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This article is a Plant Cell Advance Online Publication. The date of its first appearance online is the official date of publication. The article has been edited and the authors have corrected proofs, but minor changes could be made before the final version is published. Posting this version online reduces the time to publication by several weeks. IN BRIEF Chlorophyll Breakdown Branches Out: Identification of a Major Catabolic Route Involving Cytochrome P450 CYP89A9 At the end of their developmental lifespan, leaves put their affairs in order via the tightly regulated process of senescence. One of the most complicated “affairs” that needs to be dealt with is the disposal of chlorophyll. Central to photosynthesis during the life of the leaf, and present in large amounts, chlorophyll can be phototoxic and is thus broken down through an active catabolic pathway (reviewed in Hörtensteiner and Kräutler, 2011). The canonical pathway for chlorophyll catabolism involves conversion of chlorophyll b to chlorophyll a, removal of the phytol side chain and central magnesium ion, breakage of the chlorin ring, and further enzymatic degradation, forming a series of products termed FCCs (fluorescent chlorophyll catabolites). The cell finally commits these FCCs to the deep, which for plants does not mean burial at sea, but rather means interment in the vacuole. There, FCCs undergo non-enzymatic isomerization to NCCs (nonfluorescent chlorophyll catabolites). In addition to its function in hazardous waste disposal, chlorophyll breakdown remobilizes nitrogen, and emerging research hints that chlorophyll breakdown also plays roles in stress responses, disease resistance, and fruit ripening, although these functions remain to be explored. The chlorophyll breakdown pathway may not be as simple as we thought (reviewed in Hörtensteiner, 2012). For example, hypermodified FCCs in banana and other plants persist rather than being converted to NCCs. Also, although the canonical pathway involves formation of FCCs, which then form NCCs, Christ et al. (2013) examine Arabidopsis thaliana and identify a distinct group ofchlorophyll catabolites (see figure) called nonfluorescent dioxobilin-type chlorophyll catabolites (NDCCs). These NDCCs represent more than 90% of the chlorophyll in the green leaf, showing that they are the major chlorophyll breakdown products in Arabidopsis. Similar to FCCs, the NDCCs arise from nonenzymatic isomerization of FDCCs (fluores- www.plantcell.org/cgi/doi/10.1105/tpc.113.250513 Cytochrome P450 CYP89A9 catalyzes a novel route in chlorophyll breakdown. Chlorophyll (green) is sequentially broken down into fluorescent catabolites (blue) including FCC, which is converted to FDCC by CYP89A9 (red). FCC and FDCC are non-enzymatically converted into nonfluorescent catabolites (black) NCC and NDCC, respectively. [Modified from Christ et al., 2013; Figure 1] cent DCCs). The authors determine the molecular constitution of At-NDCC-1, the most abundant of the Arabidopsis NDCCs. The authors also identify the responsible enzyme, finding that cytochrome P450 monooxygenase CYP89A9 is required for accumulation of NDCCs. They first implicate P450s, finding that treatment with CO, which inhibits P450s, results in reduced accumulation of NDCCs and increased NCCs. CYP89A9 was identified as a candidate based on its increased expression during senescence and confirmed by examination of cyp89a9 mutants, which lack NDCCs but accumulate more NCCs. Biochemical analysis of recombinant CYP89A9 showed that it catalyzes an unusual reaction, oxidatively deformylating FCCs to form FDCCs (see figure). Intriguingly, CYP89A9 does not localize to chloroplasts, but likely localizes to the endoplasmic reticulum, indicating a potential route for chloroplast metabolites and adding evidence to previous hints of a chloroplastendoplasmic reticulum connection. The identification of this reaction and the responsible enzyme provides an unexpected branch in the known, linear chlorophyll-breakdown pathway; examining the functional and evolutionary implications of this branch will provide an interesting avenue for future research. Jennifer Mach Science Editor [email protected] REFERENCES Christ, B., Süssenbacher, I., Moser, S., Bichsel, N., Egert, A., Müller, T., Kräutler, B., and Hörtensteiner, S. (2013). Cytochrome P450 CYP89A9 is involved in the formation of major chlorophyll catabolites during leaf senescence in Arabidopsis thaliana. Plant Cell 25: doi/ 10.1105/tpc.113.112151. Hörtensteiner, S. (2012). Update on the biochemistry of chlorophyll breakdown. Plant Mol. Biol. doi/10.1007/s11103-012-9940-z. Hörtensteiner, S., and Kräutler, B. (2011). Chlorophyll breakdown in higher plants. Biochim. Biophys. Acta 1807: 977–988. The Plant Cell Preview, www.aspb.org ã 2013 American Society of Plant Biologists. All rights reserved. 1 of 1 Chlorophyll Breakdown Branches Out: Identification of a Major Catabolic Route Involving Cytochrome P450 CYP89A9 Jennifer Mach Plant Cell; originally published online May 30, 2013; DOI 10.1105/tpc.113.250513 This information is current as of June 14, 2017 Permissions https://www.copyright.com/ccc/openurl.do?sid=pd_hw1532298X&issn=1532298X&WT.mc_id=pd_hw1532298X eTOCs Sign up for eTOCs at: http://www.plantcell.org/cgi/alerts/ctmain CiteTrack Alerts Sign up for CiteTrack Alerts at: http://www.plantcell.org/cgi/alerts/ctmain Subscription Information Subscription Information for The Plant Cell and Plant Physiology is available at: http://www.aspb.org/publications/subscriptions.cfm © American Society of Plant Biologists ADVANCING THE SCIENCE OF PLANT BIOLOGY
