The cell biology of tetrapyrroles: a life and death struggle

Tetrapyrroles such as chlorophyll and heme are co-factors for essential proteins involved in a wide variety of crucial cellular functions. Nearly 2% of the proteins encoded by the Arabidopsis thaliana genome are thought to bind tetrapyrroles, demonstrating their central role in plant metabolism. Although the enzymes required for tetrapyrrole biosynthesis are well characterized, there are still major questions about the regulation of the pathway, and the transport of tetrapyrroles within cells. These issues are important, as misregulation of tetrapyrrole metabolism can lead to severe photo-oxidative stress, and because tetrapyrroles have been implicated in signaling pathways coordinating interactions between plant organelles. In this review, we discuss the cell biology of tetrapyrrole metabolism and its implications for tetrapyrroles as signaling molecules.