As cells within developing seedlings and developing plant organs differentiate, plastids embark on different patterns of differentiation according to the differentiation pathway that the cell itself takes. Proplastids have the ability to give rise to a variety of different types of plastid, which form in different types of tissue. Plas-tids can also interconvert between their different forms in many situations. Thus, for most plastid types, there are two different pathways by which they can arise: directly or by re-differentiation of an existing plastid type. Traditionally, characterisation and naming of different types of mature plastids has largely been based on the types of molecules they store or the types of pigments they accumulate, although this may not necessarily be the best system for plastid taxonomy since often plastids show a mixture of features from different types making precise naming difficult. Although distinct types of plastid differentiation do exist, a better system for their classification could be based on the biochemical and physiological properties or maybe the extent of their proteome or metabolome. Such a system could ease the difficulties by which plastids displaying intermediate pheno-types have to be named. In this chapter, the basic structure and morphology of the major types of differentiated plastids found in higher plants will be consider and subsequently, what is known of the differentiation pathways which give rise to each of the types will be discussed.
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