How Leaves Turn Yellow: Catabolism of Chlorophyll

This chapter discusses the catabolism of chlorophyll and regulation and significance of chlorophyll breakdown. The green color of chlorophyll (Chl) is due to the conjugated system of π electrons in the tetrapyrrolic macrocycle and, the loss of green color is due to its abolition. Pheide a oxygenase (PaO) pathway of Chl breakdown in senescing leaves extends over several subcellular compartments, starting with lipoprotein complexes in the thylakoids and ending with non-fluorescing Chl catabohtes (NCCs) in the vacuoles. In the course of senescence, chloroplasts (Cpls) differentiate into gerontoplasts (Gpls). Development of Gpls is controlled by nuclear genes. Regulation of Chl breakdown is associated with the expression of PaO. This oxygenase is the only catabolic enzyme whose activity is present exclusively in Chl-degrading tissues and is positively correlated with rates of Chl breakdown. In various stay-green mutants, a deficiency in the catabolic system causes a marked retention of Chl and also of apoproteins of pigment-protein complexes in the thylakoids.