Localization of ribulose bisphosphate carboxylase in the guard cells by an indirect, immunofluorescence technique

Arabidopsis guard cell chloroplasts import cytosolic ATP for starch turnover and stomatal opening

Stomatal opening requires the provision of energy in the form of ATP for proton pumping across the guard cell (GC) plasma membrane and for associated metabolic rearrangements. The source of ATP for GCs is a matter of ongoing debate that is mainly fuelled by controversies around the ability of GC chloroplasts (GCCs) to perform photosynthesis. By imaging compartment-specific fluorescent ATP and NADPH sensor proteins in Arabidopsis, we show that GC photosynthesis is limited and mitochondria are the main source of ATP. Unlike mature mesophyll cell (MC) chloroplasts, which are impermeable to cytosolic ATP, GCCs import cytosolic ATP through NUCLEOTIDE TRANSPORTER (NTT) proteins. GCs from ntt mutants exhibit impaired abilities for starch biosynthesis and stomatal opening. Our work shows that GCs obtain ATP and carbohydrates via different routes from MCs, likely to compensate for the lower chlorophyll contents and limited photosynthesis of GCCs.

Stomatal guard cells require ATP in order to fuel stomatal movements. Here the authors show that guard cell photosynthesis is limited, mitochondria are the main source of ATP and that guard cell chloroplasts import ATP via nucleotide transporters.

Stomatal Biology of CAM Plants

Recent advances in the stomatal biology of CAM plants are reviewed, and key opportunities for future progress are identified.

Organ-specific and light-induced expression of plant genes

Light plays a pivotal role in the development of plants. The photoregulation of plant genes involves recognition of light quality and quantity by phytochrome and other light receptors. Two gene families, rbcS and Cab, which code for abundant proteins active in photosynthesis, the small subunit of ribulose bisphosphate carboxylase and the chlorophyll a/b binding protein, show a 20-to 50-fold increase in transcript abundance in the light. Analyses in calli and transgenic plants of deletions of the rbcS gene and of chimeric constructions has allowed localization of two regions involved in light-induced transcription. One element is confined to a 33-base pair region surrounding the TATA box. In addition, an enhancer-like element contained within a 240-base pair fragment can confer phytochrome-induced transcription and organ specificity on nonregulated promoters.

Guard cell photosynthesis and stomatal function

Chloroplasts are a key feature of most guard cells; however, the function of these organelles in stomatal responses has been a subject of debate. This review examines evidence for and against a role of guard cell chloroplasts in stimulating stomatal opening. Controversy remains over the extent to which guard cell Calvin cycle activity contributes to stomatal regulation. However, this is only one of four possible functions of guard cell chloroplasts; other roles include supply of ATP, blue-light signalling and starch storage. Evidence exists for all these mechanisms, but is highly dependent upon species and growth/measurement conditions, with inconsistencies between different laboratories reported. Significant plasticity and extreme flexibility in guard cell osmoregulatory, signalling and sensory pathways may be one explanation. The use of chlorophyll a fluorescence analysis of individual guard cells is discussed in assessing guard and mesophyll cell physiology in relation to stomatal function. Developments in transgenic and molecular techniques have recently provided interesting, albeit contrasting, data regarding the role of these highly conserved organelles in stomatal function. Recent studies examining the link between mesophyll photosynthesis and stomatal conductance are discussed. An enhanced understanding of these processes may be fundamental in generating crop plants with greater water use efficiencies, capable of combating future climatic changes.

