Vacuolar Localization of Proteases and Degradation of Chloroplasts in Mesophyll Protoplasts from Senescing Primary Wheat Leaves

Mesophyll protoplasts isolated from primary leaves of wheat seedlings were used to follow the localization of proteases and the breakdown of chloroplasts during dark-induced senescence. Protoplasts were readily obtained from leaf tissue, even after 80% of the chlorophyll and protein had been lost. Intact chloroplasts and vacuoles could be isolated from the protoplasts at all stages of senescence. All the proteolytic activity associated with the degradation of ribulose bisphosphate carboxylase in the protoplasts could be accounted for by that localized within the vacuole. Moreover, this localization was retained late into senescence. Protoplasts isolated during leaf senescence first showed a decline in photosynthesis, then a decline in ribulose bisphosphate carboxylase activity, followed by a decline in chloroplast number. There was a close correlation between the decline in chloroplast number and the loss of chlorophyll and soluble protein per protoplast, suggesting a sequential degradation of chloroplasts during senescence. Ultrastructural studies indicated a movement of chloroplasts in toward the center of the protoplasts during senescence. Thus, within senescing protoplasts, chloroplasts appeared either to move into invaginations of the vacuole or to be taken up into the vacuole.