PHO1 expression in guard cells mediates the stomatal response to abscisic acid in Arabidopsis.
ABSTRACT Stomatal opening and closing are driven by ion fluxes that cause changes in guard cell turgor and volume. This process is, in turn, regulated by environmental and hormonal signals, including light and the phytohormone abscisic acid (ABA). Here, we present genetic evidence that expression of PHO1 in guard cells of Arabidopsis thaliana is required for full stomatal responses to ABA. PHO1 is involved in the export of phosphate into the root xylem vessels and, as a result, the pho1 mutant is characterized by low shoot phosphate levels. In leaves, PHO1 was found expressed in guard cells and up-regulated following treatment with ABA. The pho1 mutant was unaffected in production of reactive oxygen species following ABA treatment, and in stomatal movements in response to light cues, high extracellular calcium, auxin, and fusicoccin. However, stomatal movements in response to ABA treatment were severely impaired, both in terms of induction of closure and inhibition of opening. Micro-grafting a pho1 shoot scion onto wild-type rootstock resulted in plants with normal shoot growth and phosphate content, but failed to restore normal stomatal response to ABA treatment. PHO1 knockdown using RNA interference specifically in guard cells of wild-type plants caused a reduced stomatal response to ABA. In agreement, specific expression of PHO1 in guard cells of pho1 plants complemented the mutant guard cell phenotype and re-established ABA sensitivity, although full functional complementation was dependent on shoot phosphate sufficiency. Together, these data reveal an important role for phosphate and the action of PHO1 in the stomatal response to ABA.
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ABSTRACT: During stress, plant cells activate anion channels and trigger the release of anions across the plasma membrane. Recently, two new gene families have been identified that encode major groups of anion channels. The SLAC/SLAH channels are characterized by slow voltage-dependent activation (S-type), whereas ALMT genes encode rapid-activating channels (R-type). Both S- and R-type channels are stimulated in guard cells by the stress hormone ABA, which leads to stomatal closure. Besides their role in ABA-dependent stomatal movement, anion channels are also activated by biotic stress factors such as microbe-associated molecular patterns (MAMPs). Given that anion channels occur throughout the plant kingdom, they are likely to serve a general function as master switches of stress responses.Trends in Plant Science 02/2012; 17(4):221-9. · 11.81 Impact Factor
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ABSTRACT: Arabidopsis thaliana PHO1 is primarily expressed in the root vascular cylinder and is involved in the transfer of inorganic phosphate (Pi) from roots to shoots. To analyze the role of PHO1 in transport of Pi, we have generated transgenic plants expressing PHO1 in ectopic A. thaliana tissues using an estradiol-inducible promoter. Leaves treated with estradiol showed strong PHO1 expression, leading to detectable accumulation of PHO1 protein. Estradiol-mediated induction of PHO1 in leaves from soil-grown plants, in leaves and roots of plants grown in liquid culture, or in leaf mesophyll protoplasts, was all accompanied by the specific release of Pi to the extracellular medium as early as 2-3 h after addition of estradiol. Net Pi export triggered by PHO1 induction was enhanced by high extracellular Pi and weakly inhibited by the proton-ionophore carbonyl cyanide m-chlorophenylhydrazone. Expression of a PHO1-GFP construct complementing the pho1 mutant revealed GFP expression in punctate structures in the pericycle cells but no fluorescence at the plasma membrane. When expressed in onion epidermal cells or in tobacco mesophyll cells, PHO1-GFP was associated with similar punctate structures that co-localized with the Golgi/trans-Golgi network and uncharacterized vesicles. However, PHO1-GFP could be partially relocated to the plasma membrane in leaves infiltrated with a high-phosphate solution. Together, these results show that PHO1 can trigger Pi export in ectopic plant cells, strongly indicating that PHO1 is itself a Pi exporter. Interestingly, PHO1-mediated Pi export was associated with its localization to the Golgi and trans-Golgi networks, revealing a role for these organelles in Pi transport.The Plant Journal 03/2012; 71(3):479-91. · 6.58 Impact Factor
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ABSTRACT: Stomatal complexes in the epidermes of aerial plant parts are critical sites for the regulation of gas ex- change between the plant and the atmosphere. Sto- mata consist of microscopic pores, each flanked by a pair of guard cells. Guard cells can increase or de- crease the size of the pore via changes in their turgor status, hence regulating both CO2 entry into the leaf and transpiration, or the loss of water from the leaf. This Update focuses on recent progress in our under- standing of the regulation of transpiration and drought tolerance that has been garnered through the use of Arabidopsis (Arabidopsis thaliana) as a model experimental system. The coordinated regulation of gas exchange is inte- gral to land plant survival because CO2 must be able to penetrate the leaf to allow photosynthesis, yet water loss (transpiration) must be minimized to prevent des- iccation, drought stress, and plant death. Transpiration also provides the driving force for the transport of water and nutrients from the roots to the aerial tissues, and the evaporation of water from the substomatal cavity cools the plant (Lambers et al., 1998). While a number of morphological traits can contribute to the overall level of leaf gas exchange (e.g. the density and distribution of stomata, leaf epidermal structure and internal organization, cuticle thickness), the regulation of stomatal aperture size is unique in that it is a dynamic and reversible process by which water loss and CO2 influx can be rapidly fine tuned in response to a number of environmental and intrinsic signals, such as light, CO2, and the plant stress hormone abscisic acid (ABA; Schroeder et al., 2001). Because guard cells integrate and respond to a plethora of signals, they have become a model cell type in the field of plant cell signaling (Blatt, 2000; Schroeder et al., 2001; Roelfsema and Hedrich, 2005). This Update highlights recent research reports on the guard cell physiology of Arabidopsis that include some quantitative measure of stomatal function. These measures include transpiration, stomatal conductance (stomatal conductance is defined as stomatal transpi- ration divided by the vapor pressure difference be- tween the leaf and the air, and increases with increasing stomatal aperture), leaf water status, and water-use efficiency/transpiration efficiency (the ratio of photosynthetic assimilation to transpiration). By focusing the article in this manner, we hope to pro- mote the synthesis of ideas and approaches between whole-plant physiologists and molecular biologists/ geneticists. The former typically measure stomatal regulation of gas exchange and its impact on whole- plant physiology, and may treat the cellular and mo- lecular biology of guard cells as a ''black box'' that receives and reacts to inputs. The latter typically use model plant species to investigate cell and molecular regulation of guard cell function, and may employ gene expression, stomatal aperture, or a specific guard cell parameter, such as ion fluxes, as a ''readout,'' without quantifying alterations in gas exchange and concomitant whole-plant impacts. Our premise is that Arabidopsis is an excellent reference plant in which these complementary approaches can be readily com- bined, and that such an integrated approach has great potential to yield new insights into the biology of transpiration in C3 angiosperms.Plant physiology 02/2007; 143(1):19-27. · 6.56 Impact Factor