Plant Chloroplasts and Other Plastids

Abstract

Plastids are a group of organelles that are characteristic of plant cells. They have derived in an endosymbiotic event from a cyanobacterial ancestor and still exhibit many prokaryotic features. Plastids are able to perform many specialised functions that are essential for plant growth and development, such as photosynthesis, nitrate and sulfate assimilation, the synthesis of amino acids and of fatty acids, storage of carbohydrates and lipids or the formation of colours in some fruits and flowers. To accomplish this, their membrane systems exert specialised transport functions, including the import and sorting of proteins and the exchange of metabolites in case of the two envelope membranes, as well as proton and electron transport in the case of the thylakoid membranes of chloroplasts. Moreover, plastids communicate with the nucleus by retrograde signalling to adjust the expression of nuclear genes according to the metabolic and developmental state of the organelle.

Key Concepts:

  • Plastids are of prokaryotic origin.

  • Several forms of plastids exist with different functions and composition.

  • Chloroplasts, the photosynthetically active plastids, are bounded like the other flowering plant plastids by a double membrane, but contain in addition the chlorophyll‐containing thylakoid membrane system, the site of photosynthesis.

  • Plastids are essential for the assimilation of nitrate and sulfate, as well as for the synthesis of aromatic compounds.

  • In contrast to animals, the major site of fatty acid biosynthesis in plants is within the plastid.

  • Plastids import most of their several thousand different proteins because the corresponding genes are located in the nucleus.

  • Various metabolites are imported in or exported from plastids through specific translocators within the envelope.

  • Plastids communicate with the nucleus by as yet unknown messenger molecules.

Keywords: plastid; plastid division; protein import; metabolite transport; photosynthesis; starch metabolism; retrograde signalling; lipid synthesis

Figure 1.

Chloroplasts are the site of photosynthesis and fixation of atmospheric carbon dioxide. Like other plastids, chloroplasts are double membrane‐bound organelles. In addition to the outer and the inner envelope membranes, chloroplasts contain chlorophyll‐containing thylakoid membranes, the site of photosynthesis. The products of the light reaction, ATP and NADPH, are used to convert atmospheric carbon dioxide into organic compounds by enzymes of the stroma‐localised Calvin–Benson cycle. ADP, adenosine diphosphate; ATP, adenosine triphosphate; NADPH, nicotinamide–adenine dinucleotide phosphate (reduced form).

Figure 2.

Interconversion of various types of plastids. All mature plastids derive from proplastids that are converted into etioplasts in leaves if plants are grown in the dark. Upon light, etioplasts develop into chloroplasts that can reversibly differentiate into chromoplasts in flowers and fruits. Mature plastids are highly interconvertible. For example, any type of plastid can be transformed into chromoplasts which are, in turn, able to regreen into chloroplasts. Leucoplasts (e.g. amyloplasts) of nongreen tissues and even fully developed gerontoplasts can be transformed back into chloroplasts.

Figure 3.

Starch metabolism in plastids. In chloroplasts, starch biosynthesis starts with triose phosphates provided by the Calvin cycle. In plastids of nongreen tissues, glucose 6‐phosphate (G6P), imported by the glucose 6‐phosphate/phosphate translocator (GPT) is the precursor for starch biosynthesis. ATP, required for the reaction of the ADP glucose pyrophosphorylase (AGPase), is imported by the adenylate translocator (NTT). The biosynthesis of amylose and amylopectin from ADPglucose involves the combined action of granule‐bound starch synthase (GBSS), soluble starch synthases (SSSs), starch branching enzymes (SBEs) and debranching enzymes (DBEs). Starch breakdown in leaves takes place at night and is initiated by phosphorylation of the granule surface by glucan, water dikinase (GWD) and phosphoglucan, water dikinase (PWD). Subsequently, starch breakdown is catalysed by β‐amylase (BAM) and isoamylase (ISA) into maltose which can be exported from the plastid. Linear glucans can also be broken down to glucose or glucose‐1‐phosphate by the action of disproportionating enzyme (DPE) and α‐glucan phosphorylase (PHS), respectively. Starch breakdown catalysed by α‐amylase is especially important in cereals and does not require the phosphorylation of the starch granule. F6P, fructose 6‐phosphate; G1P, glucose 1‐phosphate; TrioseP, triose phosphate; Pi, inorganic phosphate; ADP, adenosine diphosphate; ATP, adenosine triphosphate.