The guard cell chloroplast: a perspective for the twenty-first century

The guard cell chloroplast is the site of perception of blue light and of photosynthetically active radiation, and of at least one of the mechanisms sensing CO₂ in the guard cell. The guard cell chloroplast has been the focus of intense controversy over its capacity for light sensing and photosynthetic carbon fixation, and the osmoregulatory mechanisms mediating stomatal movements. It is argued here that a primary reason behind these long-lived controversies is the remarkable plasticity of the guard cell, which has resulted in responses being generalized as basic properties when opposite responses appear to be the norm under different environmental or experimental conditions. Examples of guard cell plasticity are described, including variation of chlorophyll fluorescence transients over a daily course, acclimation of the guard cell responses to blue light and CO₂ , the shift from potassium to sucrose in daily courses of osmoregulation and the transduction of red light into different osmoregulatory pathways. Recent findings on the properties of the guard cell chloroplast are also presented, including the role of the chloroplastic carotenoid, zeaxanthin, in blue light photoreception, the blue-green reversibility of stomatal movements, and the involvement of phytochrome in the stomatal response to light in the orchid, Paphiopedilum.

Red light activates a chloroplast-dependent ion uptake mechanism for stomatal opening under reduced CO2 concentrations in Vicia spp

•  Under red light in ambient CO₂ guard cells of faba bean (Vicia faba) fix CO₂ and accumulate sucrose, causing stomata to open. We examined whether at [CO₂ ] low enough to limit guard cell photosynthesis stomata would open when illuminated with red (R) or far-red (FR) light. •  After illumination with R or FR in buffered KCl solutions, net stomatal opening was c. 3 µm (R and FR) in air containing 210-225 µl l^-1 CO₂ and was 5 µm (R) or 6.5 µm (FR) in air containing 40-50 µl l^-1 CO₂ . Opening was fully inhibited by 3-(3,4-dichlorophenyl)-1,1 dimethyl urea, the calmodulin antagonist W-7, the ser/thr kinase inhibitor ML-9, and sodium orthovanadate, but not by dithiothreitol, which inhibits formation of zeaxanthin, the blue light photoreceptor of guard cells. •  Stomatal opening was accompanied by K⁺ uptake and starch loss. Similar results were obtained when leaves were exposed to conditions designed to lower intercellular leaf [CO₂ ]. •  These data suggest that the guard cell chloroplasts transduce reduced [CO₂ ], activating stomatal opening through an ion uptake mechanism that depends on chloroplastic photosynthetic electron transport and that shares downstream components of the blue light signal transduction cascade.

Transpiration rate. An important factor controlling the sucrose content of the guard cell apoplast of broad bean

Evaporation of water from the guard cell wall concentrates apoplastic solutes. We hypothesize that this phenomenon provides two mechanisms for responding to high transpiration rates. First, apoplastic abscisic acid is concentrated in the guard cell wall. Second, by accumulating in the guard cell wall, apoplastic sucrose (Suc) provides a direct osmotic feedback to guard cells. As a means of testing this second hypothesized mechanism, the guard cell Suc contents at a higher transpiration rate (60% relative humidity [RH]) were compared with those at a lower transpiration rate (90% RH) in broad bean (Vicia faba), an apoplastic phloem loader. In control plants (constant 60% RH), the guard cell apoplast Suc content increased from 97 +/- 81 femtomol (fmol) guard cell pair(-1) to 701 +/- 142 fmol guard cell pair(-1) between daybreak and midday. This increase is equivalent to approximately 150 mM external, which is sufficient to decrease stomatal aperture size. In plants that were shifted to 90% RH before daybreak, the guard cell apoplast Suc content did not increase during the day. In accordance, in plants that were shifted to 90% RH at midday, the guard cell apoplast Suc content declined to the daybreak value. Under all conditions, the guard cell symplast Suc content increased during the photoperiod, but the guard cell symplast Suc content was higher (836 +/- 33 fmol guard cell pair(-1)) in plants that were shifted to 90% RH. These results indicate that a high transpiration rate may result in a high guard cell apoplast Suc concentration, which diminishes stomatal aperture size.

