MODIFICATIONS OF ETIOPLASTS IN COTYLEDONS DURING PROLONGED DARK GROWTH OF SUGAR-BEET SEEDLINGS - IDENTIFICATION OF ETIOLATION-RELATED PLASTIDIAL AMINOPEPTIDASE ACTIVITIES

We studied the effects of prolonged dark growth on proplastids and etioplasts in cotyledons of sugar beet (Beta vulgaris L.) seedlings. Differentiation of proplastids into etioplasts occurred between d 4 and d 6 after imbibition, with the typical characteristics of increased synthesis of plastidial proteins, protein and carotenoid accumulation, size increase, development of plastid membranes and of the prolamellar body, and increase of the greening capacity. However, this situation of efficient greening capacity was short-lived. The greening capacity started to decline from d 6 after imbibition. This decline was due in part to reserve depletion and glucose limitation and also to irreversible damage to plastids. Indeed, electron microscopy observations in situ showed some signs of plastidial damage, such as accumulation of plastoglobuli and membrane alterations. The biochemical characterization of purified plastids also showed a decrease of proteins per plastid. Aminopeptidase activities, and to a lesser extent; neutral endopeptidase activities, were found to increase in plastids during this degenerative process. We identified two plastidial aminopeptidases showing a sharp increase of activity at the onset of the degenerative process. One of them, an alanyl aminopeptidase, was shown to be inactivated by exposure to light or addition of exogenous glucose, thus confirming the relationship with prolonged dark growth and indicating a relationship with glucose limitation.