A New Chlorophyll Degradation Pathway

This is the authors' abstract. We don't add key points for this paper.

Chlorophyll degradation is vital during leaf senescence and fruit ripening, as it allows for recycling of nitrogen and other nutrients and for protection from buildup of phototoxic chlorophyll intermediates (Hörtensteiner, 2006). The first steps in chlorophyll breakdown are the removal of the phytol tail (dephytylation) and the central Mg atom. It has been thought that dephytylation typically occurs first, catalyzed by the enzyme chlorophyllase, which converts chlorophyll to phytol and chlorophyllide. Removal of Mg subsequently converts chlorophyllide to pheophorbide. However, Arabidopsis contains just two known chlorophyllases, CHL1 and CHL2, and it has been shown that neither of these isoforms is localized to plastids, and double knockout mutant plants still are able to degrade chlorophyll during leaf senescence (Schenk et al., 2007). Now, Schelbert et al. (pages 767–785) report the discovery of a novel plastid-localized enzyme, pheophytinase, that is essential for chlorophyll breakdown during leaf senescence. Deficiency of PPH causes a stay-green phenotype. Leaves after 5 d of dark-induced senescence are shown, left to right: wild type (Col-0), pph-1 mutant, complementation of the stay-green phenotype of pph-1 with a 35S-PPH cDNA construct, and lack of complementation using a construct harboring a mutation of the proposed active-site Ser residue (35S-PPHS221A). (Image from Figure 2 of Schelbert et al. [2009].) This work indicates that the previously accepted pathway of chlorophyll degradation must be revised. During leaf senescence, removal of Mg to form pheophytin is likely the first step, followed by removal of the phytol tail, catalyzed by PPH. Chlorophyllide, which is the last precursor of chlorophyll biosynthesis, is most likely not an intermediate of breakdown. Therefore, chlorophyll synthesis and breakdown are metabolically separated during leaf senescence. Based on patterns of expression, chlorophyllase may play a role in chlorophyll breakdown during fruit ripening (Azoulay Shemer et al., 2008) and response to pathogens and wounding (Kariola et al., 2005).