The Plastid

by Ian Tetlow & Mark Burrell

Department of Molecular & Cellular Biology, College of Biological Sciences,
University of Guelph. ON N1G 2W1 Canada.

GFP targeted plastids in an Arabidopsis plant exhibiting stromules.
Plastids are sub-cellular self-replicating organelles present in all living plant cells, and the exclusive site of many important biological processes, the most fundamental being the photosynthetic fixation of CO2 within chloroplasts, a process which is vital to life on earth. In addition, plastid metabolism is responsible for generating economically important raw materials and commodities such as starches and oils, as well as improving the nutritional status of many crop-derived products [1].
All plastids are enclosed by two membranes, the outer and the inner envelope membrane. The outer membrane represents a barrier to the movement of proteins, whilst the inner membrane is the actual permeability barrier between the cytosol and the plastid stroma and the site of specific transport systems connecting both compartments. Plastids are highly compartmentalized organelles, and certain plastid types (chloroplasts and chromoplasts) have been shown to contain lipoprotein particles termed plastoglobules which act as a lipid store during internal membrane biogenesis [2], and may also be the site for important steps in the tocopherol (vitamin E) biosynthetic pathway [3]. A number of plastid types have also been shown to initiate protrusions from the proplastid surface [4] which may be regulated by the number of plastids per cell. These protrusions, termed stromules (white arrowheads in Image-1), have also been shown to be the site of starch granule formation in non-photosynthetic plastids of wheat endosperm [5]. Many of the primary metabolic pathways are shared within different types of plastids, but perform different functions within them and are usually associated with the localization of the plastid within a specialized tissue/organ. For example, starches made inside non-photosynthetic plastids in developing seeds act as a long-term store for the next generation, whereas starches produced in leaf chloroplasts are strictly temporary carbon stores.

The classification of different plastid types is usually based on their internal structure and origin [6]. The major plastid types found in higher plants are as follows:

Proplastids (or eoplasts), are the progenitors of other plastids; these colourless plastids occur in the meristematic cells of shoots, roots, embryos, and endosperm and have no distinctive morphology, varying in shape and sometimes contain lamellae and starch granules.

Chloroplasts are the site of the photochemical apparatus and possess a distinctive internal membrane organization of thylakoid discs. The chlorophyll pigments and light reactions of photosynthesis are associated with the thylakoid membrane system. These green, lens-shaped organelles are present in all photosynthetic tissues and organs such as leaves, storage cotyledons, seed coats, embryos and the outer layers of unripe fruits. (Wikipedia : Kimball's Biology Pages)

Chromoplasts
are red-, orange-, and yellow-coloured plastids containing relatively high levels of carotenoid pigments and are commonly found in flowers, fruits, senescing leaves (also termed gerontoplasts) and certain roots. Chromoplasts often develop from chloroplasts, but may also be formed from proplastids and amyloplasts (see below). Carotenoid synthesis and/or storage in chromoplasts occur within plastoglobules, filamentous pigmented bodies, and crystals [7]. Starch is often present early in development and lost as the chromoplasts mature [8].

Etioplasts are found in leaf cells which are grown in continuous darkness, appearing yellow due to the presence of protochlorophyll, and are therefore not a normal stage of development of chloroplasts. Etioplasts are structurally simple possessing distinctive crystalline centres known as prolamellar bodies. Upon exposure to light etioplasts rapidly differentiate into chloroplasts, during which the protochlorophyll becomes converted into chlorophyll and the prolamellar body reorganizes into grana and stromal lamellae.

Leucoplasts are colourless plastids which are distinct from proplastids in that they have lost their progenitor function. Within this group are amyloplasts, elaioplasts/oleoplasts and proteinoplasts which are the sites of synthesis of starch, lipids and proteins respectively. • Wikipedia Link

Amyloplasts are characterized by the presence of one or more starch granules and are found in roots (where they may be involved in the detection of gravity) and storage tissues such as cotyledons, endosperm, and tubers. Amyloplasts are also capable of redifferentiating into other plastid types, for example in the re-greening of potato tubers where cell layers deep within the tuber undergo chloroplast formation [9].

Photoheterotrophic plastids are a specialized form of differentiated plastid which allows the embryos of certain seeds such as legumes to acquire a higher energy state. Photoheterotrophic plastids differ from leaf chloroplasts in both morphology and physiology [10]. Photoheterotrophic plastids import carbon, mainly in the form of glucose-6-phosphate, pyruvate and phosphoenolpyruvate, and in addition, are photosynthetically active [11]. Despite the low photosynthetic capacity of heterotrophic plastids [10], it is thought that photosynthetic CO2 fixation allows production of both energy-rich compounds and oxygen, which is especially important for embryogenesis in oxygen-limited tissues [12].
FP probes targeted to Plastids
FP probes for Plastids

References

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