The control of chlorophyll catabolism and the status of yellowing as a biomarker of leaf senescence

The pathway of chlorophyll catabolism during leaf senescence is known in a fair amount of biochemical and cell biological detail. In the last few years, genes encoding a number of the catabolic enzymes have been characterized, including the key ring-opening activities, phaeophorbide a oxygenase (PaO) and red chlorophyll catabolite reductase (RCCR). Recently, a gene that modulates disassembly of chlorophyll-protein complexes and activation of pigment ring-opening has been isolated by comparative mapping in monocot species, positional cloning exploiting rice genomics resources and functional testing in Arabidopsis. The corresponding gene in pea has been identified as Mendel's I locus (green/yellow cotyledons). Mutations in this and other chlorophyll catabolic genes have significant consequences, both for the course of leaf senescence and senescence-like stress responses, notably hypersensitivity to pathogen challenge. Loss of chlorophyll can occur via routes other than the PaO/RCCR pathway, resulting in changes that superficially resemble senescence. Such 'pseudosenescence' responses tend to be pathological rather than physiological and may differ from senescence in fundamental aspects of biochemistry and regulation.

Chlorophyll degradation during senescence

The catabolic pathway of chlorophyll (Chl) during senescence and fruit ripening leads to the accumulation of colorless breakdown products (NCCs). This review updates an earlier review on Chl breakdown published here in 1999 ( 69 ). It summarizes recent advances in the biochemical reactions of the pathway and describes the characterization of new NCCs and their formation inside the vacuole. Furthermore, I focus on the recent molecular identification of three chl catabolic enzymes, chlorophyllase, pheophorbide a oxygenase (PAO), and red Chl catabolite reductase (RCCR). The analysis of Chl catabolic mutants demonstrates the importance of Chl breakdown for plant development and survival. Mutants defective in PAO or RCCR develop a lesion mimic phenotype, due to the accumulation of breakdown intermediates. Thus, Chl breakdown is a prerequisite to detoxify the potentially phototoxic pigment within the vacuoles in order to permit the remobilization of nitrogen from Chl-binding proteins to proceed during senescence.

Chlorophyll breakdown in Chlorella protothecoides: characterization of degreening and cloning of degreening-related genes

Chlorella protothecoides cultures grown in a nitrogen-free bleaching medium (BM-N) in the dark rapidly degraded chlorophyll (Chl) to red catabolites. This degreening process was investigated under different growth conditions. Supply of nitrogen to the culture medium (BM+N) inhibited bleaching and the synthesis of catabolites as did the addition to BM-N of cycloheximide or a chelator, 2,2'-bipyridyl. In contrast, chloramphenicol or the protease inhibitor E64 had no effect. During bleaching, Chl breakdown was accompanied by the degradation of cellular proteins such as light-harvesting complex II, cytochrome f and protochlorophyllide oxido-reductase. During growth in BM-N, protease activity increased and proteins immunologically detectable with an antibody against a senescence-enhanced cysteine protease accumulated. cDNAs from BM-N and BM+N cells were used for differential and subtractive screening to isolate cDNAs representing genes with degreening-enhanced expression (dee) in C. protothecoides. Several different dees were identified with different patterns of expression during Chlorella growth but which were all expressed at higher levels during bleaching. Among these, dee4 was most abundant and its expression was exclusive in BM-N cultures. Analysis of the dee sequences showed that they encode different proteins including a novel amino acid carrier (dee4), ferritin, ATP-dependent citrate lyase, a Ca2+-binding protein, MO25, ubiquinone-cytochrome c-reductase and several new proteins.

Molecular cloning, functional expression and characterisation of RCC reductase involved in chlorophyll catabolism

Red chlorophyll catabolite (RCC) reductase (RCCR) and pheophorbide (Pheide) a oxygenase (PaO) catalyse the key reaction of chlorophyll catabolism, porphyrin macrocycle cleavage of Pheide a to a primary fluorescent catabolite (pFCC). RCCR was purified from barley and a partial gene sequence was cloned (pHvRCCR). The gene was expressed at all stages of leaf development and in roots. By comparison with different databases, genomic sequences and expressed sequence tags similar to RCCR were found in phylogenetically diverse species, and activity of RCCR was demonstrated in two of them, Arabidopsis thaliana and Marchantia polymorpha. The gene of A. thaliana (AtRCCR) was employed for molecular cloning, heterologous expression and the production of polyclonal antibodies. With recombinant RCCR, the major product of RCC reduction was pFCC-1, but small quantities of its C1 epimer, pFCC-2, also accumulated. The reaction required reduced ferredoxin and was sensitive to oxygen. AtRCCR encoded a 35 kDa protein which was used for chloroplast import experiments. Upon transport, it was processed to a mature form of 31 kDa. The significance of cloning of RCCR is discussed in respect to the evolution of chlorophyll catabolism and to the cloning of PaO.

