The Diversity of Plastid Form and Function

Plastids are semiautonomous organelles found, in one form or another, in practically all plant and algal cells, several taxa of marine mollusks and at least one phylum of parasitic protists. The members of the plastid family play pivotal roles in photosynthesis, amino acid and lipid synthesis, starch and oil storage, fruit and flower coloration, gravity sensing, stomatal functioning, and environmental perception. Plastids arose via an endosym biotic event in which a protoeukaryotic cell engulfed and retained a photosynthetic bacterium. This polyphyletic event occurred multiple times between roughly 1.5 to 1.6 billion years ago. Although most of the algal genes were transferred to the nuclear genome, plastids have retained a complete protein synthesizing machinery and enough information to code for about 100 of their approximately 2,500 proteins; all other plastid proteins are coded for by the nuclear genome and imported from the cytoplasm. Plastids divide via fission prior to cytokinesis and are equally apportioned between the two daughter cells, along with the rest of the cytoplasmic contents.