Chlorophyll breakdown is an important catabolic process of leaf senescence and fruit ripening. Structure elucidation of colorless linear tetrapyrroles as (final) breakdown products of chlorophyll was crucial for the recent delineation of a chlorophyll breakdown pathway which is highly conserved in land plants. Pheophorbide a oxygenase is the key enzyme responsible for opening of the chlorin macrocycle of pheophorbide a characteristic to all further breakdown products. Degradation of chlorophyll was rationalized by the need of a senescing cell to detoxify the potentially phototoxic pigment, yet recent investigations in leaves and fruits indicate that chlorophyll catabolites could have physiological roles. This review updates structural information of chlorophyll catabolites and the biochemical reactions involved in their formation, and discusses the significance of chlorophyll breakdown. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

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.Chlorophyll (chl) degradation is an integral part of leaf senescence and fruit ripening. The fate of chl during senescence has been well established in recent years (for review, see Matile et al., 1999; Hörtensteiner, 1999; Hö rtensteiner and Kräutler, 2000;Kräutler, 2003;Eckhardt et al., 2004). Thereby, chl is converted to colorless nonfluorescent chl catabolites (NCCs; Fig. 1) in a pathway that is probably active in all higher plants (Pružinská et al., 2003;Gray et al., 2004). Structure elucidation of NCCs from different species has unraveled a common tetrapyrrolic skeleton with an oxygenolytically opened porphyrin macrocycle (Kräutler, 2003). Peripheral modifications at several side chains within different NCCs (Fig. 1, R 1 -R 3 ) are species specific (Berghold et al., 2002(Berghold et al., , 2004, and hence have been proposed to occur rather late in the pathway (Hö rtensteiner, 1999). Indeed, a primary chl breakdown product (primary fluorescent chl catabolite-1 [pFCC-1]), which exhibits a blue fluorescence, could be identified as a common product of porphyrin ring cleavage ( Fig. 1; Mü hlecker et al., 1997). Thus, the sequence of reactions is the removal of phytol and magnesium (Mg) by chlorophyllase and Mg-dechelatase, respectively, followed by the conversion of pheophorbide (pheide) a to pFCC-1, which requires the activity of two enzymes, pheide a oxygenase (PAO) and red chl catabolite (RCC) reductase (RCCR; Rodoni et al., 1997;Hö rtensteiner, 1999).PAO is a chloroplast envelope-bound Rieske-type iron-sulfur oxygenase, which is identical to lethal leaf spot 1 (LLS1) from maize (Zea mays) and accelerated cell death 1 (ACD1) from Arabidopsis (Arabidopsis thaliana; Pružinská et al., 2003;Yang e...

Fall colors have always been fascinating and are still a remarkably puzzling phenomenon associated with the breakdown of chlorophyll (Chl) in leaves. As discovered in recent years, nongreen bilin‐type Chl catabolites are generated, which are known as the phyllobilins. Collaborative chemical‐biological efforts have led to the elucidation of the key Chl‐breakdown processes in senescent leaves and in ripening fruit. Colorless and largely photoinactive phyllobilins are rapidly produced from Chl, apparently primarily as part of a detoxification program. However, fluorescent Chl catabolites accumulate in some senescent leaves and in peels of ripe bananas and induce a striking blue glow. The structural features, chemical properties, and abundance of the phyllobilins in the biosphere suggest biological roles, which still remain to be elucidated.

regeneration were carried out as described.I7. Developing shoots were selected on 100 pgmL-' kanamycin.Fifteen kanamycin resistant plants were tested for cyanamide hydratase a~tivity!~] Enzyme activity was found in all plants, the level of expression ranging from 0.03 to 0.79 units per mg protein in the cell extract. The highest value corresponds to about a fifth of the specific activity measured in induced Myrothecium verrucaria extracts. Although the specific activity was highest in the roots, the total enzyme activity per gram fresh weight was distinctly higher in the leaves than in the other plant organs (Table 1). No activity was found in plants transgenic for cah-.