Figure 4.

Exchange of substrates between plastids and the cytosol to connect metabolic processes in both compartments. The net product of photosynthetic carbon dioxide fixation, triose phosphate (trioseP), is exported via the triose phosphate/phosphate translocator (TPT). Nitrogen assimilation requires the import of carbon (2‐oxoglutarate, 2‐OG, by the oxoglutarate/malate translocator, DiT1) and the export of amino acids (glutamate, Glu, by the glutamate/malate translocator, DiT2). Phosphoenolpyruvate (PEP), imported via the PEP/phosphate translocator (PPT), is the starting molecule for the shikimic acid pathway leading to the formation of aromatic amino acids that are exported via as yet unknown transporters. Xylulose 5‐phosphate is also imported from the cytosol via a Xyl 5‐P translocator (XPT) to support Calvin cycle activity under conditions where intermediates are withdrawn for other biosynthetic processes, such as the shikimic acid pathway. The biosynthesis of isoprenoids, for example carotenoids, requires the uptake of pyruvate that can also be used for the synthesis of fatty acids that can subsequently be exported from the plastids for use in other compartments. Fatty acid synthesis is also driven by acetate. The ATP/ADP translocator (NTT) provides ATP for green plastids used in chlorophyll biosynthesis during night, and nongreen plastids which use ATP for biosynthetic processes, for example starch biosynthesis. In amyloplasts, starch synthesis starts with glucose 6‐phosphate (G6P) that is imported via the glucose 6‐phosphate/phosphate translocator (GPT). G6P can also be exchanged for trioseP, the product of the oxidative pentose phosphate pathway (OPPP) that leads to the generation of reducing power in form of NADPH. In some grasses, starch can alternatively be synthesised from ADPglucose (ADPGlc) that is imported by an ADPGlc translocator. The products of starch degradation, for example glucose, are exported by a maltose translocator (MEX1) and hexose translocator (HT), respectively. GS/GOGAT, glutamine synthetase/glutamate synthase; Mal, malate; DOXP, deoxyxylulose 5‐phosphate; Pi, inorganic phosphate; ADP, adenosine diphosphate.

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Further Reading

Benning C (2009) Mechanisms of lipid transport involved in organelle biogenesis in plant cells. Annual Review of Cell and Developmental Biology 25: 71–91.

Block MA, Douce R, Joyard J and Rolland N (2007) Chloroplast envelope membranes: a dynamic interface between plastids and the cytosol. Photosynthesis Research 92: 225–244.

Gould SB, Waller RF and McFadden GI (2008) Plastid evolution. Annual Review in Plant Biology 59: 491–517.

Jarvis P (2008) Targeting of nucleus‐encoded proteins to chloroplasts in plants. New Phytologist 179: 257–285.

Kleine T, Voigt C and Leister D (2009) Plastid signalling to the nucleus: messengers still lost in the mists? Trends in Genetics 25: 185–192.

Lindahl M and Kieselbach T (2009) Disulphide proteomes and interactions with thioredoxin on the track towards understanding redox regulation in chloroplasts and cyanobacteria. Journal of Proteomics 72: 416–438.

Waters MT and Langdale JA (2009) The making of a chloroplast. EMBO Journal 28: 2861–2873.

Weber AP and Fischer K (2007) Making the connections – the crucial role of metabolite transporters at the interface between chloroplast and cytosol. FEBS Letters 581: 2215–2222.

Yang Y, Glynn JM, Olson BJ, Schmitz AJ and Osteryoung KW (2008) Plastid division: across time and space. Current Opinion in Plant Biology 11(6): 577–584.

Zeeman SC, Smith SM and Smith AM (2007) The diurnal metabolism of leaf starch. Biochemical Journal 401: 13–28.

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How to Cite close
Finkemeier, Iris, and Leister, Dario(Jun 2010) Plant Chloroplasts and Other Plastids. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001678.pub2]