Chlorophyll breakdown in oilseed rape

Chlorophyll catabolism accompanying leaf senescence is one of the most spectacular natural phenomena. Despite this fact, the metabolism of chlorophyll has been largely neglegted until recently. Oilseed rape has been used extensively as a model plant for the recent elucidating of structures of chlorophyll catabolites and for investigation of the enzymic reactions of the chlorophyll breakdown pathway. The key reaction which causes loss of green color is catalyzed in a two-step reaction by pheophorbide a oxygenase and red chlorophyll catabolite reductase. In this Minireview, we summarize the actual knowledge about catabolites and enzymes of chlorophyll catabolism in oilseed rape and discuss the significance of this pathway in respect to chlorophyll degradation during Brassica napus seed development.

Chlorophyll breakdown in higher plants and algae

Leaf senescence is accompanied by the metabolism of chlorophyll (Chl) to nonfluorescent catabolites (NCCs). The pathway of Chl degradation comprises several reactions and includes the occurrence of intermediary catabolites. After removal of phytol and the central Mg atom from Chl by chlorophyllase and Mg dechelatase, respectively, the porphyrin macrocycle of pheophorbide (Pheide) a is cleaved. This two-step reaction is catalyzed by Pheide a oxygenase and RCC reductase and yields a primary fluorescent catabolite (pFCC). After hydroxylation and additional species-specific modifications, FCCs are tautomerized nonenzymically to NCCs inside the vacuole. Different subcellular compartments participate in Chl catabolism and, thus, transport processes across membranes are required. This review focuses on the catabolites and the individual reactions of Chl breakdown in higher plants. In addition, the pathway is compared to Chl conversion to red catabolites in an alga, Chlorella protothecoides. Finally, the significance and regulation of Chl degradation are discussed.

NCC malonyltransferase catalyses the final step of chlorophyll breakdown in rape (Brassica napus)

Of the three final products of chlorophyll breakdown that in senescing cotyledons of oilseed rape are accumulated progressively, the nonfluorescent Bn-NCC-1 is the most abundant catabolite. It represents the malonylester of the minor catabolite Bn-NCC-3. The in vitro malonylation of Bn-NCC-3 into Bn-NCC-1 was investigated. Extracts from senescent as well as from presenescent cotyledons contained corresponding activities in the presence of malonyl-coenzyme A as the co-substrate. Malonyltransferase activity exhibited pH- and activation optima at 8 and 34 degrees, respectively, and it was saturable with an apparent Michaelis constant of 58 microM for Bn-NCC-3. The partially purified enzyme recognized chlorophyll catabolites as substrate specifically, provided that they had a free hydroxyl group in the ethyl side chain of pyrrole B.

The key step in chlorophyll breakdown in higher plants. Cleavage of pheophorbide a macrocycle by a monooxygenase

Chlorophyll breakdown in green plants is a long-standing biological enigma. Recent work has shown that pheophorbide a (Pheide a) derived from chlorophyll (Chl) is converted oxygenolytically into a primary fluorescent catabolite (pFCC-1) via a red Chl catabolite (RCC) intermediate. RCC, the product of the ring cleavage reaction catalyzed by Pheide a oxygenase, which is suggested to be the key enzyme in Chl breakdown in green plants, is converted into pFCC-1 by a reductase. In the present study, an in vitro assay comprising 18O2 Pheide a oxygenase and RCC reductase yielded labeled pFCC-1. Fast atom bombardment-mass spectrometric analysis of the purified pFCC-1 product revealed that only one of the two oxygen atoms newly introduced into Pheide a in the course of the cleavage reaction is derived from molecular oxygen. Analysis of the fragment ions located the oxygen atom derived from molecular oxygen on the formyl group of pyrrole B. This finding demonstrates that the cleavage of Pheide a in vascular plants is catalyzed by a monooxygenase. Chlorophyll breakdown is therefore indicated to be mechanistically related in higher plants and in the green alga Chlorella protothecoides.

