Regreening of senescent Nicotiana leaves II. Redifferentiation of plastids

Mancera et al., 1999). The reappearance of chlorophyll (Chl ) correlated with the reappearance of the Chl biosynSingle senescent leaves attached to decapitated thesis enzyme, NADPH-protochlorophyllide oxidoreducshoots of Nicotiana rustica L. regreened, especially tase (POR). Development of functional thylakoid when treated with cytokinin. Regreening caused an membranes during regreening was indicated by the increase in leaf thickness, due to cell expansion. reappearance of the thylakoid component, lightSenescent leaf plastids (gerontoplasts) were smaller harvesting chlorophyll a/b-binding protein. The nature of than green chloroplasts, with degenerated membrane the plastids in which these regreening events occur is systems and stroma, and larger plastoglobuli. At explored in the present paper. advanced senescence, micrographs showed disintegSenescent leaves contain partially dismantled chlororating gerontoplasts, reduced numbers of plastids plasts that function to salvage or compartmentalize were counted, and regreening became variable. The degraded material. These senescent plastids are sometimes redevelopment of grana and stroma in regreening called ‘chromoplasts’ because of their content of residual plastids was accelerated by cytokinin. All plastids in pigments (Tuquet and Newman, 1980; Thomson and regreening leaves were identifiable as redifferentiating Platt-Aloia, 1987). However, the term ‘gerontoplast’ has gerontoplasts because of their content of plastoglobuli been advocated to designate the specific plastid developand starch. Immunogold labelling showed significant ment of senescing leaves (Matile, 1992; Thomas, 1997). association of POR with etioplasts in cotyledons, but Gerontoplasts and chromoplasts have ultrastructural feawith mature plastids in regreening leaves. No protures in common, such as a reduced thylakoid system and plastids or dividing chloroplasts were observed in prominent plastoglobuli. However, chromoplasts that regreening leaves. Plastid numbers declined during develop from proplastids, amyloplasts or young chlorosenescence and did not increase again during plasts retain the capacity for division and biosynthesis, regreening. It is concluded that the chloroplasts of whereas gerontoplasts have been suggested to have lost regreening leaves arose by redifferentiation of these capacities. Reported characteristics of gerontoplasts gerontoplasts. include reductions in size, electron density and buoyant density, along with loss of ribosome and DNA contents