Early Events in Plastid Protein Degradation in<i>stay-green</i><i>Arabidopsis</i>Reveal Differential Regulation beyond the Retention of LHCII and Chlorophyll

An individually darkened leaf model was used to study protein changes in the Arabidopsis mutant stay-green1 (sgr1) to partially mimic the process of leaf covering senescence that occurs naturally in the shaded rosettes of Arabidopsis plants. Utilizing this controlled and predictable induced senescence model has allowed the direct comparison of sgr1 with Col-0 during the developmental period preceding the retention of chlorophyll and light harvesting complex II (LHCII) in sgr1 and the induction of senescence in Col-0. Quantitative proteomic analysis of soluble leaf proteins from sgr1 and Col-0 before the initiation of senescence has revealed a range of differences in plastid soluble protein abundance in sgr1 when compared to Col-0. Changes were also observed in membrane located machinery for photosystem II (PSII), in Calvin cycle components, proteins involved in redox control of the stromal compartment and ammonia assimilation that differentiated sgr1 during the early stages of the senescence process. The changes in PSII abundance were accompanied with a lower capacity of photosynthetic CO(2) assimilation in sgr1 than Col-0 after return of plants to lighted conditions following 3 and 5 days of darkness. A light-harvesting chlorophyll-a/b binding protein (LHCB2) was retained during the later stages of senescence in sgr1 but this was accompanied by an enhanced loss of oxygen evolving complex (OEC) subunits from PSII, which was confirmed by Western blotting, and an enhanced stability of PSII repair proteins in sgr1, compared to Col-0. Together these data provide insights into the significant differences in the steady-state proteome in sgr1 and its response to senescence, showing this cosmetic stay-green mutant is in fact significantly different to wild-type plants both before and during leaf senescence.