An ABC-transporter of Arabidopsis thaliana has both glutathione-conjugate and chlorophyll catabolite transport activity

An ABC-transporter of Arabidopsis thaliana exhibiting high sequence similarity to the human (MRP1) and yeast (YCF1) glutathione-conjugate transporters has been analysed and used to complement a cadmium-sensitive yeast mutant (DTY168) that also lacks glutathione-conjugate transport activity. Comparison of the hydrophobicity plots of this A. thaliana MRP-like protein with MRP1 and YCF1 demonstrates that the transmembrane domains are conserved, even at the N-terminus where sequence identity is low. Cadmium resistance is partially restored in the complemented ycf1 mutant, and glutathione-conjugate transport activity can be observed as well. The kinetic properties of the A. thaliana MRP-like protein (AtMRP3) are very similar to those previously described for the vacuolar glutathione-conjugate transporter of barley and mung bean. Furthermore, a hitherto undescribed ATP-dependent transport activity could be correlated with the gene product, i.e. vesicles isolated from the complemented yeast, but not from DTY168 or the wild type, take up the chlorophyll catabolite Bn-NCC-1. The results indicate that the product of the MRP-like gene of A. thaliana is capable of mediating the transport of the two different classes of compounds.

How plants dispose of chlorophyll catabolites. Directly energized uptake of tetrapyrrolic breakdown products into isolated vacuoles

During the yellowing of leaves the porphyrin moiety of chlorophyll is cleaved into colorless linear tetrapyrrolic catabolites, which eventually are deposited in the central vacuoles of mesophyll cells. In senescent cotyledons of rape, Brassica napus, three nonfluorescent chlorophyll catabolites (NCCs), accounting for practically all the chlorophyll broken down, were found to be located in the vacuoles (vacuoplasts) prepared from protoplasts. Transport of catabolites across the tonoplast was studied with vacuoles isolated from barley mesophyll protoplasts in conjunction with a radiolabeled NCC, Bn-NCC-1, prepared from senescent rape cotyledons. The uptake of Bn-NCC-1 into vacuoles was against a concentration gradient and strictly dependent on MgATP and it followed saturation kinetics with a Km of approximately 100 microM. Although the hydrolysis of ATP was required, transport was apparently independent of the vacuolar proton pumps: accumulation of the NCC occurred both in the presence of the H+-ATPase inhibitor bafilomycin and after destroying the DeltapH between the vacuolar sap and the medium. ATP could be replaced by GTP or UTP, and the transport was inhibited in the presence of vanadate. Chlorophyll catabolites isolated from senescent barley leaves competed with the rape-specific substrate for uptake into the vacuoles. Compounds such as the glutathione conjugate of N-ethylmaleimide and taurocholate, which are known to be transported across the tonoplast in a primary active mode, did not significantly inhibit uptake of Bn-NCC-1. Although the heme catabolites biliverdin and bilirubin inhibited the uptake of the NCC, this effect is caused by unspecific binding to the vacuolar membrane rather than to the specific inhibition of carrier-mediated transport. Taken together, the results demonstrate that barley mesophyll vacuoles are constitutively equipped with a directly energized carrier that transports tetrapyrrolic catabolites of chlorophyll into the vacuole.

Early salt stress effects on the differential expression of vacuolar H(+)-ATPase genes in roots and leaves of Mesembryanthemum crystallinum

In Mesembryanthemum crystallinum, the salt stress-induced metabolic switch from C3 photosynthesis to Crassulacean acid metabolism is accompanied by major changes in gene expression. However, early effects of salt exposure (i.e. prior to Crassulacean acid metabolism induction) on genes coding for vacuolar transport functions have not yet been studied. Therefore, the expression of vacuolar H(+)-ATPase genes was analyzed in different organs of 4-week-old plants stressed with 400 mM NaCl for 3, 8, or 24 h. Partial cDNAs for the subunits A, B, and c were cloned and used as homologous probes for northern blot analysis. In control plants, the mRNA levels for the different subunits showed organ-specific differences. In fully expanded leaves, subunit c mRNA was very low but increased transiently during the light period. Plant organs also differed in their salt-stress response. In roots and young leaves, mRNA levels for all three subunits increased about 2-fold compared to control plants, whereas in fully expanded leaves only subunit c mRNA responded to salt. The results indicate that the expression of vacuolar H(+)-ATPase genes does not always involve a fixed stoichiometry of mRNAs for the different subunits and that the mRNA level for subunit c is particularly sensitive to developmental and environmental changes.

