Isolation and characterization of MdATG18a, a WD40-repeat AuTophaGy-related gene responsive to leaf senescence and abiotic stress in Malus

Autophagy is an important means for recycling damaged cell components or degrading various proteins when plants are under starvation or oxidative stress. AuTophaGy-related gene 18 (ATG18), a phosphoinositide-binding protein that functions through the WD40 domain, is required for vesicle formation in autophagy. Here, we isolated and characterized MdATG18a from Malus domestica. Its full-length cDNA is a complete 1281 bp open reading frame (ORF) of nucleotides encoding a polypeptide of 426 amino acids. The deduced protein contains two WD40 domains. Phylogenetic analysis confirmed that MdATG18a clusters with Arabidopsis members AtATG18a, c, d, and e. The 3D structure has a seven-bladed β-propeller and two putative phosphatidylinositol 3-phosphate (PI3P) binding sites. When transiently expressed in onion epidermal cells, the MdATG18a-GFP fusion protein was localized in the nucleus and cell membrane. Prediction of a 1824 bp promoter region cloned from apple genome DNA indicated several key cis-regulatory elements involved in plant responses to abiotic stresses. MdATG18a was expressed in all tested tissues and showed obvious up-regulation under leaf senescence, drought, heat, oxidative stress, nitrogen starvation or ER stress. Taken together, our findings suggest that MdATG18a plays a role in responses to leaf senescence and abiotic stresses, which might be indicative of autophagy involved in mechanism regulation for stress tolerance in apple.