Cell wall arabinan is essential for guard cell function.
(1/12)
Stomatal guard cells play a key role in the ability of plants to survive on dry land, because their movements regulate the exchange of gases and water vapor between the external environment and the interior of the plant. The walls of these cells are exceptionally strong and must undergo large and reversible deformation during stomatal opening and closing. The molecular basis of the unique strength and flexibility of guard cell walls is unknown. We show that degradation of cell wall arabinan prevents either stomatal opening or closing. This locking of guard cell wall movements can be reversed if homogalacturonan is subsequently removed from the wall. We suggest that arabinans maintain flexibility in the cell wall by preventing homogalacturonan polymers from forming tight associations. (+info)
The effects of manipulating phospholipase C on guard cell ABA-signalling.
(2/12)
Studies using stably transformed tobacco plants containing very low levels of PI-PLC in their guard cells show that this enzyme plays a role in the events associated with the inhibition of stomatal opening by ABA, but not in the cellular reactions that are responsible for ABA-induced stomatal closure. However, Commelina communis guard cells microinjected with the InsP3 antagonist, heparin, fail to close on addition of ABA. There are three possible explanations for this apparent data mismatch. The differences may be indicative of species-specific signalling pathways, the presence of a PI-PLC isoform(s) that is not down-regulated in these transgenic lines and/or they may reflect differences between short-term (acute) administration of an inhibitor and long-term (chronic) effects of gene manipulation. It is possible that the guard cell is a robust signalling system that is able to adapt or compensate for the chronic loss of PI-PLC, but which is unable to adjust quickly to acute loss of this component. It would be interesting to investigate this possibility further using either transient manipulation of gene expression or through the use of an inducible promoter. (+info)
Comparative structure and pollen production of the stamens and pollinator-deceptive staminodes of Commelina coelestis and C. dianthifolia (Commelinaceae).
(3/12)
BACKGROUND AND AIMS: Flowers of Commelina coelestis and C. dianthifolia provide pollen alone as a floral reward, and rely on visual cues to attract pollinators. Three stamen types, all producing pollen, occur in each of these species: two cryptically coloured lateral stamens, a single cryptically coloured central stamen and three bright yellow staminodes that sharply contrast with the blue to purple corolla. The objective was to compare the stamen structure and pollen characteristics of each of the three stamen types, and to test the hypothesis that the staminodes are poor contributors of viable pollen for the siring of seed. The pollination roles of the three stamen types and the breeding systems of both species were also explored. METHODS: Light, fluorescence and scanning electron microscopy were utilized to examine stamen morphology and pollen structure and viability. Controlled hand pollinations were used to explore the breeding system of each species. Filament and style lengths were measured to investigate herkogamy and autogamy. KEY RESULTS: Pollen from all stamen morphs is viable, but staminode pollen has significantly lower viability. Pollen polymorphism exists both (a) between the lateral and central stamens and the staminodes, and (b) within each anther. Lateral and central stamens have thicker endothecia with a greater number of secondary cell wall thickenings than the staminodes. CONCLUSIONS: Both species are entomophilous and facultatively autogamous. Lateral stamen pollen is important for cross-pollination, central stamen pollen is utilized by both species as a pollinator reward and for delayed autogamy in C. dianthifolia, and the staminodes mimic, by means of both colour and epidermal features, large amounts of pollen to attract insects to the flowers. Pollen from all three anther morphs is capable of siring seed, although staminode pollen is inferior. The thin staminode endothecium with fewer secondary thickenings retards staminode dehiscence. (+info)
Lateral diffusion of CO2 in leaves is not sufficient to support photosynthesis.
(4/12)
Lateral diffusion of CO(2) was investigated in photosynthesizing leaves with different anatomy by gas exchange and chlorophyll a fluorescence imaging using grease to block stomata. When one-half of the leaf surface of the heterobaric species Helianthus annuus was covered by 4-mm-diameter patches of grease, the response of net CO(2) assimilation rate (A) to intercellular CO(2) concentration (C(i)) indicated that higher ambient CO(2) concentrations (C(a)) caused only limited lateral diffusion into the greased areas. When single 4-mm patches were applied to leaves of heterobaric Phaseolus vulgaris and homobaric Commelina communis, chlorophyll a fluorescence images showed dramatic declines in the quantum efficiency of photosystem II electron transport (measured as F(q)'/F(m)') across the patch, demonstrating that lateral CO(2) diffusion could not support A. The F(q)'/F(m)' values were used to compute images of C(i) across patches, and their dependence on C(a) was assessed. At high C(a), the patch effect was less in C. communis than P. vulgaris. A finite-volume porous-medium model for assimilation rate and lateral CO(2) diffusion was developed to analyze the patch images. The model estimated that the effective lateral CO(2) diffusion coefficients inside C. communis and P. vulgaris leaves were 22% and 12% of that for free air, respectively. We conclude that, in the light, lateral CO(2) diffusion cannot support appreciable photosynthesis over distances of more than approximately 0.3 mm in normal leaves, irrespective of the presence or absence of bundle sheath extensions, because of the CO(2) assimilation by cells along the diffusion pathway. (+info)
Species-dependent changes in stomatal sensitivity to abscisic acid mediated by external pH.
