Plastid interconversions

Although chloroplast differentiation from proplastids, as directed by light, appears the central tenet of plastid biogenesis, there are many examples in which plastids can redifferentiate from pre-existing plastid types and form a different type of plastid (Fig. 2). Such interconversions are controlled by cellular developmental processes as well as environmental or hormonal signals and demonstrate an extreme plasticity in the plastid's functionality within the cell. Although several of these interconversion processes have been described, little is known of the exact molecular control of such redifferentiation processes. The best studied interconversion is that of chloroplasts redifferentiating into chromoplasts during fruit ripening, as discussed previously in this article. In tomatoes and peppers, the chro-moplast differentiation pathway has a clear endpoint in mature ripe fruit, but in other systems such as orange citrus fruit ripening and maturation of pumpkins, the

Plastids Interconversion
Fig. 2. A general scheme for interconversions of plastid types in different plant tissues. Although various routes for plastid interconversions are arrowed, it is likely that in various specific instances in different tissues, the majority of plastid types can interconvert to a different type.

orange chromoplasts are capable of reverting back to green chloroplasts. Application of the hormone gibberellin further promotes this process in oranges (Thomson et al. 1967). Another plastid redifferentiation pathway, which has major agronomic consequences, is the formation of chloroplasts from amyloplasts in the tissues of potato tubers as a result of illumination (Virgin and Sundquist 1992; Lju-bicic et al. 1998). Although significant efforts are made to prevent such tuber greening during potato storage, the reason why this amyoplast-chloroplast interconversion is enabled in potato storage cells but prevented in other amyloplasts containing tissues, such as endosperm, is unclear. In reality, plastid interconversion is a common process during the development of complex tissues. For instance during the development of the anther, there is a complex pattern of interconver-

sion between proplastids, amyloplasts, chromoplasts, chloroplasts, leucoplasts, and elaioplasts which varies in its nature according to the specific tissue type within the anther (Clement and Pacini 2001). In the face of such complex interactions, it could be more prudent to consider that no plastid differentiation pathways are terminal and that all plastids have the ability to change between different states according to the precise information derived from the cell.

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