The ATG Autophagic Conjugation System in Maize: ATG Transcripts and Abundance of the ATG8-Lipid Adduct Are Regulated by Development and Nutrient Availability    

Abstract

Plants employ sophisticated mechanisms to recycle intracellular constituents needed for growth, development, and survival under nutrient-limiting conditions. Autophagy is one important route in which cytoplasm and organelles are sequestered in bulk into vesicles and subsequently delivered to the vacuole for breakdown by resident hydrolases. The formation and trafficking of autophagic vesicles are directed in part by associated conjugation cascades that couple the AUTOPHAGY-RELATED8 (ATG8) and ATG12 proteins to their respective targets, phosphatidylethanolamine and the ATG5 protein. To help understand the importance of autophagy to nutrient remobilization in cereals, we describe here the ATG8/12 conjugation cascades in maize (Zea mays) and examine their dynamics during development, leaf senescence, and nitrogen and fixed-carbon starvation. From searches of the maize genomic sequence using Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) counterparts as queries, we identified orthologous loci encoding all components necessary for ATG8/12 conjugation, including a five-member gene family expressing ATG8. Alternative splicing was evident for almost all Atg transcripts, which could have important regulatory consequences. In addition to free ATG8, its membrane-associated, lipidated form was detected in many maize tissues, suggesting that its conjugation cascade is active throughout the plant at most, if not all, developmental stages. Levels of Atg transcripts and/or the ATG8-phosphatidylethanolamine adduct increase during leaf senescence and nitrogen and fixed-carbon limitations, indicating that autophagy plays a key role in nutrient remobilization. The description of the maize ATG system now provides a battery of molecular and biochemical tools to study autophagy in this crop under field conditions.

Comparative analyses of ubiquitin-like ATG8 and cysteine protease ATG4 autophagy genes in the plant lineage and cross-kingdom processing of ATG8 by ATG4

“…S2). 32 We found more examples of identical ATG8 sequences in potato (StPGSC0003DMP400025414 with StPGSC0003DMP400038670) and soybean (Gm09g00630 with Gm15g11510). These results indicate that ATG4 and ATG8 genes are conserved among plant lineages.…”

“…30,31 However, little is known about ATG genes or autophagy mechanism in other plant species. Although a few ATG8 orthologs have been used as markers in some plants, [32][33][34][35][36][37] there is no systematic analysis of ATG4 and ATG8 in agronomically important crop plants.…”

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

“…As a control, we compared the expression pattern of ATG in cotyledons (see Supplemental Figure 5 online). We focused on genes encoding the ATG8 conjugation system because the expression of these genes is often regulated developmentally (van der Graaff et al, 2006;Chung et al, 2009). A preliminary search in the Genevestigator database (http://www.genevestigator.com) revealed that ATG3, ATG8b, ATG8g, and ATG8i showed higher expression levels in hypocotyls than in cotyledons.…”

“…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.…”

Role of plant autophagy in stress response

“…Two putative ATG13 genes were found in Arabidopsis, but their similarity is restricted to a relatively small region of the protein and it is currently unclear if they are true ATG13 orthologs (Bassham et al, 2006). Recently, rice and maize ATG genes have also been identified and have been characterized (Su et al, 2006;Chung et al, 2009;Xia et al, 2011).…”