Autophagy: Pathways for Self-Eating in Plant Cells

Abstract

Plants have developed sophisticated mechanisms to survive when in unfavorable environments. Autophagy is a macromolecule degradation pathway that recycles damaged or unwanted cell materials upon encountering stress conditions or during specific developmental processes. Over the past decade, our molecular and physiological understanding of plant autophagy has greatly increased. Most of the essential machinery required for autophagy seems to be conserved from yeast to plants. Plant autophagy has been shown to function in various stress responses, pathogen defense, and senescence. Some of its potential upstream regulators have also been identified. Here, we describe recent advances in our understanding of autophagy in plants, discuss areas of controversy, and highlight potential future directions in autophagy research.

Evidence for autophagy-dependent pathways of rRNA turnover inArabidopsis

“…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. The turnover of ribosomes through autophagy or other mechanisms may be important for homeostasis and the resources recycled through this process, such as bases, nucleosides, nitrogen, or energy, may be needed to maintain housekeeping functions in normal cells.…”

“…ATG8 isoforms decorate both the inner and outer surface of a completed autophagosome. 6,8,25 Fusion of this protein with GFP allows visualization of autophagosomes present in the cytoplasm and autophagic bodies present in the vacuole; thus GFP-ATG8 has been used extensively as an autophagosome and autophagic body marker. [26][27][28] It was unknown if the potential accumulation of autophagosomes observed in rns2-2 was a result of increased autophagosome formation or decreased autophagic body degradation in the vacuole.…”

“…Macroautophagy is the best-characterized form of autophagy and is well studied in plants. 6 Macroautophagy, referred to here as autophagy, involves de novo formation of double-membrane vesicles in the cytoplasm through the action of autophagy-related (ATG) proteins. The initial autophagic compartment, called a phagophore, encapsulates cargo targeted for degradation; the phagophore expands and seals to form an autophagosome, which is trafficked to and fuses with the vacuole, resulting in single-membrane autophagic bodies within the lumen.…”

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

“…Among these, the machinery required for autophagosome formation is constituted by several core complexes: the ATG1/Unc-51-like kinase complex, the phosphoinositide 3-kinase (PI3K) complex, the Atg9 reservoir and its trafficking machinery, and two ubiquitin-like conjugation systems, including Atg12 and Atg8 (Xie and Klionsky, 2007; Despite tremendous progress made in our understanding of the molecular mechanisms underlying autophagic pathways in yeast and mammals, autophagy studies in plants are still in their infancy. Although most of the ATG genes required for autophagy have been identified in plants ( Avin-Wittenberg et al, 2012;Liu and Bassham, 2012), the molecular mechanism whereby ATG proteins regulate autophagosome formation in plant cells remains to be examined. Models for plant autophagosome formation are primarily deduced from those in yeast or mammals and are poorly characterized Li and Vierstra, 2012;Liu and Bassham, 2012).…”

“…Although most of the ATG genes required for autophagy have been identified in plants ( Avin-Wittenberg et al, 2012;Liu and Bassham, 2012), the molecular mechanism whereby ATG proteins regulate autophagosome formation in plant cells remains to be examined. Models for plant autophagosome formation are primarily deduced from those in yeast or mammals and are poorly characterized Li and Vierstra, 2012;Liu and Bassham, 2012). However, the identity of nascent autophagosome structures such as PAS is pivotal to several crucial questions regarding autophagosome biogenesis, including the membrane deformation mechanism and the origin of the autophagosome membrane.…”