Analysis of the promotors of the single-copy genes for plastocyanin and subunit delta of the chloroplast ATP synthase from spinach

The promotors of the single-copy genes for subunit delta of the chloroplast ATP synthase (atpD) and plastocyanin (PC) from spinach have been sequenced, dissected and analysed in transgenic F0 and F1 tobacco plants using the bacterial GUS gene as a reporter for promotor activity. The transcription of these genes is photo-controlled. The results have been compared with those obtained for the spinach rbcS-1 gene, one of the light-regulated genes encoding the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase, and for the cauliflower mosaic virus (CaMV) 35S RNA promotor. We find that the 5' upstream regions of about 1200 nucleotides contain all the sequences required for light regulation, organ-, tissue- and development-specific expression, and that they are structurally diverse. Their cis-acting elements are functionally defined. The proximal regions of the spinach promotors contain potential TATA, CAAT and T-cyt boxes at appropriate positions, but only sequence elements with low similarity to published light-responsive elements. Positive light-stimulated regions, regions with constitutive, light-independent enhancing effects and with 'silencer'-like activity in complete darkness are found in proximal and far upstream promotor segments. Highest activity of these promotors is correlated with the presence of chloroplasts but is not confined to photosynthetic tissue. Surprisingly, expression is observed in the phloem regions of transgenic leaves, leaf and floral stems, in the vascular area of anthers and in pollen. No histochemical staining has been detected in roots. The distal region of atpD located between -1137 and -590 contains elements for expression in the outer phloem, the region from -590 to -185 for activity in the inner phloem of floral stems. Similar tissue-specific patterns are observed with a fusion between the caufliflower mosaic virus 35S RNA promotor and the GUS gene.

Two immunological approaches to the detection of ribulose-1,5-bisphosphate carboxylase in guard cell chloroplasts

Two immunological approaches were used to determine if ribulose bisphosphate carboxylase oxygenase (RuBisCo) is present in guard cell chloroplasts. Immunocytochemistry on thin plastic sections using tissue samples that were processed using traditional glutaraldehyde/osmium fixation and then restored to antigenicity with metaperiodate treatment, resulted in labeling over wild-type mesophyll and guard cell plastids of several green and white variegated Pelargonium chimeras. The density of immunogold labeling in guard cell chloroplasts was only about one-seventh of that noted in mesophyll chloroplasts on a square micron basis. Because guard cell chloroplasts are much smaller than mesophyll chloroplasts, and occur at lower quantities/cell, the relative differences in RuBisCo concentration between the cell types indicate that guard cells have only 0.48% of the RuBisCo of mesophyll cells. No reaction was noted over 70S ribosomeless plastids of these chimeras even though adjacent green chloroplasts were heavily stained, indicating the high specificity of the reaction for RuBisCo. Spurr's resin gave the most successful colloidal gold labeling in terms of low background staining and structural detail but L. R. White's resin appeared to be superior for antigen retention. In the white leaf edges of the white and green Pelargonium chimeras, the only green, functional chloroplasts are in the guard cells. When either whole tissue or plastid enriched extracts from this white tissue were electrophoresed, blotted, and probed with anti-RuBisCo a large subunit band was detected, identical to that in the green tissue. These data indicate that a low, but detectable, level of RuBisCo is present in guard cell chloroplasts.

Light Activation of NADP-Malate Dehydrogenase in Guard Cell Protoplasts from Vicia faba L

Light-induced swelling of guard cell protoplasts (GCP) from Vicia faba was accompanied by increases in content of K(+) and malate. DCMU inhibited the increase of K(+) and malate, and consequently swelling.Effect of light on the activity of selected enzymes that take part in malate formation was studied. When isolated GCP were illuminated, NADP-malate dehydrogenase (NADP-MDH) was activated, and the activity reached a maximum within 5 minutes. The enzyme activity underwent 5- to 6-fold increase in the light. Upon turning off the light, the enzyme was inactivated in 5 minutes NAD-MDH and phosphoenolpyruvate carboxylase (PEPC) were not influenced by light. The rapid light activation of NADP-MDH was inhibited by DCMU, suggesting that the enzyme was activated by reductants from the linear electron transport in chloroplasts. An enzyme localization study by differential centrifugation indicates that NADP-MDH is located in the chloroplasts, NAD-MDH in the cytosol and mitochondria, and PEPC in the cytosol. After light activation, the activity of NADP-MDH in guard cells was 10 times that in mesophyll cells on a chlorophyll basis. The physiological significance of light-dependent activation of NADP-MDH in guard cells is discussed in relation to stomatal movement.

