MdATG18a overexpression improves tolerance to nitrogen deficiency and regulates anthocyanin accumulation through increased autophagy in transgenic apple

Abstract

Nitrogen (N) availability is an essential factor for plant growth. Recycling and remobilization of N have strong impacts on crop yield and quality under N deficiency. Autophagy is a critical nutrient-recycling process that facilitates remobilization under starvation. We previously showed that an important AuTophaGy (ATG) protein from apple, MdATG18a, has a positive role in drought tolerance. In this study, we explored its biological role in response to low-N. Overexpression of MdATG18a in both Arabidopsis and apple improved tolerance to N-depletion and caused a greater accumulation of anthocyanin. The increased anthocyanin concentration in transgenic apple was possibly due to up-regulating flavonoid biosynthetic and regulatory genes (MdCHI, MdCHS, MdANS, MdPAL, MdUFGT, and MdMYB1) and higher soluble sugars concentration. MdATG18a overexpression enhanced starch degradation with up-regulating amylase gene (MdAM1) and up-regulated sugar metabolism related genes (MdSS1, MdHXKs, MdFK1, and MdNINVs). Furthermore, MdATG18a functioned in nitrate uptake and assimilation by up-regulating nitrate reductase MdNIA2 and 3 high-affinity nitrate transporters MdNRT2.1/2.4/2.5. MdATG18a overexpression also elevated other important MdATG genes expression and autophagosomes formation under N-depletion, which play key contributions to above changes. Together, these results demonstrate that overexpression of MdATG18a enhances tolerance to N-deficiencies and plays positive roles in anthocyanin biosynthesis through greater autophagic activity.

Comprehensive Analysis of Autophagy-Related Genes in Sweet Orange (Citrus sinensis) Highlights Their Roles in Response to Abiotic Stresses

“…Most ATGs were first identified and functionally characterized by mutagenesis studies in yeast [2]. More than 30 ATGs have now been identified in yeast, and 23, including ATG1-10, [12][13][14][16][17][18]20,27,29,31,TOR,VPS15,and VPS34, are considered to be the core ATGs participating in autophagy [3,4]. In Arabidopsis, around 40 homologues to these core ATGs have been identified, except for ATG14, 17,27,29, and 31 with no homologues [1,3].…”

“…For example, an apple MdATG8i (Malus domestica, Md) was significantly up-regulated in response to nitrogen depletion and oxidative stress, and overexpression of MdATG8i in apple callus conferred enhanced tolerance to nutrient-limited conditions [17]. MdATG3a, MdATG3b, MdATG7b, and MdATG18a of apple were shown to confer tolerance to nitrogen/carbon starvation, drought, salt, or osmotic stresses by ectopic-expressing in Arabidopsis and apple [18][19][20][21]. Li et al reported that a banana MaATG8f improved drought stress tolerance in transgenic Arabidopsis through modulating reactive oxygen species (ROS) metabolism, abscisic acid biosynthesis, and autophagy activity [22].…”

Autophagy: An Intracellular Degradation Pathway Regulating Plant Survival and Stress Response

“…However, even under nitrogen rich conditions, the seed yield and 15 N reallocation into the seeds are much lower in atg12 than in wild type (Li et al, 2015a). Recently, a study illustrated that the overexpression of ATG18a in apple results in increased tolerance to nitrogen starvation due to increased autophagy under these conditions (Sun et al, 2018). Additionally, under carbon starvation, the Arabidopsis atg5 and atg7 mutants exhibit delayed growth, reduced amino acid levels, increased respiration, and decreased flux to net protein synthesis.…”

“…In Arabidopsis, the atg5, atg7, and ATG18a RNAi mutants display hypersensitization to drought stress (Zhou et al, 2013). On the other hand, overexpression of ATG18a in apple results in higher autophagy activity and increased drought tolerance, indicating the key role for autophagy in responses to drought (Sun et al, 2018). Recently, Medicago truncatula dehydrin MtCAS31 (cold acclimation-specific 31) was found to interact with both ATG8a and a negative drought stress regulator MtPIP2;7 to improve drought tolerance by facilitating autophagic degradation of MtPIP2;7, demonstrating that MtCAS31 functions as a positive regulator of drought-induced autophagy (Li et al, 2019b).…”

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

“…The maize autophagy mutant atg12 has been used to reveal the specific aspects of maize metabolism that are controlled by autophagy (McLoughlin et al, 2018). In our previous study, we characterized MdATG18a in apple, and demonstrated that it enhances plant resistance to drought stress (Sun et al, 2018a), nitrogen depletion (Sun et al, 2018b), and heat stress (Huo et al, 2020a).…”