Mesophyll chloroplast

Unique subcellular compartmentation is also present in quinolizidine alkaloid biosynthesis, which occurs in the mesophyll chloroplasts of some legumes.158 One of the enzymes catalyzing the last two acylations of the pathway in Lupinus albus occurs in the cytoplasm, whereas the other resides in the mitochondria/59 Although the quinolizidine nucleus appears to be synthesized in the chloroplast, subsequent modifications can occur only after alkaloid intermediates are transported to the cytosol and mitochondia. Quinolizidine alkaloids appear to accumulate in vacuoles of epidermal cells where their defensive properties are most effective. 

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). 

Hallberg, M and Larsson, C. 1983. Metabolism of labelled 3-phosphoglycerate with mesophyll protoplasts and purified mesophyll chloroplasts from the C4 plant Digitaria sanguinalis. In Proceedings of the Vlth International Congress on Photosynthesis (C. Sybesma, ed.), Vol. III., pp. 417-420. The Hague, M. Nijhoff/W. Junk. 

Subsequent metabolism of oxaloacetate (OAA) varies according to species. Three main types of C4 pathway are recognized, of which the most extensively studied is that shown by plants such as Zea mays (corn) (Fig. 2). In these plants (here called type-1 C4 plants) OAA is reduced to malate via NADP-malate dehydrogenase in mesophyll chloroplasts. Malate is then transported to bundle sheath chloroplasts and oxidatively decarboxylated by NADP-malic enzyme to produce pyruvate, CO2 and NADPH. Pyruvate is recycled to the mesophyll cells while the CO2 and NADPH are used in the RPP cycle in the bundle sheath chloroplast. The original C3 carbon acceptor (PEP) is regenerated from pyruvate in the mesophyll chloroplast by the activity of pyruvate, Pj dikinase [8] (Eq. 5). 

Although the bundle sheath chloroplasts contain all the enzymes of the RPP cycle, there is now evidence that some of the 3-PGA formed by the activity of rubisco is exported to the mesophyll cells [9]. Bundle sheath chloroplasts of maize are deficient in photosystem II activity and apparently cannot produce sufficient NADPH to reduce all of the 3-PGA formed to triose phosphate. Responsibility for this step is thus shared with the mesophyll chloroplasts which recycle triose phosphate to the bundle sheath as DHAP. This transport of 3-PGA from bundle sheath to mesophyll permits the synthesis of sucrose in the mesophyll cell cytoplasm. The evidence suggests that the mesophyll cells are the major site of sucrose synthesis [10-13]. Sucrose phosphate synthetase, one of the regulatory enzymes of sucrose synthesis and fructose 6-phosphate, 2-kinase (Fru-6-P,2K), the enzyme synthesizing fructose 2,6-bisphosphate — a potent regulator of cytoplasmic sucrose synthesis (see Section 5.4.1) — are both almost completely confined to the mesophyll cells. 

Ohnishi J, Flugge UI, Heldt HW and Kanai R (1990) Involvement of No in active uptake of pyruvate in mesophyll chloroplasts of some Q plants. Plant Physiol 94 950-959. 

FIGURE 1 Electron micrographs of mesophyll chloroplasts of somatic hybrids. 

Whether sulphate reduction takes place only in the mesophyll chloroplasts of C4 plants has not been determined. 

The distribution of carbonic anhydrase is also in accord with the alkaline stress theory. This enzyme is highly active in C3 chloroplasts whereas it is absent from C4 bundle-sheath chloroplasts conducting carboxlation - nor is it needed there. Bicarbonate generated on nitrite reduction in the mesophyll chloroplasts could be transferred to the cytosol at a leisurely rate for PEP carboxylase activity there. ... 

The metabolic capabilities of the chloroplasts of the mesophyll and bimdle sheath cells of C4 plants are quite different. The PSII-defident bundle sheath chloroplasts of C4 plants of group (i) were mentioned earlier. That apart, the chloroplasts of both cell types generate NADPH and ATP by non-cyclic photophosphorylation. However, only the bundle sheath chloroplasts of all three groups of C4 plants have a functional Calvin cycle. Rubisco and several other Calvin cycle enzymes are absent from the mesophyll chloroplasts. 

QAs are biosynthesized in the green tissues of the plant, in the mesophyll chloroplasts, then transported via the phloem and stored in all organs of the plant, including seeds in which they cause the bitter taste. 

Acetyl-CoA carboxylase is recognised generally as being a key enzyme in acyl lipid formation. In plants it has been shown to be important as a regulatory enzyme in light-driven lipid synthesis [1] and has a high flux control coefficient under such conditions [2]. Recently, characterisation of different isoforms of acetyl-CoA carboxylase from plants has been made. In Poaceae such as maize there are two multifunctional proteins [3]. By contrast, the dicotyledon pea contains a multienzyme complex form of acetyl-CoA carboxylase in the mesophyll chloroplasts but a multifunctional protein Isoform In the cytosol of epidermal cells [4]. 

Huber SC and Edwards GE (1977). Transport in C4-mesophyll chloroplasts. Characterization of a pymvate carrier. Biochim. Biophys. Acta 462 583-602. 

NADP-malate dehydrogenase in green leaves is regulated in vivo in a manner similar to the other key mesophyll chloroplast enzyme, pyruvate phosphate-dikinase, i.e. it is rapidly inactivated in the dark and reactivated upon illumination." Maize plants kept in darkness... 

CONCENTRATION OF LONG-CHAIN ACYL-(ACYL-CARRIER PROTEIN) IN MESOPHYL CHLOROPLASTS FROM THE CHILLING-SENSITIVE PLANT AMARANTHUS LIVIDUS L... 

Based on this information and assuming a general stromal volume of 25 ul/nng Chi (6) the chloroplast levels of laoth metabolites have Iseen estimated (Table I, B) and compared with the substrate requirements of ACS and PDC from kinetic measurements in rapidly prepared stromal extracts (Table II, la). These calculations point to a predominant role of acetate in acetyl-CoA synthesis of spinach chloroplasts and to an increasing involvement of pyruvate in pea and maize mesophyll chloroplasts (6). The recently observed, light-driven pyruvate uptake by maize mesophyll chloroplasts (11) even favors PDC activities within these organelles. 

Tremolieres A, Dainese P, Bassi R. Heterogenous lipid distribution among chlorophyll-binding proteins of photosystem II in maize mesophyll chloroplasts. Eur J Biochem 1994 221 721-730. 

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