Bundle sheath

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). 

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. 

Lindbeck AGC, Rose RJ, Lawrence ME, Possingham JV. The chloroplast nucleoids of the bundle sheath and meosophyll cells of Zea mays. Physiol Plant 1989 75 7-12. 

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 pyruvate formed by decarboxylation of malate in bundle-sheath cells is transferred back to the mesophyll cells, where it is converted to PEP by an unusual enzymatic reaction catalyzed by pyruvate phosphate dikinase (Fig. 20-23b). This enzyme is called a dikinase because two different molecules are simultaneously phosphorylated by one molecule of ATP pyruvate to PEP, and phosphate to pyrophosphate. The pyrophosphate is subsequently hydrolyzed to phosphate, so two high-energy phosphate groups of ATP are used in regenerating PEP. The PEP is now ready to receive another molecule of C02 in the mesophyll cell. 

The PEP carboxylase of mesophyll cells has a high affinity for HCCU (which is favored relative to C02 in aqueous solution and can fix C02 more efficiently than can rubisco). Unlike rubisco, it does not use 02 as an alternative substrate, so there is no competition between C02 and 02. The PEP carboxylase reaction, then, serves to fix and concentrate C02 in the form of malate. Release of C02 from malate in the bundle-sheath cells yields a sufficiently high local concentration of C02 for rubisco to function near its maximal rate, and for suppression of the enzyme s oxygenase activity. 

Once C02 is fixed into 3-phosphoglycerate in the bundle-sheath cells, the other reactions of the Calvin cycle take place exactly as described earlier. Thus in C4 plants, mesophyll cells carry out C02 assimilation by the C4 pathway and bundle-sheath cells synthesize starch and sucrose by the C3 pathway. 

In C4 plants, the carbon-assimilation pathway minimizes photorespiration C02 is first fixed in mesophyll cells into a four-carbon compound, which passes into bundle-sheath cells and releases C02 in high concentrations. The released C02 is fixed by rubisco, and the remaining reactions of the Calvin cycle occur as in C3 plants. 

Malic enzyme, found in the bundle-sheath cells of C4 plants, carries out a reaction that has a counterpart in the citric acid cycle. What is the analogous reaction Explain your choice. 

The C4 cycle for concentration of carbon dioxide. The C4 plants reduce their rate of photorespiration by using a C02 concentrating mechanism that enables them to avoid the competition from 02. All species of C4 plants have a characteristic internal leaf anatomy in which a single dense layer of dark green cells surrounds the vascular bundles in the leaves. This bundle sheath is surrounded by a loosely packed layer of... 

The overall effect is to transport C02 from the mesophyll cells into the bundle sheath cells along with two reducing equivalents, which appear as NADPH following the action of the malic enzyme. The C02, the NADPH, and additional NADPH generated in the chloroplasts of the bundle sheath cells are then used in the Calvin-Benson cycle reactions to synthesize 3-phospho-glycerate and other materials. Of the C02 used in the bundle sheath cells, it is estimated that 85% comes via the C4 cycle and only 15% enters by direct diffusion. The advantage to the cell is a higher C02 tension, less competition with 02, and a marked reduction in photorespiration. 

What tricarboxylic acid cycle enzyme is analogous to the malate enzyme of bundle-sheath cells ... 

Mesophyll cells use C02 from the air to convert phospho-enolpyruvate to oxaloacetate (fig. 15.28). Oxaloacetate is reduced to malate, which then moves to the bundle sheath cells that surround the vascular structures in the interior of the leaf. Here malate is decarboxylated to pyruvate in an oxidative reaction that reduces NADP+ to NADPH. The pyruvate returns to the mesophyll cells, where it is phos-phorylated to phosphoenolpyruvate. This phosphorylation is driven by splitting of ATP to AMP and pyrophosphate and subsequent hydrolysis of the pyrophosphate to phosphate. 

