Chloroplast Protein Degradation: Involvement of Senescence-Associated Vacuoles

Abstract

Senescence, the last developmental phase in the life of a leaf, is characterized by massive degradation of chloroplast proteins and redistribution of the released amino acids and peptides to other parts of the plant. Chloroplast protein degradation plays an important role in the nitrogen economy of plants. Loss of chloroplast proteins is associated with cessation of protein synthesis and an increase in rates of protein degradation. For some photosynthesis-associated proteins, there is clear evidence for degradation within the plastid itself. For example, chloroplastic FtsH metalloproteases and DegP serine-proteases are involved in the breakdown of the D1 protein upon photoinhibition of photosystem II, and these same proteases might degrade D1 during senescence. The involvement of chloroplast proteases in the degradation of Rubisco, the most abundant leaf protein, is less clear. Senescence-associated vacuoles (SAVs) are a class of small, acidic, lytic vacuoles that occur in senescing leaf cells. They develop in chloroplast-containing cells (i.e., mesophyll and guard cells) and are characterized by high peptidase activity, particularly of cysteine-type proteases. SAVs seem to be different from "Rubisco Containing Bodies", and development of SAVs does not depend on the operation of the autophagic pathway. A role for SAVs in chloroplast protein degradation can be implied from the fact that stromal proteins of the chloroplast and PSI are re-located to SAVs during senescence. In vitro, cysteine-type proteases within SAVs degrade the chloroplast proteins contained in these vacuoles. SAVs may be part of an extra-plastidial degradation pathway for chloroplast stroma and PSI proteins. In vitro, cysteine-type proteases within SAVs degrade the chloroplast proteins contained in these vacuoles. SAVs may be part of an extra-plastidial degradation pathway for chloroplast stroma and PSI proteins.