How Plants Dispose of Chlorophyll Catabolites

Abstract

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 K m of approximately 100 M. 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 ⌬pH 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.

Nitric Oxide Deficiency Accelerates Chlorophyll Breakdown and Stability Loss of Thylakoid Membranes during Dark-Induced Leaf Senescence in Arabidopsis

“…PAO catalyzes the cleavage of the porphyrin ring, resulting in the red chlorophyll catabolite, which is further reduced in to pFCC by RCCR. The resulting primary fluorescent catabolite pFCCs are exported from the plastid and after further modification they are imported into their final destination, the vacuole by a primary active transport system [14], [15].…”

Chlorophyll Breakdown in Tobacco: On the Structure of Two Nonfluorescent Chlorophyll Catabolites

“…1). The tetrapyrrolic NCCs are deposited in the vacuoles of senescent leaves [17] [18] and have been suggested to represent the final stages of senescence- Fig. 1.…”

Enhanced Multispecificity of Arabidopsis Vacuolar Multidrug Resistance-associated Protein-type ATP-binding Cassette Transporter, AtMRP2

“…Reciprocally, AtMRP2-dependent DNP-GS uptake is not inhibited appreciably by the inclusion of Bn-NCC-1 in the uptake medium (17 (17). Although it was initially surprising to find that AtMRP2 can transport both Bn-NCC-1 and GS conjugates, because the functionality, and by implication the pump, responsible for MgATP-energized Bn-NCC-1 uptake by barley vacuoles had earlier been concluded to be different from that responsible for GS conjugate uptake in that GS conjugates did not compete with Bn-NCC-1 for uptake (19), it is now clear, at least in the case of some AtMRPs, that lack of competition between candidate substrates does not automatically preclude their transport by the same transporter.…”