The C-4 pathway requires the cooperation of two types of cells. Mesophyll cells (left) take up C02 from the air and export malate to the bundle sheath cells (right). The bundle sheath cells return pyruvate to the mesophyll cells and fix the C02 using ribulose bisphosphate carboxylase and the reductive pentose cycle. 

Aloe latex contains anthraquinones and is completely different from aloe gel, a colorless gelatin obtained from the central portion of the aloe leaf. The mucilaginous parenchymous tissue is excised from fresh leaves and immediately utilized for pharmaceutical preparations, or lyophilized and kept dry until use. During extraction of the gel, it is practically impossible to prevent contamination by the leaf latex as the leaves are cut. On the other hand, in intact leaves, anthraquinones may diffuse into gel from the bundle sheath cells. To reduce such contamination, the starting material must be from varieties of aloe with a reduced anthraquinone content. 

PEP carboxylase is concentrated in special mesophyll cells in the outer part of the leaf. This means that the cells most exposed to the atmosphere are the most efficient at converting C02 into organic products. Photosynthesis involving Rubisco is more prominent in the bundle sheath cells located in the inner part of the leaf around the veins that carry compounds between different parts of the plant. 

After PEP carboxylase makes the oxaloacetate, it is transported to the bundle sheath cells. First, NADPH reduces it to malate, and it is then transported to the bundle sheath cells. In the bundle sheath cells, malic enzyme cleaves the malate to pyruvate and C02 for Rubisco. This generates NADPH as well, so the C4 cycle consumes no reducing equivalents. Pyruvate is transported from the bundle sheath back to the mesophyll cells where it is rephosphorylated to phosphoenolpyruvate, expending the equivalent of two ATP high-energy phosphates. ... 

Sedimentation of amyloplasts within the cell has been correlated with the capacity of the plant to perceive gravity. The buoyant mass of amyloplasts present in specialized cells in the center of the root cap and in the stem (depending on the plant species, in the endodermis, the bundle sheath, or in the parenchyma to the inside of the vascular bundle) would allow the amyloplasts to sediment inside the cell, where the cytosol would have a relatively low viscosity. This sedimentation would translate into a signal of an unknown nature, maybe through pressure onto a sensitive part of the cell or acting as a mechano transducer, etc. Whatever the nature of the signal, it eventually results in the asymmetry of the organ and its curvature. The isolation of starchless mutants of Arabidopsis thaliana and Nicotiana sylvestris has made... 

The C4 cycle can be viewed as an ATP-dependent C02 pump that delivers C02 from the mesophyll cells to the bundle-sheath cells, thereby suppressing photorespiration (Hatch and Osmond, 1976). The development of the C4 syndrome has resulted in considerable modifications of inter- and intracellular transport processes. Perhaps the most striking development with regard to the formation of assimilates is that sucrose and starch formation are not only compartmented within cells, but in C4 plants also may be largely compartmented between mesophyll and bundle-sheath cells. This has been achieved together with a profound alteration of the Benson-Calvin cycle function, in that 3PGA reduction is shared between the bundle-sheath and mesophyll chloroplasts in all the C4 subtypes. Moreover, since C4 plants are polyphyletic in origin, several different metabolic and structural answers have arisen in response to the same problem of how to concentrate C02. C4 plants have three distinct mechanisms based on decarboxylation by NADP+-malic enzyme, by NAD+-malic enzyme, or by phosphoenolpy-ruvate (PEP) carboxykinase in the bundle-sheath (Hatch and Osmond, 1976). 

Down ton and Hawker (1973), showed that starch, starch synthase, and ADPGlc PPase were much higher in bundle-sheath cells than in mesophyll cells on a protein basis or on a chlorophyll basis. The mesophyll cell is able to synthesize starch on exposure of the leaf to continuous light for approximately 2.5 days. Under these conditions, starch synthase levels in the mesophyll cell increased. Thus the mesophyll cell is capable of starch synthesis under certain conditions. Later reports on other C4 plants (e.g., nutsledge leaves, Chen et al., 1974 Digitaria pentzii, Mbaku et aL, 1978) also indicate that both tissues are capable of starch synthesis. 

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