Immunological evidence for the presence of ribulose bisphosphate carboxylase in guard cell chloroplasts

Ribulose bisphosphate carboxylase (Rubisco) has been found in Vicia faba L. guard cell chloroplasts by two immunological methods, using antibodies raised against highly purified subunits of ribulose bisphosphate carboxylase. Indirect cytoimmunofluorescence revealed binding of antibodies against both the small and the large subunits of ribulose bisphosphate carboxylase. Binding was observed only after partial digestion of guard cell walls by 4% Cellulysin to facilitate antibody penetration. After electrophoresis of a homogenate of guard cell protoplasts, the presence of both subunits was also revealed by immunolabeling technique. Positive response required the inhibition of proteolysis which appeared to be active upon homogenization.

Photosynthetic electron transport in guard cells of diverse species

Guard cells of plants representing 18 species were assayed qualitatively for potential to conduct photosynthetic linear electron transport. These plants included C(3) pteridophytes, C(3) and C(4) monocots, and C(3), C(4), and Crassulacean acid metabolism dicots. By use of a microfluorospectrophotometer, guard cell samples in epidermal peels were isolated optically. Chlorophyll fluorescence was monitored from the onset of excitation light. For guard cells of all these species, fluorescence intensity increased during illumination. When samples were preincubated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron, however, there was a more rapid increase in fluorescence. These results indicate that all tested guard cells conduct photosynthetic electron transport through the reaction center of photosystem II.

Guard Cell Starch Biosynthesis Regulated by Effectors of ADP-Glucose Pyrophosphorylase

ADP-glucose pyrophosphorylase catalyzes the regulated step of starch bioynthesis in mesophyll chloroplasts. This enzyme is activated by a high ratio of the concentrations of 3-P-glycerate to inorganic phosphate (Pi) in light. In contrast, starch in guard cell chloroplasts is degraded when stomata open, which usually occurs in light. We have investigated the biochemical causes for this contrasting phenomenon.Vicia faba L. leaflets were sampled in darkness and after various periods of illumination. The samples were quick-frozen and freeze-dried. Guard cells and other cells were dissected out, weighed, and assayed for ADP-glucose pyrophosphorylase activity, 3-P-glycerate, and Pi. In the pyrophosphorolytic direction, ADP-glucose pyrophosphorylase specific activity in guard cells was 2.7 moles per kilogram protein per hour, which was comparable to the values obtained for palisade and spongy cells. The specific activity in epidermal cells was 4-fold lower. Under our assay conditions, the guard cell enzyme activity was 5-fold higher in the presence of 3-P-glycerate and 5-fold lower with Pi (i.e. similar to the results obtained with extracts of fresh leaflet). During three minutes of illumination, 3-P-glycerate concentration in palisade cells increased 2.5-fold to 10 millimoles per kilogram dry mass. The concentration of 3-P-glycerate in guard cells was 20-fold lower and unaffected by illumination. The concentration of Pi was approximately 17 millimoles per kilogram dry mass in palisade cells, but was 10-fold higher in guard cells. These overall cellular Pi concentrations were unaffected by illumination. We conclude that starch biosynthesis in guard cells is not activated by light because of the low and constant 3-P-glycerate concentration there. We interpret this last to be a consequence of the absence of the photosynthetic carbon reduction pathway in chloroplasts of these cells.

