Chlorophyll breakdown in senescent leaves: demonstration of Mg‐dechelatase activity

Abstract

The action of Mg‐dechelatase was brought to light by incubating senescent rape cotyledons or chloroplasts under conditions which prevented the oxidative cleavage of chlorophyll‐porphyrin. The accumulation of chlorophyllide and pheophorbide taking place under such conditions was considered as a measure of apparent activities of chlorophyllase and dechelatase, respectively. In excised cotyledons metal chelators such as 2,2′‐dipyridyl and o‐phenanthroline caused a marked accumulation of pheophorbide a, without affecting the apparent activity of chlorophyllase. Treatment of cotyledons with an inhibitor of cytoplasmic protein synthesis d‐2‐(4‐methyl‐2,6‐dinitroanilino)‐N‐methyl‐propionamide (d‐MDMP) caused a reduced accumulation of pheophorbide a in the presence of dipyridyl, suggesting that the appearance and maintenance of Mg‐dechelatase activity in senescent cotyledons requires continuous cytoplasmic protein synthesis. In isolated senescent chloroplasts (gerontoplasts) the cleavage of chlorophyll‐porphyrin requires the supplementation with glucose‐6‐phosphate (Glc6P). Upon the incubation of gerontoplasts in the absence of Glc6P, a conspicuous accumulation of pheophorbide a occurred. Much smaller pools of pheophorbide a were produced when porphyrin cleavage was allowed in the presence of Glc6P. These phenomena were not observed in pre‐senescent chloroplasts. In contrast to the apparent Mg‐dechelatase activity, chlorophyllase activity did not change in a senescent‐specific fashion. The lysis of gerontoplasts by freezing and thawing caused an enhancement of apparent chlorophyllase activity whereas the activity of Mg‐dechelatase was lower than in the intact organelles. In the pre‐senescent chloroplasts, lysis evoked a small apparent Mg‐dechelatase activity, suggesting that in a latent form this enzyme may be present even before the onset of foliar senescence. Copyright © 1993, Wiley Blackwell. All rights reserved