Tetrapyrrole biosynthesis has been the subject of numerous studies over several decades. In recent years scientific interest in plant tetrapyrrole biosynthesis has included both genetical and biochemical elucidation of almost every enzymatic step of the pathway and has focused to an increasing extent on the regulatory mechanism of the entire metabolic pathway, but in particular of key steps, such as synthesis of 5-aminolevulinate, magnesium chelatase or protochlorophyllide oxidoreductase.