Immunocytochemical evidence for a peroxisomal localization of manganese superoxide dismutase in leaf protoplasts from a higher plant

The controversial question of the intracellular location of manganese-containing superoxide dismutase in higher plants was examined under a new experimental approach by applying the more rigorous and specific methods of immunocytochemistry to protoplasts isolated fromPisum sativum L. leaves. Manganese superoxide dismutase (EC 1.15.1.1) was purified to homogeneity from 15 kg of leaves ofPisum sativum L. Rabbits were immunized with the mangano enzyme and the antibody specific for pea manganese superoxide dismutase was purified and found not to contain antigenic sites in common with (i) human manganese superoxide dismutase, (ii) iron superoxide dismutase from eitherEscherichia coli or higher plants, or (iii) plant or animal cuprozinc-superoxide dismutase.Pisum sativum L. manganese superoxide dismutase only appears to have antigenic determinants similar to other manganese superoxide dismutases from higher land plants. The antibody to pea Mn-superoxide dismutase was used to locate the enzyme in protoplasts isolated from young pea leaves by indirect immunofluorescence, and by electron microscopy using the unlabelled antibody peroxidase-antiperoxidase method. Results from immunofluorescence showed that chloroplasts were devoid of specific fluorescence which appeared scattered over the cytosolic spaces among chloroplasts, and demonstrate the absence of manganese superoxide dismutase inside chloroplasts. The metalloenzyme was found to be localized only in peroxisomes, whereas mitochondria, the traditionally accepted site for this enzyme in many eukaryotic organisms, did not show any specific staining. The possible subcellular roles of manganese superoxide dismutase inPisum sativum L. leaves are discussed in the light of its peroxisomal location.

Profile of Basic Carbon Pathways in Guard Cells and Other Leaf Cells of Vicia faba L

Guard cells and three other cell types from Vicia faba L. ;Longpod' leaflets were assayed for enzymes that catalyze one step in each of five major carbon pathways in green plants: the photosynthetic carbon reduction pathway (ribulose-bisphosphate carboxylase, EC 4.1.1.39), the photosynthetic carbon oxidation pathway (hydroxypyruvate reductase, EC 1.1.1.81), glycolysis ([NAD] glyceraldehyde-P dehydrogenase, EC 1.2.1.12), the oxidative pentose-P pathway (6-P-gluconate dehydrogenase, EC 1.1.1.44), and the tricarboxylic acid pathway (fumarase, EC 4.2.1.2). Neither ribulose-bisphosphate carboxylase nor hydroxypyruvate reductase could be detected in guard cells or epidermal cells; high levels of these activities were present in mesophyll cells. The specific activity of fumarase (protein basis) was about 4-fold higher in guard cells than in epidermal, palisade parenchyma or spongy parenchyma cells. (NAD) glyceraldehyde-P and 6-P-gluconate dehydrogenases also were present at high protein specific activities in guard cells (2- to 4-fold that in meosphyll cells).It was concluded that the capacity for metabolite flux through the catabolic pathways is high in guard cells. In addition, other support is provided for the view that photoreduction of CO(2) by these guard cells is absent.

Taxonomic survey for the presence of ribulose-1,5-bisphosphate carboxylase activity in guard cells

Guard cell pairs were dissected from freeze-dried leaves of plants representing 15 families, including monocots, dicots, and pteridophytes. All three major photosynthetic carbon pathways (C(2), C(4), and Crassulacean acid metabolism) were represented. These individual guard cell pairs were assayed quantitatively for ribulose-1,5-bisphosphate carboxylase specific activity. Assay sensitivity averaged 0.08 picomoles of ribulose-P(2) dependent P-glycerate formation (i.e. 100-fold more sensitive than required to detect the activity present in a single Vicia faba mesophyll cell). The calculated specific activities for guard cells and mesophyll cells averaged 4 and 472 millimoles per kilogram dry weight per hour, respectively. For all species surveyed, (a) the enzyme activity calculated for guard cells was below the detection limit of the assay, or (b) the specific activity (weight or cell basis) calculated for guard cells was less than 1% of the specific activity calculated for adjacent mesophyll cells. Based on this survey, the generalization is made that the photosynthetic carbon reduction pathway is absent, or virtually so, in guard cell chloroplasts.