Extra‐plastidial degradation of chlorophyll and photosystem I in tobacco leaves involving ‘senescence‐associated vacuoles’

Abstract

Summary Chlorophyll (Chl) loss is the main visible symptom of senescence in leaves. The initial steps of Chl degradation operate within the chloroplast, but the observation that ‘senescence‐associated vacuoles’ (SAVs) contain Chl raises the question of whether SAVs might also contribute to Chl breakdown. Previous confocal microscope observations (Martínez et al., 2008) showed many SAVs containing Chl. Isolated SAVs contained Chl a and b (with a Chl a/b ratio close to 5) and lower levels of chlorophyllide a. Pheophytin a and pheophorbide a were formed after the incubation of SAVs at 30°C in darkness, suggesting the presence of Chl‐degrading activities in SAVs. Chl in SAVs was bound to a number of ‘green bands’. In the most abundant green band of SAVs, Western blot analysis showed the presence of photosystem I (PSI) Chl‐binding proteins, including the PsaA protein of the PSI reaction center and the apoproteins of the light‐harvesting complexes (Lhca 1–4). This was confirmed by: (i) measurements of 77‐K fluorescence emission spectra showing a single emission peak at around 730 nm in SAVs; (ii) mass spectrometry of the most prominent green band with the slowest electrophoretic mobility; and (iii) immunofluorescence detection of PsaA in SAVs observed through confocal microscopy. Incubation of SAVs at 30°C in darkness caused a steady decrease in PsaA levels. Overall, these results indicate that SAVs may be involved in the degradation of PSI proteins and their associated chlorophylls during the senescence of leaves.

Autophagy-Related 2 Regulates Chlorophyll Degradation under Abiotic Stress Conditions in Arabidopsis

“…Recently, three pathways of chloroplast degradation were revealed: senescence-associated vacuoles (SAVs), chloroplast vesiculation, and autophagy [ 3 , 12 , 13 , 14 ]. SAVs are small protein lytic compartments containing chloroplast stroma-targeted fluorescent proteins that can be transported to vacuoles for degradation [ 15 ].…”

Biochemical and Comparative Transcriptome Analyses Reveal Key Genes Involved in Major Metabolic Regulation Related to Colored Leaf Formation in Osmanthus fragrans ‘Yinbi Shuanghui’ during Development

“…The mechanism underlying leaf color variation is complex and mainly related to chlorophyll, carotenoid, secondary metabolite synthesis, photosynthesis, and chloroplast development. Remarkably, the leaf color changes in the leaf mutant and senescence process are usually related to the chlorophyll metabolism pathway [11,12]. However, the change in leaf color during leaf senescence is later followed by leaf drop, and the leaf color in coloration mutants start with the initial stage of leaf color formation and would keep this color during leaf growth and development.…”

Chloroplast Protein Degradation in Senescing Leaves: Proteases and Lytic Compartments

“…SAVs are bound by a single membrane, and their diameter ranges from 500 to 800 nm. SAVs were also shown to contain stromal (e.g., Rubisco and Glutamine synthetase II) and thylakoid (e.g., PsaA and Lhcas) proteins, indicating trafficking between the plastid and SAVs in senescing leaves (Martínez et al, 2008; Gomez et al, 2019). Remarkably, SAVs are devoid of PSII components (Gomez et al, 2019).…”

“…SAVs were also shown to contain stromal (e.g., Rubisco and Glutamine synthetase II) and thylakoid (e.g., PsaA and Lhcas) proteins, indicating trafficking between the plastid and SAVs in senescing leaves (Martínez et al, 2008; Gomez et al, 2019). Remarkably, SAVs are devoid of PSII components (Gomez et al, 2019). The involvement of SAVs in chloroplast protein degradation was indicated by in vitro autodigestion of chloroplast proteins contained within isolated SAVs (Martínez et al, 2008; Gomez et al, 2019), and by in vivo experiments where incubation of leaf disks with the cysteine protease inhibitor E-64 completely abolished the protease activity of SAVs and concomitantly reduced Rubisco degradation (Carrión et al, 2013).…”