Mass Exodus from Senescing Soybean Chioroplasts

During leaf senescence, chioroplast proteins, lipids and pigmentsundergo massive degradation releasing large amounts of nutrientsfor reuse elsewhere. Understandably, the chloroplast-degradingmachinery has been considered to operate within the chloroplastitself. However, most of those lipases and proteases that increaseduring senescence and have been localized occur in the vacuoleor cytoplasm rather than in the chloroplast. In chloroplastsof senescing (monocarpic) soybean (Glycine max) leaves, numerousplastoglobuli (lipid-protein globules) protruded through thechloroplast envelope and emerged into the cytoplasm, where theseglobules acquired a polygonal coat and eventually disintegrated.The fluorescence characteristics of these cytoplasmic globulesindicated that they contained chlorophyll or chlorophyll derivatives.The secreted globules were specific to senescing cells and wereabsent in old leaves of the ‘stay green’ genotypeGGd₁d₁d₂d₂ which shows a generalized inhibition of chloroplastdegradation. These observations suggest that the globules secretedby the chloroplast carry photosynthetic components to the cytoplasmor vacuole where they are degraded. This blebbing from the chloroplastsuggests the occurrence of a novel pathway for the degradationof photosynthetic components in senescing leaves, and it opensnew approaches to the study of chloroplast breakdown and itsregulation. ¹ Permanent address: Instituto de Fisiología Vegetal,Universidad Nacional de La Plata, c. c. 327, 1900-La Plata,Argentina.