Mesophyll cells

Air pollutants may enter plant systems by either a primary or a secondary pathway. The primary pathway is analogous to human inhalation. Figure 8-2 shows the cross section of a leaf. Both of the outer surfaces are covered by a layer of epidermal cells, which help in moisture retention. Between the epidermal layers are the mesophyll cells—the spongy and palisade parenchyma. The leaf has a vascular bundle which carries water, minerals, and carbohydrates throughout the plant. Two important features shown in Fig. 8-2 are the openings in the epidermal layers called stomates, which are controlled by guard cells which can open and close, and air spaces in the interior of the leaf. 

Sulfur dioxide Bleached spots, bleached areas between veins, chlorosis insect injury, winter and drought conditions may cause similar markings Middle-aged leaves most sensitive oldest least sensitive Mesophyll cells 0.3 785 8 hr... 

Nifrogen dioxide Irregular, white or brown collapsed lesions on intercostal tissue and near leaf margin Middle-aged leaves most sensitive Mesophyll cells 2.5 4700 4 hr... 

Hydrogen fluoride Tip and margin bums, dwarfing, leaf abscission narrow brown-red band separafes necrotic from Youngest leaves most sensitive Epidermis and mesophyll cells 0.1 (ppb) 0.08 wi. i... 

Mature leaves most sensitive Epidermis and mesophyll cells 0.10 290 2 hr... 

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

FIGURE 22.30 Essential features of the coinpartinenCation and biochemistry of die Hatch-Slack padiway of carbon dioxide uptake in C4 plants. Carbon dioxide is fixed into organic linkage by PEP carboxylase of meso-phyll cells, forming OAA. Eidier malate (die reduced form of OAA) or aspartate (the ami-iiated form) serves as die carrier transpordiig CO9 to the bundle slieadi cells. Within die bundle slieadi cells, CO9 is liberated by decar-boxyladon of malate or aspartate die C-3 product is returned to die mesophyll cell. 

Sharkey, T.D. Badger, M.R. (1982). Effects of water stress on photosynthetic electron transport, photophosphorylation and metabolite levels of Xanthium strumarium mesophyll cells. Planta, 156, 199-206. 

Harvey, D.M.R., Hall, J.L., Flowers, T.J. Kent, B. (1981). Quantitative ion localisation within Suaeda maritima leaf mesophyll cells. Planta, 151, 555-60. 

Steudle, E., Smith, J.A.C. Liittge, U. (1980). Water relations parameters of individual mesophyll cells of the crassulacean acid metabolism plant Kalanchoe diagre-montiana. Plant Physiology, 66, 1155-63. 

Pantoja, O. Willmer, C.M. (1986). Pressure effects on membrane potentials of mesophyll cell protoplasts and epidermal cell protoplasts of Commelina communis. Journal of Experimental Botany, 37, 315-20. 

Fig 6 Immunodot-blots of culture-medium aliquots sampled during the time-course of differentiation of Zinnia mesophyll cells to tracheary elements and dotted on to nitrocellulose. I = Inductive medium, N = Non-inductive medium. The JIM 7 epitope dries in a series of concentric rings on the nitrocellulose, indicating a mixed population of pectins. During the time-course, the rhamnose content of inductive culture medium increases dramatically compared with non-inductive medium. 

Stacey, N.J., Roberts, K., Carpita, N.C., Wells, B. and McCann, M.C. (1995) Dynamic changes in cell surface molecules are very early events in the differentiation of mesophyll cells from Zinnia elegans into tracheary elements. The Plant Journal, in press. 

Fig. 2. Accumulation of PHA inclusions in Arabidopsis transgenic plants. Transgenic cell expressing the poly(3HB) pathway in the plastid showing accumulation of poly(3HB) inclusions (arrows) in the chloroplast of a leaf mesophyll cell. Bar represents 1 pm... 

Most protocols for the isolation of protoplasts from Cowpea, Petunia, or Tobacco mesophyll cells are based on [93, 94] and others [95-97], After the isolation of protoplasts from the cell type of choice, the transfection is carried out as follows (based on [51, 96]) ... 

In contrast to the exterior localization of cutin, suberin can be deposited in both external and internal tissues. External deposition occurs in the periderm of secondary roots and stems and on cotton fibers, whereas internal deposition occurs in the root endodermis and the bundle sheath of monocots. The Casparian strip of the root en-dodermis contains suberin, which produces a barrier isolating the apoplast of the root cortex from the central vascular cylinder. Suberin also produces a gas-impermeable barrier between the bundle sheath and mesophyll cells in C4 plants. The bark of trees contains periderm-derived cork cells that have a high suberin content. 

