The import and export business in plastids: transport processes across the inner envelope membrane.

Plastids are the characteristic organelles of plant and algae cells and are, like mitochondria, of endosymbiontic origin. They can be traced back to a single cyanobacterial ancestor that was engulfed by a primitive protist about 1.5 billion years ago (Yoon et al., 2004). Higher plants possess different types of plastids, among them undifferentiated proplastids, colorless etioplasts, green photosynthetically active chloroplasts, colored chromoplasts that are responsible for the yellow, orange, and red colors of flowers and fruits, starch-storing amyloplasts, and lipid-storing elaioplasts. The differentiation into these plastid types depends on the tissue and the developmental stage of the plant. Plastids show a tremendous biochemical versatility. They are the site of carbon, sulfur, and nitrogen assimilation, and they synthesize fatty acids, many amino acids, and a plethora of other primary and secondary metabolites. It is, therefore, reasonable to consider plastids as metabolic factories of plant cells. Plastids of red and green algae and of higher plants are enclosed by two membranes, the outer and inner envelope membranes that separate the stroma from the cytosol. The integration of plastids into the metabolism of plant cells is achieved by a large set of proteins transporting numerous inorganic and organic molecules across both membranes. Transport across the outer envelope membrane is enabled by a set of b-barrel proteins that form selective pores (Bölter and Soll, 2001). In contrast, the transport across the inner envelope is catalyzed by a large set of a-helical membrane transporters, which are highly specific for their substrates. In this Update, I will give a short overview of the current knowledge of transport processes across the plastid inner envelope membrane and of the approaches used for the identification and characterization of plastid membrane transporters. Furthermore, I will discuss the evolution of these proteins and future challenges and opportunities in the field. For more detailed information about particular transporters, the reader is referred to several comprehensive reviews covering plastid transport processes (e.g. Weber and Fischer, 2007; Linka and Weber, 2010). FROM THE BEGINNINGS OF PLASTID TRANSPORT TOWARD THE PERMEOME OF PLASTIDS