Chloroplast Protein Degradation: Involvement of Senescence-Associated Vacuoles

Senescence-Associated Vacuoles, a Specific Lytic Compartment for Degradation of Chloroplast Proteins?

“…The ultrastructural changes described above are the consequence of wide-range macromolecule degradation processes. Ultrastructural changes in thylakoids parallel the degradation of photosystem proteins [ 12 , 13 , 14 ], thylakoid galactolipids [ 12 , 15 ] and loss of chlorophylls [ 12 , 16 , 17 ], which result in impaired photosynthetic electron transport [ 13 , 18 ]. The stromal proteins of plastids, including Rubisco and other components of the C3 photosynthetic pathway [ 12 , 15 ], glutamine sunthetase II [ 19 ], and even sulfur assimilating enzymes [ 20 ] are also degraded, and their breakdown may start even before changes in the photochemical apparatus (e.g., chlorophyll loss) can be detected.…”

“…In spite of the intense search for within-the-chloroplast degradation of Rubisco and other stromal proteins, there is still no convincing evidence to implicate chloroplast proteases in Rubisco degradation (reviewed in [ 14 , 32 ]). Moreover, recent findings showing Rubisco and other chloroplast proteins in vesicular structures outside the plastid ( i .…”

“…Likewise, subcellular fractionation combined with the use of an activity-based probe for cysteine proteases, DCG-04 [ 56 , 57 ] detected a large part of the cysteine protease activity of senescing cells in a fraction enriched in SAVs [ 54 ]. Up-regulation of cysteine protease genes is a common observation in many transcriptomic studies of leaf senescence in several plant species (reviewed in [ 14 ]), which suggests that SAVs contain some of the senescence up-regulated proteases of the cell. In line with this, the senescence-specific cysteine protease SAG12 appears to be located preferentially in SAVs [ 53 , 54 ] ( Figure 2 ), confirming fluorescent (R6502) and activity-based probe (DCG04) results about the prevalence of cysteine proteases in these lytic vacuoles.…”

Chloroplast Protein Degradation in Senescing Leaves: Proteases and Lytic Compartments

“…Given the crucial role of protein degradation in the decline of photosynthetic protein levels, detection of proteases whose activity or expression increases during leaf senescence might contribute to identify putative candidate genes to manipulate senescence. Transcriptomic studies have consistently shown, across a range of different plant species, that some of the genes up-regulated during senescence (“senescence-associated genes,” SAGs) are proteases (e.g., Parrott et al, 2007; Breeze et al, 2011; Sekhon et al, 2012; Costa et al, 2013; Zhang et al, 2014). Most proteases associated with senescence are serine and cysteine (Cys) proteases, but some are aspartic proteases and metalloproteases (Roberts et al, 2012; Díaz-Mendoza et al, 2014).…”

“…The chloroplast seems a logical place to harbor proteases involved in the degradation of photosynthetic proteins. The major chloroplast protease families (Clp, FtsH, DegP) display mostly constitutive expression and seem to be involved in protein quality control and maintenance of homeostasis rather than in massive protein degradation (van Wijk, 2015), although some members of these families can be up-regulated during leaf senescence (Costa et al, 2013). The FtsH6 metalloprotease has been linked to senescence as it was first shown to degrade Lhcb3 in vitro (Zelisko et al, 2005), but this result could not be confirmed in vivo (Wagner et al, 2011).…”