Mass exodus from senescing soybean chloroplasts

Abstract

During leaf senescence, chloroplast proteins, lipids and pigments undergo massive degradation releasing large amounts of nutrients for reuse elsewhere. Understandably, the chloroplast-degrading machinery has been considered to operate within the chloroplast itself. However, most of those lipases and proteases that increase during senescence and have been localized occur in the vacuole or cytoplasm rather than in the chloroplast. In chloroplasts of senescing (monocarpic) soybean (Glycine max) leaves, numerous plastoglobuli (lipid-protein globules) protruded through the chloroplast envelope and emerged into the cytoplasm, where these globules acquired a polygonal coat and eventually disintegrated. The fluorescence characteristics of these cytoplasmic globules indicated that they contained chlorophyll or chlorophyll derivatives. The secreted globules were specific to senescing cells and were absent in old leaves of the 'stay green' genotype GGd1d1d2d2 which shows a generalized inhibition of chloroplast degradation. These observations suggest that the globules secreted by the chloroplast carry photosynthetic components to the cytoplasm or vacuole where they are degraded. This blebbing from the chloroplast suggests the occurrence of a novel pathway for the degradation of photosynthetic components in senescing leaves, and it opens new approaches to the study of chloroplast breakdown and its regulation.