density (PFD), vapour pressure deficit (VPD), and temperature in the leaf microclimate. The negative correlation between PFD/VPD/temperature and g(s) determined in well-watered Forsythia plants exposed to a naturally varying summer microclimate was eliminated by spraying the plants with relatively alkaline but not acidic buffers, providing evidence for a novel pH-based signalling mechanism linking the aerial microclimate with stomatal aperture. Increasing the pH of the foliar spray only reduced g(s) in plants of the abscisic acid (ABA)-deficient flacca mutant of tomato when ABA was simultaneously sprayed onto leaves or injected into stems. In well-watered Forsythia plants exposed to a naturally varying summer microclimate (variable PFD, VPD, and temperature), xylem pH and leaf ABA concentration fluctuated but were positively correlated. Manipulation of foliar apoplastic pH also affected the response of g(s) and leaf growth to ABA injected into stems of intact Forsythia plants. The techniques used here to control physiology and water use in intact growing plants could easily be applied in a horticultural context.

Ethylene limits abscisic acid- or soil drying-induced stomatal closure in aged wheat leaves

The mechanism of age-induced decreased stomatal sensitivity to abscisic acid (ABA) and soil drying has been explored here. Older, fully expanded leaves partly lost their ability to close stomata in response to foliar ABA sprays, and soil drying which stimulated endogenous ABA production, while young fully expanded leaves closed their stomata more fully. However, ABA- or soil drying-induced stomatal closure of older leaves was partly restored by pretreating plants with 1-methylcyclopropene (1-MCP), which can antagonize ethylene receptors, or by inoculating soil around the roots with the rhizobacterium Variovorax paradoxus 5C-2, which contains 1-aminocyclopropane-1-carboxylic acid (ACC)-deaminase. ACC (the immediate biosynthetic precursor of ethylene) sprays revealed higher sensitivity of stomata to ethylene in older leaves than younger leaves, despite no differences in endogenous ACC concentrations or ethylene emission. Taken together, these results indicate that the relative insensitivity of stomatal closure to ABA and soil drying in older leaves is likely due to altered stomatal sensitivity to ethylene, rather than ethylene production. To our knowledge, this is the first study to mechanistically explain diminished stomatal responses to soil moisture deficit in older leaves, and the associated reduction in leaf water-use efficiency.

Food production: reducing water consumption by manipulating long-distance chemical signalling in plants

One of the great challenges for the future is the production of sufficient food for a dramatically growing population during a period of climate change where water supplies are dwindling, and phytotoxic air pollutants such as ozone are increasing. By 2050 global grain demand is expected to double (Postel, 1998; Tilman et al., 2002; Schiermeier, 2008). Greater food production, however, will require larger areas of irrigated land unless the water use efficiency (WUE) of crop plants can be increased. To increase and sustain food production the following ecophysiological, genetic, and agronomic actions have been suggested (Postel, 1998; Tilman et al., 2002; Schiermeier, 2008).

Stomata are less responsive to environmental stimuli in high background ozone in Dactylis glomerata and Ranunculus acris

Two mesotrophic grassland species, Ranunculus acris and Dactylis glomerata were exposed to a range of ozone treatments (16.2-89.5 ppb 24 h mean) and two watering regimes under naturally fluctuating photosynthetically active radiation (PAR), vapour pressure deficit (VPD) and temperature. Stomatal conductance was measured throughout the experiments, and the combined data set (>1000 measurements) was analysed for effects of low and high ozone on responses to environmental stimuli. We show that when D. glomerata and R. acris were grown in 72.6-89.5 ppb ozone the stomata consistently lose the ability to respond, or have reduced response, to naturally fluctuating environmental conditions in comparison to their response in low ozone. The maximum stomatal conductance (g(max)) was also significantly higher in the high ozone treatment for D. glomerata. We discuss the hypotheses for the reduced sensitivity of stomatal closure to a changing environment and the associated implications for ozone flux modelling.