(5/12)
The direct effects of pH changes and/or abscisic acid (ABA) on stomatal aperture were examined in epidermal strips of Commelina communis L. and Arabidopsis thaliana. Stomata were initially opened at pH 7 or pH 5. The stomatal closure induced by changes in external pH and/or ABA (10 microM or 10 nM) was monitored using video microscopy and quantified in terms of changes in stomatal area using image analysis software. Measurements of aperture area enabled stomatal responses and, in particular, small changes in stomatal area to be quantified reliably. Both plant species exhibited a biphasic closure response to ABA: an initial phase of rapid stomatal closure, followed by a second, more prolonged, phase during which stomata closure proceeded at a slower rate. Changes in stomatal sensitivity to ABA were also observed. Comparison of these effects between C. communis and A. thaliana demonstrate that this differential sensitivity of stomata to ABA is species-dependent, as well as being dependent on the pH of the extracellular environment. (+info)
Osmotic effects on vacuolar ion release in guard cells.
(6/12)
Tracer flux experiments in isolated guard cells of Commelina communis L. suggest that the vacuolar ion content is regulated and is reset to a reduced fixed point by abscisic acid (ABA) with no significant change in cytoplasmic content. The effects of changes in external osmotic pressure were investigated by adding and removing mannitol from the bathing solution. Two effects were distinguished. In the new steady state of volume and turgor, the vacuolar ion efflux was sensitive to turgor: efflux increased at high turgor and reduced at lower turgor after the addition of mannitol. These changes were inhibited by phenylarsine oxide and are likely to involve the same channel that is involved in the response to ABA. After a hypoosmotic transfer, there was an additional effect: a fast transient stimulation of vacuolar efflux during the period of water flow into the cell; the size of this hypopeak increased with the size of the hypoosmotic shock, with increased water flow. No corresponding transient in reduced vacuolar efflux was observed upon hyperosmotic transfer. The fast hypopeak was not inhibited by phenylarsine oxide and appears to involve a different ion channel from that involved in the resting efflux, the response to ABA, or the turgor sensitivity. Thus, the tonoplast can sense an osmotic gradient and respond to water flow into the vacuole by increased vacuolar ion efflux, thereby minimizing cytoplasmic dilution. An aquaporin is the most likely sensor and may also be involved in the signal transduction chain. (+info)
Modification of leaf apoplastic pH in relation to stomatal sensitivity to root-sourced abscisic acid signals.
(7/12)
The confocal microscope was used to determine the pH of the leaf apoplast and the pH of microvolumes of xylem sap. We quantified variation in leaf apoplast and sap pH in relation to changes in edaphic and atmospheric conditions that impacted on stomatal sensitivity to a root-sourced abscisic acid signal. Several plant species showed significant changes in the pH of both xylem sap and the apoplast of the shoot in response to environmental perturbation. Xylem sap leaving the root was generally more acidic than sap in the midrib and the apoplast of the leaf. Increasing the transpiration rate of both intact plants and detached plant parts resulted in more acidic leaf apoplast pHs. Experiments with inhibitors suggested that protons are removed from xylem sap as it moves up the plant, thereby alkalinizing the sap. The more rapid the transpiration rate and the shorter the time that the sap resided in the xylem/apoplastic pathway, the smaller the impact of proton removal on sap pH. Sap pH of sunflower (Helianthus annuus) and Commelina communis did not change significantly as soil dried, while pH of tomato (Lycopersicon esculentum) sap increased as water availability in the soil declined. Increasing the availability of nitrate to roots also significantly alkalinized the xylem sap of tomato plants. This nitrogen treatment had the effect of enhancing the sensitivity of the stomatal response to soil drying. These responses were interpreted as an effect of nitrate addition on sap pH and closure of stomata via an abscisic acid-based mechanism. (+info)
Structure of commelinin, a blue complex pigment from the blue flowers of Commelina communis.
(8/12)
The X-ray crystal structure of natural commelinin is investigated. The results demonstrate that commelinin is a tetranuclear (4 Mg(2+)) metal complex, in which two Mg(2+) ions chelate to six anthocyanin molecules, while the other two Mg(2+) ions bind to six flavone molecules, stabilizing the commelinin complex, a new type of supramolecular complex. (+info)