The APG8/12-activating Enzyme APG7 Is Required for Proper Nutrient Recycling and Senescence in Arabidopsis thaliana

Abstract

The vacuole/lysosome serves an important recycling function during starvation and senescence in eukaryotes via a process called autophagy. Here bulk cytosolic constituents and organelles become sequestered in specialized autophagic vesicles, which then deliver their cargo to the vacuole for degradation. In yeasts, genetic screens have identified two novel post-translational modification pathways remarkably similar to ubiquitination that are required for autophagy. From searches of the Arabidopsis genome, we have identified gene families encoding proteins related to both the APG8 and ؊12 polypeptide tags and orthologs for all components required for their attachment. A single APG7 gene encodes the ATP-dependent activating enzyme that initiates both conjugation pathways. Phenotypic analysis of an APG7 disruption indicates that it is not essential for normal growth and development in Arabidopsis. However, the apg7-1 mutant is hypersensitive to nutrient limiting conditions and displays premature leaf senescence. mRNAs for both APG7 and APG8 preferentially accumulate as leaves senesce, suggesting that both conjugation pathways are up-regulated during the senescence syndrome. These findings show that the APG8/12 conjugation pathways have been conserved in plants and may have important roles in autophagic recycling, especially during situations that require substantial nitrogen and carbon mobilization.Plants, like other organisms, have developed sophisticated mechanisms for recycling intracellular constituents during periods of growth, developmental remodeling, and nutrient-limiting conditions (1, 2). Especially critical is the degradation of protein, given the importance of reused amino acids to the nitrogen and carbon economy. Selective protein removal is accomplished primarily by the ubiquitin (Ub) 1 /26 S proteasome pathway (3, 4). In this pathway, the covalent attachment of Ubs is used as a signal to target specific proteins for degradation by the 26 S proteasome. Another major recycling system employs the vacuole, or its animal equivalent, the lysosome, as a lytic organelle (5, 6). Here cytosolic proteins are delivered to the vacuole and then degraded by a wide variety of vacuolar proteases. In some situations, entire organelles can be targets (7). Unlike the Ub/26 S proteasome pathway, vacuolar proteolysis is for the most part non-selective, thus targeting proteins indiscriminately. As a consequence protein turnover by the vacuole is thought to play less of a role in cellular regulation and a more prominent role under conditions when rapid remobilization and resorption of nutrients are crucial. These conditions include nutrient deprivation and environmental stress and developmental periods that require extensive cell and organ remodeling such as senescence and programmed cell death (5, 6).The vacuole degrades cytoplasm by three unique but overlapping mechanisms, chaperone-assisted import, microautophagy, and macroautophagy (6 -9). Chaperone-assisted import is activated during starvation; it employs Hsp70-related...

Large‐scale identification of leaf senescence‐associated genes

“…Our ®nding that the senescence-related cDNA clone At3g15580 is identical to the APG8 gene suggests its association to a novel autophagic pathway in plants (Doelling et al, 2002). During autophagy, bulk cytosolic constituents and organelles are sequestered in specialized autophagic vesicles and are delivered into the vacuoles for their degradation.…”

“…The APG8 gene encodes a component of this autophagic pathway in yeast. An Arabidopsis mutant altered in another component (APG7) of the autophagic pathway displayed premature senescence under nutrient-limiting conditions (Doelling et al, 2002). Recently, a novel autophage gene (AtAPG9) related to senescence has been identi®ed (Hanaoka et al, 2002) but has not yet been found in our subtraction library.…”

The pleiotropic role of autophagy: from protein metabolism to bactericide

“…In contrast, plant autophagy mutants show only minimal abnormalities, that is, accelerated senescence and reduced seed number. [50][51][52] This might be because plants have multiple backup systems for starvation and also develop a sophisticated root system to continuously absorb nutrients from the soil.…”

Autophagy-Related Proteins Are Required for Degradation of Peroxisomes inArabidopsisHypocotyls during Seedling Growth

“…First, electron microscopic observation suggested pexophagy in castor bean (Ricinus communis) endosperm (Vigil, 1970), although similar observations have not been reported. Second, Atg genes encoding core autophagic machineries are highly conserved in various eukaryotes, including dicots (Hanaoka et al, 2002;Doelling et al, 2002) and monocots (Chung et al, 2009), and reverse genetic studies using Arabidopsis demonstrated their functions in plant autophagy (reviewed by Kim et al, 2012). Of four conserved core autophagic complexes, the ATG8 conjugation system has been the most extensively studied.…”