The stay-green revolution: Recent progress in deciphering the mechanisms of chlorophyll degradation in higher plants

Plants control the abundance of pigments within their cells depending on the developmental stage of particular tissues or organs and in response to environmental cues. Chlorophyll is degraded in plants as a result of natural or induced senescence, at the onset of fruit ripening, and in response to pathogen infection. Many of the steps that contribute to chlorophyll degradation, including the identity of breakdown intermediates and the enzymes that catalyze their conversions have been identified during the last decade. In particular, knowledge at the molecular level has advanced considerably over the past two years with the identity of the long sought STAY-GREEN (SGR/SID) gene revealed, together with the realization that several stay-green mutants of various species are also defective in SGR orthologs. Genetic analysis of additional stay-green mutants of rice has also led to the discovery of chlorophyll b reductase. Furthermore, recent functional analysis of putative chlorophyllase knockout mutants of Arabidopsis found that they were only very slightly compromised in their ability to degrade chlorophyll and did not have reduced chlorophyllase activity. These data suggest the need to re-examine the initial step of the chlorophyll degradation pathway. This review focuses on these discoveries and other recent research that has helped to shape our current understanding of chlorophyll degradation.