Senescent higher plants degrade their chlorophylls (Chls) to polar colorless tetrapyrrolic Chl catabolites, which accumulate in the vacuoles. In extracts from degreened leaves of the tree Cercidiphyllum japonicum an unpolar catabolite of this type was discovered. This tetrapyrrole was named Cj-NCC-2 and was found to be identical with the product of a stereoselective nonenzymatic isomerization of a ''fluorescent'' Chl catabolite. This (bio-mimetic) formation of the ''nonfluorescent'' catabolite Cj-NCC-2 took place readily at ambient temperature and at pH 4.9 in aqueous solution. The indicated nonenzymatic process is able to account for a crucial step during Chl breakdown in senescent higher plants. Once delivered to the acidic vacuoles, the fluorescent Chl catabolites are due to undergo a rapid, stereoselective isomerization to the ubiquitous nonfluorescent catabolites. The degradation of the Chl macrocycle is thus indicated to rely on just two known enzymes, one of which is senescence specific and cuts open the chlorin macroring. The two enzymes supply the fluorescent Chl catabolites, which are ''programmed'' to isomerize further rapidly in an acidic medium, as shown here. Indeed, only small amounts of the latter are temporarily observable during senescence in higher plants.

Nonfluorescent chlorophyll catabolites (NCCs) were described as products of chlorophyll breakdown in Arabidopsis thaliana. NCCs are formyloxobilin-type catabolites derived from chlorophyll by oxygenolytic opening of the chlorin macrocycle. These linear tetrapyrroles are generated from their fluorescent chlorophyll catabolite (FCC) precursors by a nonenzymatic isomerization inside the vacuole of senescing cells. Here, we identified a group of distinct dioxobilin-type chlorophyll catabolites (DCCs) as the major breakdown products in wild-type Arabidopsis, representing more than 90% of the chlorophyll of green leaves. The molecular constitution of the most abundant nonfluorescent DCC (NDCC), At-NDCC-1, was determined. We further identified cytochrome P450 monooxygenase CYP89A9 as being responsible for NDCC accumulation in wild-type Arabidopsis; cyp89a9 mutants that are deficient in CYP89A9 function were devoid of NDCCs but accumulated proportionally higher amounts of NCCs. CYP89A9 localized outside the chloroplasts, implying that FCCs occurring in the cytosol might be its natural substrate. Using recombinant CYP89A9, we confirm FCC specificity and show that fluorescent DCCs are the products of the CYP89A9 reaction. Fluorescent DCCs, formed by this enzyme, isomerize to the respective NDCCs in weakly acidic medium, as found in vacuoles. We conclude that CYP89A9 is involved in the formation of dioxobilin-type catabolites of chlorophyll in Arabidopsis.

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SummaryEukaryotic microalgae and prokaryotic cyanobacteria are the major components of the phytoplankton. Determining factors that govern growth of these primary producers, and how they interact, is therefore essential to understanding aquatic ecosystem productivity. Over half of microalgal species representing marine and freshwater habitats require for growth the corrinoid cofactor B12, which is synthesized de novo only by certain prokaryotes, including the majority of cyanobacteria. There are several chemical variants of B12, which are not necessarily functionally interchangeable. Cobalamin, the form bioavailable to humans, has as its lower axial ligand 5,6-dimethylbenzimidazole (DMB). Here, we show that the abundant marine cyanobacterium Synechococcus synthesizes only pseudocobalamin, in which the lower axial ligand is adenine. Moreover, bioinformatic searches of over 100 sequenced cyanobacterial genomes for B12 biosynthesis genes, including those involved in nucleotide loop assembly, suggest this is the form synthesized by cyanobacteria more broadly. We further demonstrate that pseudocobalamin is several orders of magnitude less bioavailable than cobalamin to several B12-dependent microalgae representing diverse lineages. This indicates that the two major phytoplankton groups use a different B12 currency. However, in an intriguing twist, some microalgal species can use pseudocobalamin if DMB is provided, suggesting that they are able to remodel the cofactor, whereas Synechococcus cannot. This species-specific attribute implicates algal remodelers as novel and keystone players of the B12 cycle, transforming our perception of the dynamics and complexity of the flux of this nutrient in aquatic ecosystems.