title: Conversion of chlorophyll b to chlorophyll a precedes magnesium dechelation for protection against necrosis in Arabidopsis.
abstract: Chlorophyll is a deleterious molecule that generates reactive oxygen species and must be converted to nontoxic molecules during plant senescence. The degradation pathway of chlorophyll a has been determined; however, that of chlorophyll b is poorly understood, and multiple pathways of chlorophyll b degradation have been proposed. In this study, we found that chlorophyll b is degraded by a single pathway, and elucidated the importance of this pathway in avoiding cell death. In order to determine the chlorophyll degradation pathway, we first examined the substrate specificity of 7-hydroxymethyl chlorophyll a reductase. 7-hydroxymethyl chlorophyll a reductase reduces 7-hydroxymethyl chlorophyll a but not 7-hydroxymethyl pheophytin a or 7-hydroxymethyl pheophorbide a. These results indicate that the first step of chlorophyll b degradation is its conversion to 7-hydroxymethyl chlorophyll a by chlorophyll b reductase, although chlorophyll b reductase has broad substrate specificity. In vitro experiments showed that chlorophyll b reductase converted all of the chlorophyll b in the light-harvesting chlorophyll a/b protein complex to 7-hydroxymethyl chlorophyll a, but did not completely convert chlorophyll b in the core antenna complexes. When plants whose core antennae contained chlorophyll b were incubated in the dark, chlorophyll b was not properly degraded, and the accumulation of 7-hydroxymethyl pheophorbide a and pheophorbide b resulted in cell death. This result indicates that chlorophyll b is not properly degraded when it exists in core antenna complexes. Based on these results, we discuss the importance of the proper degradation of chlorophyll b.
authors: Institute of Low Temperature Science, Hokkaido University, N19 W8, Sapporo 060-0819, Japan.
authors: Yousuke
authors: Y
authors: Shimoda
authors: Hisashi
authors: H
authors: Ito
authors: Ayumi
authors: A
authors: Tanaka
mesh_terms: D000429:Alcohol Oxidoreductases / Q000235:genetics / Q000378:metabolism*; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D016923:Cell Death; D002734:Chlorophyll / Q000031:analogs & derivatives* / Q000378:metabolism; D000077194:Chlorophyll A; D045342:Light-Harvesting Protein Complexes / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism*; D010641:Phenotype; D045331:Photosystem I Protein Complex / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: C098029:7-hydroxymethyl chlorophyll; D029681:Arabidopsis Proteins; D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; D002734:Chlorophyll; C037184:chlorophyll b; D000429:Alcohol Oxidoreductases; C102575:chlorophyll b reductase; D008274:Magnesium; D000077194:Chlorophyll A