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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Chlorophyll Breakdown in Senescent Arabidopsis Leaves. Characterization of Chlorophyll Catabolites and of Chlorophyll Catabolic Enzymes Involved in the Degreening Reaction¹

Adriana Pruinská, Gaby Tanner², Sylvain Aubry, Iwona Anders, Simone Moser, Thomas Müller, Karl-Hans Ongania, Bernhard Kräutler, Ji-Young Youn, Sarah J. Liljegren and Stefan Hörtensteiner^*

Institute of Plant Sciences, University of Bern, CH–3013 Bern, Switzerland (A.P., G.T., S.A., I.A., S.H.); Institute of Organic Chemistry and Center of Molecular Biosciences, University of Innsbruck, A–6020 Innsbruck, Austria (S.M., T.M., K.-H.O., B.K.); and Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599 (J.-Y.Y., S.J.L.)

During senescence, chlorophyll (chl) is metabolized to colorless nonfluorescent chl catabolites (NCCs). A central reaction of the breakdown pathway is the ring cleavage of pheophorbide (pheide) a to a primary fluorescent chl catabolite. Two enzymes catalyze this reaction, pheide a oxygenase (PAO) and red chl catabolite reductase. Five NCCs and three fluorescent chl catabolites (FCCs) accumulated during dark-induced chl breakdown in Arabidopsis (Arabidopsis thaliana). Three of these NCCs and one FCC (primary fluorescent chl catabolite-1) were identical to known catabolites from canola (Brassica napus). The presence in Arabidopsis of two modified FCCs supports the hypothesis that modifications, as present in NCCs, occur at the level of FCC. Chl degradation in Arabidopsis correlated with the accumulation of FCCs and NCCs, as well as with an increase in PAO activity. This increase was due to an up-regulation of Pao gene expression. In contrast, red chl catabolite reductase is not regulated during leaf development and senescence. A pao1 knockout mutant was identified and analyzed. The mutant showed an age- and light-dependent cell death phenotype on leaves and in flowers caused by the accumulation of photoreactive pheide a. In the dark, pao1 exhibited a stay-green phenotype. The key role of PAO in chl breakdown is discussed.

² Present address: Institute of Medical Molecular Genetics, University of Zurich, Schorenstrasse 16, CH–8603, Schwerzenbach, Switzerland.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.065870.

^* Corresponding author; e-mail shorten{at}ips.unibe.ch; fax 41–31–631–49–42.

Received May 24, 2005; returned for revision June 16, 2005; accepted June 20, 2005.

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