Mirocha CJ, Schauerhamer B, Pathre SV (1974) Isolation, detection, and quantification of zearalenone in maize and barley. J Assoc Anal Chem 57 1104-1110 Muller R, Baier M, Kaiser WM (1991) Differential stimulation of PEP-carboxylation in guard cells and mesophyll cells by ammonium or fusicoccin. J Exp Bot 42 215-220 Murashige T, Skoog F (1962) A revised medium for rapid growth and bioassays with tobacco tissue cultures. Physiol Plant 15 473-497... 

Figure 2. Scanning electron micrograph of a mesophyll cell of a dormant cotyledon of Buffalo gourd (Cucurbita foetidissima). Tissue was fixed in aqueous glutaraldehyde, dehydrated with ethanol and critically point dried. Note cell wall (W) and intracellular components including protein bodies (P) and emptied spherosomes that appear as a cytoplasmic reticulum.

Figure 3. Transmission electron micrographs of mesophyll cells of dormant cotyledons of A, Cucurbita foetidissima B, Cucurbita pepo C, Cucurbita palmata D, Cucurbita digitata E, Apodanthera undulata" Note cell wall (W), protein body (P), spherosome (S), globoid (G), and crystalloid (X). In each micrograph, the bar represents five microns. Reproduced from reference 14.

A number of histologic and histochemical changes in current-year needles of ponderosa pine were detected after five to seven daily exposures to ozone at 0.45 ppm for 12 h each day. Chloroplasts and carbohydrate stain accumulated in the peripheral portions of mesophyll cells concurrently, the homogeneous distribution of proteins and nucleic acids was disrupted, and add phosphatase activity increased. Cell wall destruction occurred in mesophyll cells after appredable intracellular damage. 

Atmospheric CO2 first moves through the stomata, dissolves into leaf water and enters the outer layer of photosynthetic cells, the mesophyll cell. Mesophyll CO2 is directly converted by the enzyme ribulose biphosphate carboxylase/oxygenase ( Rubisco ) to a six carbon molecule that is then cleaved into two molecules of phosphoglycerate (PGA), each with three carbon atoms (plants using this photosynthetic pathway are therefore called C3 plants). Most PGA is recycled to make ribulose biphosphate, but some is used to make carbohydrates. Free exchange between external and mesophyll CO2 makes the carbon fixation process less efficient, which causes the observed large C-depletions of C3 plants. 

C4 plants incorporate CO2 by the carboxylation of phosphoenolpyruvate (PEP) via the enzyme PEP carboxylase to make the molecule oxaloacetate which has 4 carbon atoms (hence C4). The carboxylation product is transported from the outer layer of mesophyll cells to the inner layer of bundle sheath cells, which are able to concentrate CO2, so that most of the CO2 is fixed with relatively little carbon fractionation. 

In conclusion, the main controls on carbon fractionation in plants are the action of a particular enzyme and the leakiness of cells. Because mesophyll cells are permeable and bundle sheath cells are less permeable, C3 vs C4 plants have C-depletions of -18%c versus -4%c relative to atmospheric CO2 (see Fig. 2.10). 

The induction of PAL activity at the onset of vascular differentiation can be shown by the use of plant tissue cultures (37-39). Xylem cells with secondary and lignified walls are differentiated over a time course of 3-14 days by the application of the plant growth factors naphthylene acetic acid (NAA) and kinetin in the ratio 5 1 (1.0 mg/liter NAA, 0.2 mg/liter kinetin) to tissue cultures of bean cells (Phaseolus vulgaris) (37,40). The time for differentiation varies with the type of culture, solid or suspension, and with the frequency and duration of subculture, but for any one culture it is relatively constant (37,41,42). At the time of differentiation when the xylem vessels form, the activity of PAL rises to a maximum. The rising phase of the enzyme activity was inhibited by actinomycin D and by D-2,4-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide (MDMP) applied under carefully controlled conditions (42). This indicated that both transcription and translation were necessary for the response to the hormones. Experiments using an antibody for PAL and a cDNA probe for the PAL-mRNA have also shown that there is an increase in the amount of transcript for PAL during the formation of lignin when Zinnia mesophyll cells are induced to form xylem elements in culture (Lin and Northcote, unpublished work). 

Figure 6. A Photomicrograph (x 51,000) of caffeine treated leaf epidermal cell showing electron-dense deposits on cell wall and membrane vesicles fusing with the plasmalemma (arrows). B Immunofluorescence labeling of flavonoids in cell walls of leaf epidermal strips (arrows) and autofluorescent stomata (x 62.5). C Immunogold labeling of the walls of a mesophyll cell (left, x 41,000). Ch, chloroplast EC, epidermal cell G, Golgi IS, intercellular space MC, mesophyll cell (right, control x 19,500).

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