Abstract
Plastoglobules are globular compartments found in plastids. Before initial proteomic studies were published, these particles were often viewed as passive lipid droplets whose unique role was to store lipids coming from the thylakoid turn-over, or to accumulate carotenoids in the chromoplasts. Yet, two proteomic studies, published concomitantly, suggested for the first time that plastoglobules are more than “junk cupboards” for lipids. Indeed, both studies demonstrated that plastoglobules do not only include structural proteins belonging to the plastoglobulin/fibrillin family, but also contain active enzymes. The specific plastoglobule localization of these enzymes has been confirmed by different approaches such as immunogold localization and GFP protein fusions, thus providing evidence that plastoglobules actively participate in diverse pathways of plastid metabolism. These proteomic studies have been the basis for numerous recent works investigating plastoglobule function. However, a lot still needs to be discovered about the molecular composition and the role of plastoglobules. In this chapter, we will describe how the proteomic approaches have launched new perspectives on plastoglobule functions.
Highlights
In addition to the network of thylakoid membranes which are the site of photosynthesis, plastids contain in their soluble phase, the stroma, some enigmatic lipoprotein bodies named the plastoglobules
Plastoglobules are thought to be lipid reservoirs in greening tissue (Kessler et al, 1999), allowing the rapid formation of thylakoids. For example they may be involved in the formation of thylakoid membranes in de-etiolating plastids: etioplasts with poorly developed thylakoids have more plastoglobules than chloroplasts, but the plastoglobule abundance decreases during thylakoid biogenesis (Sprey and Lichtenthaler, 1966; Lichtenthaler and Peveling, 1967; Lichtenthaler, 1968)
Our conception of the plastoglobules has dramatically changed from insipid passive lipid droplets inside plastids to particles with active role at the crossroad of diverse metabolic pathways, for example in vitamin biosynthesis
Summary
In addition to the network of thylakoid membranes which are the site of photosynthesis, plastids contain in their soluble phase, the stroma, some enigmatic lipoprotein bodies named the plastoglobules (cf. Figure 1). Numerous studies have described an increase of plastoglobule size and/or number under various environmental conditions (for review, see Bréhélin et al, 2007; Bréhélin and Kessler, 2008), such as drought (Rey et al, 2000), salt stress (Locy et al, 1996; Ben Khaled et al, 2003), or in the presence of heavy metals (Baszynski et al, 1980). Based on these ultrastructural observations, the involvement of the plastoglobules in plant responses to stress has been suggested, but biochemical or physiological evidence is missing. DECIPHERING THE NATURE AND ROLES OF PLASTOGLOBULES: FROM ULTRASTRUCTURAL BASED SPECULATIONS TO PROTEOMIC INDICATIONS The progress made in plant electron microscopy allowed the first descriptions of plastoglobules: Hodge et al (1955) observed the presence of “dense spherical bodies” in stroma of maize mesophyll chloroplasts while Falk (1960) reported the existence in Ficus elastica chloroplasts of “osmiophilic spheres” and “magnoglobuli” ranging from 0.13 to 2.5 μm in diameter. Menke (1962) stated that the chemical composition of the “spherical inclusions known as www.frontiersin.org
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