Recent advances in the cell biology of chlorophyll catabolism

Chlorophyll (chl) catabolism occurs throughout the plant life cycle, in germination, vegetative growth and reproductive development. It is highly sensitive to the environment, varying markedly with the season and in response to both biotic and abiotic stresses. On a global scale, chl breakdown in aquatic systems is quantitatively and ecologically at least as significant as the more visible processes that occur in terrestrial environments. The catabolic pathway that accounts for the disappearance of chl from green tissues during senescence, ripening and other terminal developmental events leads to the accumulation of colourless products (non fluorescent chl catabolites or NCCs. The chemistry of NCCs and the coloured and fluorescent intermediates that precede them is described. The enzyme sequence responsible for NCC formation comprises activities that convert chl a to chl b. Chi a is dephytylated by chlorophyllase (chlase), and a dechelatase activity removes Mg from chlorophyllide (chlide) a to produce phaeophorbide (phaeide) a Phaeide a oxygenase (PaO), the key enzyme in degreening, opens the tetrapyrrole macrocycle to produce a red bilin, RCC. In a channelled reaction, a reductase normally converts RCC immediately to a colourless fluorescent product, FCC. Further modifying enzymes, differing in different species, are responsible for metabolizing FCC to NCCs. Current biochemical understanding of the pathway, its relationship to chl biosynthesis and the significance of other possible catabolic routes are discussed. The cellular organization of the PaO pathway is considered in terms of the traffic between membranes and compartments and the metabolism of pigment-binding proteolipids. Recently, genes for components of the pathway have been cloned, including chlases, RCC reductase and the ABC- type transporters responsible for loading FCCs into the vacuole. Mutants and other variants modifying chl catabolism are described in the context of genetic regulation of the process and approaches to isolating more genes for steps in the pathway and associated control elements. Finally, some ideas about the evolutionary significance of chl degradation are presented.