Plastid Structure, Diversification and Interconversions II. Land Plants

Abstract

In the course of endosymbiogenesis, the photosynthetic prokaryotes engulfed and retained by different heterotrophic organisms have partially lost their independence during evolution and became semi-autonomous organelles, the chloroplasts. The chloroplast represents the most ancestral form of plastids that has parallelly evolved in several algal groups (reviewed in [1]) and in land plants. After briefly discussing plastid morphology, we review the most important ultrastructural features of the plastids of land plants. Then we discuss how plastids got gradually specialized in parallel with the increasing developmental and/or organizational complexity of the plant body. The plastids of non-photosynthetic tissues and cells do not need to produce and maintain a photosynthetic apparatus, but have adjusted their metabolism to the major function of the host cell (and tissue). This way, different plastid forms specialized for other functions such as storage (e.g. starch storing leucoplasts called amyloplast) or carotenoid synthesis (chromoplasts) have developed. However, the classical ultrastructural characterization and classification of plastids is often problematic. First of all, the term plastid refers to the extremely high plasticity of this organelle, and its capacity to be readily transformed from one type into another one upon different environmental or developmental stimuli. Therefore, transitional (or if persistent, intermediate) plastids with morphological features characteristic for two different plastid types can be often observed. Sometimes plastids with similar ultrastructure can have different specific functions and basically different metabolism, and should be therefore treated separately. After having recalled the different plastid types of land plants we present a dynamic model about their interconversions.