The Apple Autophagy-Related Gene MdATG9 Confers Tolerance to Low Nitrogen in Transgenic Apple Callus

Abstract

Autophagy is an efficient degradation system for maintaining cellular homeostasis when plants are under environmental stress. ATG9 is the only integral membrane protein within the core ATG machinery that provides a membrane source for autophagosome formation. In this study, we isolated an <i>ATG9</i> homologs gene in apple, <i>MdATG9</i>, from <i>Malus domestica</i>. The analysis of its sequence, subcellular localization, promoter cis-elements, and expression patterns revealed the potential function of <i>MdATG9</i> in response to abiotic stressors. Overexpression of <i>MdATG9</i> in apple callus conferred enhanced tolerance to nitrogen depletion stress. During the treatment, other important <i>MdATG</i>s were expressed at higher levels in transgenic callus than in the wild type. Furthermore, more free amino acids and increased sucrose levels were found in <i>MdATG9</i>-overexpression apple callus compared with the wild type in response to nitrogen starvation, and the expression levels of <i>MdNRT1.1</i>, <i>MdNRT2.5</i>, <i>MdNIA1</i>, and <i>MdNIA2</i> were all increased higher in transgenic lines. These data suggest that, as an important autophagy gene, <i>MdATG9</i> plays an important role in the maintenance of amino acids and sugars in response to nutrient starvation in apple.