Abstract
Autophagy is a highly conserved intracellular degradation pathway that breaks down damaged macromolecules and/or organelles. It is involved in plant development and senescence, as well as in biotic and abiotic stresses. However, the autophagy process and related genes are largely unknown in citrus. In this study, we identified 35 autophagy-related genes (CsATGs—autophagy-related genes (ATGs) of Citrus sinensis, Cs) in a genome-wide manner from sweet orange (Citrus sinensis). Bioinformatic analysis showed that these CsATGs were highly similar to Arabidopsis ATGs in both sequence and phylogeny. All the CsATGs were randomly distributed on nine known (28 genes) and one unknown (7 genes) chromosomes. Ten CsATGs were predicted to be segmental duplications. Expression patterns suggested that most of the CsATG were significantly up- or down-regulated in response to drought; cold; heat; salt; mannitol; and excess manganese, copper, and cadmium stresses. In addition, two ATG18 members, CsATG18a and CsATG18b, were cloned from sweet orange and ectopically expressed in Arabidopsis. The CsATG18a and CsATG18b transgenic plants showed enhanced tolerance to osmotic stress, salt, as well as drought (CsATG18a) or cold (CsATG18b), compared to wild-type plants. These results highlight the essential roles of CsATG genes in abiotic stresses.
Highlights
Autophagy is a highly conserved intracellular degradation process that occurs in eukaryotes including yeasts, animals, and plants [1,2]
On the basis of known functional classification of AtATGs, all CsATGs were classified in the categories of the ATG1/13 kinase complex, PI3K complex, ATG9/2/18 complex, ubiquitin-like ATG8 and PE conjugation pathway, ubiquitin-like ATG12 and ATG5 conjugation pathway, and SNARE (Table 1)
Among the 35 CsATGs, 15 contained only one member, whereas 4 CsATGs were encoded by gene families
Summary
Autophagy is a highly conserved intracellular degradation process that occurs in eukaryotes including yeasts, animals, and plants [1,2]. During this process, damaged macromolecules and/or organelles are sequestered into a double-membrane vesicle (called an autophagosome) and delivered into a vacuole (yeast and plants) or a lysosome (animals) for breakdown. According to the reported characterizations of ATGs in yeast and Arabidopsis, these ATGs can be divided into the following functional groups: (1) the ATG1/13 kinase complex consisting of ATG1, ATG13, ATG20, and TOR (target of rapamycin kinase), mainly functioning on autophagy induction and initiation; (2) the PI3K (phosphatidylinositol 3 kinase) complex consisting of ATG6, VPS15 (vacuolar protein sorting-associated protein), and VPS34 that is involved in vesicle nucleation and autophagosome formation; (3) the ATG9/2/18 complex consisting of ATG9, ATG2, and ATG18 that is responsible for the delivery of membranes for autophagosome formation; (4) the ubiquitin-like ATG8-PE (phosphatidylethanolamine) conjugation pathway (including ATG3, ATG4, ATG7, and ATG8) and ATG12–ATG5 conjugation pathway (including ATG5, ATG7, ATG10, ATG12, and ATG16) that are involved in the elongation of autophagic vesicles; and (5) the VTI12 (vesicle transport v-soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) protein) family belonging to the SNARE group, which contributes to the fusion of autophagosomes with vacuoles [1,2,3,4,6]
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