Accumulation of plastidial alanine-aminopeptidase in relation to plastid damage in cotyledons of dark-grown sugar beet seedlings

During prolonged dark-growth of sugar beet (Beta vulgaris L.) seedlings, the etioplasts of cotyledons undergo a degenerative process shortly after the proplastid to etioplast transition. Two plastidial aminopeptidase activities were previously identified as early markers of this degenerative process which results in a decrease of greening capacity. One of these activities was shown to be due to one enzyme, plastidial alanine-aminopeptidase, which was purified to homogeneity and characterized. In the present study, the effects of dark development, light, and exogenous glucose on enzyme activity and protein level of plastidial alanine-aminopeptidase were investigated. Changes of enzyme activity during prolonged dark-growth were found to be correlated with changes of protein level. Significant increases of both activity and protein levels occurred at the onset of the degenerative process affecting etioplasts. By contrast, the activity and protein levels of plastidial alanine-aminopeptidase were negatively affected by light and by exogenous glucose, thus suggesting a type of expression that is related to prolonged dark-growth and etioplast ageing.