The Autophagic Degradation of Chloroplasts via Rubisco-Containing Bodies Is Specifically Linked to Leaf Carbon Status But Not Nitrogen Status in Arabidopsis

Abstract

Autophagy is an intracellular process facilitating the vacuolar degradation of cytoplasmic components and is important for nutrient recycling during starvation. We previously demonstrated that chloroplasts can be partially mobilized to the vacuole by autophagy via spherical bodies named Rubisco-containing bodies (RCBs). Although chloroplasts contain approximately 80% of total leaf nitrogen and represent a major carbon and nitrogen source for new growth, the relationship between leaf nutrient status and RCB production remains unclear. We examined the effects of nutrient factors on the appearance of RCBs in leaves of transgenic Arabidopsis (Arabidopsis thaliana) expressing stroma-targeted fluorescent proteins. In excised leaves, the appearance of RCBs was suppressed by the presence of metabolic sugars, which were added externally or were produced during photosynthesis in the light. The light-mediated suppression was relieved by the inhibition of photosynthesis. During a diurnal cycle, RCB production was suppressed in leaves excised at the end of the day with high starch content. Starchless mutants phosphoglucomutase and ADP-Glc pyrophosphorylase1 produced a large number of RCBs, while starch-excess mutants starch-excess1 and maltose-excess1 produced fewer RCBs. In nitrogen-limited plants, as leaf carbohydrates were accumulated, RCB production was suppressed. We propose that there exists a close relationship between the degradation of chloroplast proteins via RCBs and leaf carbon but not nitrogen status in autophagy. We also found that the appearance of non-RCB-type autophagic bodies was not suppressed in the light and somewhat responded to nitrogen in excised leaves, unlike RCBs. These results imply that the degradation of chloroplast proteins via RCBs is specifically controlled in autophagy.

Stress-Induced Chloroplast Degradation in Arabidopsis Is Regulated via a Process Independent of Autophagy and Senescence-Associated Vacuoles

“…Even whole chloroplasts have been shown to be transported to the vacuole through the autophagy-dependent process in individually darkened leaves Wada et al, 2009). Interestingly, RCB-mediated chloroplast degradation was highly activated by carbon rather than nitrogen shortage (Izumi et al, 2010;Izumi and Ishida, 2011). This observation might be partially explained by studies showing that autophagy also participates in chloroplast starch degradation by engulfing small starch granule-like structures from chloroplasts and transporting them to the vacuole for subsequent degradation .…”

“…Two pathways have been demonstrated for the degradation of chloroplast stromal proteins: autophagy Wada et al, 2009;Izumi et al, 2010) and senescence-associated vacuoles (SAVs) (Otegui et al, 2005;Martínez et al, 2008a;Carrión et al, 2013). Autophagy is a well-known system for the bulk degradation of intracellular proteins and organelles (Ohsumi, 2001;Bassham, 2009).…”

Autophagic Recycling Plays a Central Role in Maize Nitrogen Remobilization

“…Another is the possibility that ATG8/ATG12-independent autophagic routes become relevant (Nishida et al, 2009;Reyes et al, 2011). Alternatively, given the fact that autophagic turnover of chloroplast constituents by RBC is stimulated by fixed-C and not by N stress (Izumi et al, 2010;Izumi and Ishida, 2011), measuring chloroplast turnover in limiting fixed-C might be required to detect the effects of the atg12 mutations.…”

“…As examples, the transcript levels of multiple chloroplast proteases and various components of the autophagic machinery are known to rise early during leaf senescence in Arabidopsis thaliana and various cereals (Parrott et al, 2007;Liu et al, 2008;Phillips et al, 2008;Ruuska et al, 2008;Breeze et al, 2011;Avila-Ospina et al, 2014;Penfold and Buchanan-Wollaston, 2014) and is concomitant with the export of major protein stores like Rubisco and glutamine synthetase into SAVs with intense proteolytic activity (Otegui et al, 2005;Martínez et al, 2008). In addition, direct connections between autophagy and the turnover of chloroplast and mitochondrial constituents was provided by the study of autophagy mutants during leaf senescence (Ishida et al, 2008;Wada et al, 2009;Izumi et al, 2010;Li et al, 2014). For chloroplast turnover in particular, a special type of autophagy has been proposed whereby Rubisco-containing bodies bud from stromule projections and are then delivered to vacuoles for breakdown (Ishida et al, 2008;Spitzer et al, 2015).…”

Analysis of the role of RNS2 and autophagy in the turnover of ribosomal RNA in Arabidopsis thaliana

“…Furthermore, autophagy is involved in reducing both the size and number of chloroplasts during senescence through an RCB mechanism rather than whole chloroplast degradation alone ). Nutrient availability also has a differential effect on the autophagic degradation of RCBs compared to non-selective autophagy (Izumi et al 2010). Non-selective autophagy is upregulated under nutrient starvation and various abiotic stresses (Bassham 2009).…”