Factors affecting the re-formation of vacuoles in evacuolated protoplasts and the expression of the two vacuolar proton pumps

The re-formation of vacuoles in miniprotoplasts (evacuolated mesophyll protoplasts) of tobacco was investigated under different conditions. When a constant osmolarity was maintained, increasing the concentration of NaCl in the medium enhanced the regeneration of vacuoles compared to the control (0.5 M mannitol used as osmoticum). An enhanced growth rate of miniprotoplasts could also be observed under low-osmolarity conditions, by substitution of NaCl for KCl or NaNO3, or with different effectors (glycinebetaine and methyljasmonate). Using the polymerase chain reaction, one cDNA fragment of the B-subunit of the vacuolar ATPase and two fragments of the tonoplast-bound pyrophosphatase (PPase) of tobacco were cloned. Southern blot analyses indicates that for both proteins more than one gene is present in tobacco. During the regeneration of vacuoles the transcript level of the PPase increased earlier than that of the B-subunit of the vacuolar ATPase under all conditions tested (0.5 M mannitol, 0.3 M mannitol, and 0.25 M NaCl, respectively). Under salt-stress conditions (0.25 M NaCl used as osmoticum), the expression level of both proton pumps is enhanced compared to the control. This increase is not specifically due to salt stress but generally to an increased growth rate of the vacuole, since under low-osmolarity conditions the expression of the vacuolar pumps is enhanced, too.

Direct energization of bile acid transport into plant vacuoles

Bile acids were shown to be transported into barley mesophyll vacuoles. Uptake of the cholate conjugates taurocholate and glycocholate is strictly ATP-dependent. Uptake of taurocholate is a saturable process (Km = 40 microM) and is inhibited by vanadate but not by bafilomycin, a specific inhibitor of the vacuolar H(+)-ATPase. Together with the observation that the non-hydrolyzable ATP analog AMPPNP (5'-adenylyl beta,gamma-imidodiphosphate) does not stimulate, but rather inhibits, the ATP-dependent uptake of taurocholate, and that a 3-fold accumulation of the bile acid is observed in the presence of bafilomycin, these results suggest that taurocholate is transported into the vacuole by a primary active process as is the case for its canalicular secretion in rat liver (Nishida, T., Gatmaitan, Z., Che, M., and Arias, I. M. (1991) Proc. Natl. Acad. Sci. U. S. A. 88, 6590-6594). Taurocholate uptake is inhibited by other bile acids and is slightly stimulated by glutathione S-conjugates. The different responses of the glutathione S-conjugate (Martinoia, E., Grill, E., Tommasini, R., Kreuz, K., and Amrhein, N. (1993) Nature 364, 247-249) and the taurocholate transporters, respectively, to substrates, oligomycin, GTP, and UTP suggest the presence of at least two ATPases specifically involved in the transport of conjugates across the tonoplast. As cholate and its conjugates have so far not been reported to occur in plants, the physiological function of the novel transport ATPase described here is presently unknown.

Reappearance of hydrolytic activities and tonoplast proteins in the regenerated vacuole of evacuolated protoplasts

Mesophyll protoplasts of tobacco (Nicotiana tabacum L. cv. Xanthi) were evacuolated by centrifugation in a density gradient. Evacuolation resulted in the quantitative loss of vacuolar hydrolytic activities. The evacuolated miniprotoplasts were cultivated under different conditions, and the regeneration of the central vacuole was investigated by light and electron microscopy as well as by the determination of activities of vacuolar marker enzymes. Vacuoles and hydrolytic activities, as well as cell wall material reappeared faster when the cells were cultivated at low osmotic strength. A newly synthesized tonoplast polypeptide could be detected using a polyspecific serum raised against tonoplast proteins of barley (Hordeum vulgare L.). Both vacuolar proton pumps, the ATPase as well as the pyrophosphatase appear to be newly synthesized during the regeneration of the vacuole.