First published online July 22, 2005; 10.1104/pp.105.060673
Plant Physiology 138:2097-2110 (2005)
© 2005 American Society of Plant Biologists
ENVIRONMENTAL STRESS AND ADAPTATION
Autophagic Nutrient Recycling in Arabidopsis Directed by the ATG8 and ATG12 Conjugation Pathways1
Allison R. Thompson,
Jed H. Doelling2,
Anongpat Suttangkakul and
Richard D. Vierstra*
Department of Genetics, University of Wisconsin, Madison, Wisconsin 53706
Autophagy is an important mechanism for nonselective intracellular breakdown whereby cytosol and organelles are encapsulated in vesicles, which are then engulfed and digested by lytic vacuoles/lysosomes. In yeast, this encapsulation employs a set of autophagy (ATG) proteins that direct the conjugation of two ubiquitin-like protein tags, ATG8 and ATG12, to phosphatidylethanolamine and the ATG5 protein, respectively. Using an Arabidopsis (Arabidopsis thaliana) atg7 mutant unable to ligate either tag, we previously showed that the ATG8/12 conjugation system is important for survival under nitrogen-limiting growth conditions. By reverse-genetic analyses of the single Arabidopsis gene encoding ATG5, we show here that the subpathway that forms the ATG12-ATG5 conjugate also has an essential role in plant nutrient recycling. Similar to plants missing ATG7, those missing ATG5 display early senescence and are hypersensitive to either nitrogen or carbon starvation, which is accompanied by a more rapid loss of organellar and cytoplasmic proteins. Multiple ATG8 isoforms could be detected immunologically in seedling extracts. Their abundance was substantially elevated in both the atg5 and atg7 mutants, caused in part by an increase in abundance of several ATG8 mRNAs. Using a green fluorescent protein-ATG8a fusion in combination with concanamycin A, we also detected the accumulation of autophagic bodies inside the vacuole. This accumulation was substantially enhanced by starvation but blocked in the atg7 background. The use of this fusion in conjunction with atg mutants now provides an important marker to track autophagic vesicles in planta.
1 This work was supported by the National Research Initiative of the U.S. Department of Agriculture Cooperative State Research, Education and Extension Service (grant no. 2002–01452 to R.D.V.), by the National Institutes of Health (Postdoctoral Fellowship to J.H.D.), by a Thailand Predoctoral Fellowship (to A.S.), and by the Wisconsin Alumni Research Foundation and Louis and Elsa Thomsen Wisconsin Distinguished Predoctoral Fellowships (to A.R.T.).
2 Present address: Department of Genetics and Developmental Biology, 1090 Agricultural Science Building, West Virginia University, Morgantown, WV 26506–6108.
Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.060673.
* Corresponding author; e-mail vierstra{at}wisc.edu; fax 608–262–2976.
Received February 2, 2005;
returned for revision May 12, 2005;
accepted May 24, 2005.
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