Chloroplast Protein Degradation: Involvement of Senescence Associated Vacuoles

COSTA M. LORENZA ; MARTINEZ, DANA; GOMEZ FACUNDO; CARRIÓN CRISTIAN; GUIAMÉT JUAN JOSÉ

Chloroplast development during leaf growth and senescence

Springer

Año: 2011;

Senescence, the last developmental phase in the life of a leaf, is characterized bymassive degradation of chloroplast proteins and redistribution of the released aminoacidsand peptides to other parts of the plant. Chloroplast protein degradation plays animportant role in the nitrogen economy of plants.Loss of chloroplast proteins is associated with cessation of protein synthesis and anincrease in rates of protein degradation. For some photosynthetic proteins, there is clearevidence for degradation within the plastid itself. For example, the chloroplastic FtsH6metalloprotease is involved in the breakdown of the Lhcb apoproteins of the lightharvestingcomplex associated to photosystem II. The involvement of chloroplastproteases 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 thatoccur in senescing leaf cells. They develop in chloroplast-containing cells (i.e.,mesophyll and guard cells) and are characterized by high peptidase activity, particularlycysteine-type proteases. A role for SAVs in chloroplast protein degradation can beimplied from the fact that stromal proteins of the chloroplast and PSI are re-located toSAVs during senescence. In vitro, cysteine-type proteases within SAVs degrade thechloroplast proteins contained in these vacuoles. Since development of SAVs is notaffected in an autophagy mutant, for some chloroplast proteins this may represent anextra-plastidial degradation pathway independent from the autophagic machinery.