What to Eat: Evidence for Selective Autophagy in Plants^F

Abstract

Autophagy is a macromolecular degradation pathway by which cells recycle their contents as a developmental process, housekeeping mechanism, and response to environmental stress. In plants, autophagy involves the sequestration of cargo to be degraded, transport to the cell vacuole in a double-membrane bound autophagosome, and subsequent degradation by lytic enzymes. Autophagy has generally been considered to be a non-selective mechanism of degradation. However, studies in yeast and animals have found numerous examples of selective autophagy, with cargo including proteins, protein aggregates, and organelles. Recent work has also provided evidence for several types of selective autophagy in plants. The degradation of protein aggregates was the first selective autophagy described in plants, and, more recently, a hybrid protein of the mammalian selective autophagy adaptors p62 and NBR1, which interacts with the autophagy machinery and may function in autophagy of protein aggregates, was described in plants. Other intracellular components have been suggested to be selectively targeted by autophagy in plants, but the current evidence is limited. Here, we discuss recent findings regarding the selective targeting of cell components by autophagy in plants.Keywords: Autophagy; degradation; stress; vacuole.

A BAR-Domain Protein SH3P2, Which Binds to Phosphatidylinositol 3-Phosphate and ATG8, Regulates Autophagosome Formation in Arabidopsis

“…Since SH3P2 has a membrane-remodeling domain, it is likely that an interaction between SH3P2 and ATG8 promotes membrane deformation for developing autophagosome. Recently, selective autophagy has been identified in plants, and the ATG8 family interacting motif has emerged as the molecular basis for ATG8 to recognize cargo(s) or other proteins in regulating autophagy (Suttangkakul et al, 2011;Svenning et al, 2011;Vanhee et al, 2011;Derrien et al, 2012;Floyd et al, 2012;Honig et al, 2012). Since SH3P2 signals were detected inside the vacuole after autophagy induction (Figures 1Bc and 1Bd), such an SH3P2-ATG8 interaction may target SH3P2 for degradation to regulate the turnover of autophagy.…”

Evidence for autophagy-dependent pathways of rRNA turnover inArabidopsis

“…7,8 Both nonselective and selective autophagy pathways exist in many eukaryotes. [8][9][10][11][12][13][14] Recently, a selective type of autophagy termed ribophagy was described in yeast cells undergoing nitrogen starvation. Genetic analyses showed that ribophagy specifically targets ribosomes for degradation during nutritional stress in a process that depends on components of the basal, nonselective autophagy machinery, such as the scaffold protein Atg17 and E1-like protein Atg7.…”

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

“…Regardless of this evidence, the existence of a ribophagylike process in plants is not yet conclusive [52], and the potential involvement of NnSR1 in RNA degradation requires further study.…”

“…Basal autophagy is always present in plants, turning over cytoplasmic components and functioning as a quality control mechanism. 52,76 Autophagy is upregulated during the developmental processes of senescence and cell death as well as during biotic and abiotic stresses such as oxidative stress, drought, nutrient starvation, and pathogen infection. 6,36,[77][78][79][80][81] Upregulation of autophagy in rns2-2 could be caused by a lack of ribosome turnover that results in the accumulation of toxic products and oxidative stress, or it could be the result of a starvation response, since rRNA represents a large proportion of cellular resources.…”

“…4A and B). Autophagy is present at a low basal level in WT seedlings, 51,52 as indicated by a few fluorescent puncta present in root cells. rns2-2 has increased autophagy, with multiple puncta.…”