Characterization of an Autophagy-Related Gene MdATG8i from Apple

Systematic analysis and expression of Gossypium ATG8 family reveals the roles of GhATG8f responding to salt stress in cotton

KEY MESSAGE

Comprehensive analysis of Gossypium ATG8 family indicates that GhATG8f could improve salt tolerance of cotton by increasing SOD, POD and CAT activity and proline accumulation. In plants, autophagy is regulated by several genes that play important roles in initiating and controlling the process. ATG8, functioning as a protein similar to ubiquitin, is involved in crucial tasks throughout the autophagosome formation process. In this research, we conducted an extensive and all-encompassing investigation of 64 ATG8 genes across four varieties of cotton. According to the subcellular localization prediction results, 49 genes were found in the cytoplasm, 6 genes in the chloroplast, 1 gene in the peroxisome, 5 genes in the nucleus, and 3 genes in the extracellular region. Phylogenetic analysis categorized a total of 5 subfamilies containing sixty-four ATG8 genes. The expression of the majority of GhATG8 genes was induced by salt, drought, cold, and heat stresses, as revealed by RNA-seq and real-time PCR. Analysis of cis-elements in the promoters of GhATG8 genes revealed the predominant presence of responsive elements for plant hormones and abiotic stress, suggesting that GhATG8 genes might have significant functions in abiotic stress response. Furthermore, we additionally performed a gene interaction network analysis for the GhATG8 proteins. The salt stress resistance of cotton was reduced due to the downregulation of GhATG8f expression, resulting in decreased activity of CAT, SOD, and POD enzymes, as well as decreased fresh weight and proline accumulation. In summary, our research is the initial exploration of ATG8 gene components in cotton, providing a basis for future investigations into the regulatory mechanisms of ATG8 genes in autophagy and their response to abiotic stress.

Role of autophagy-related proteins ATG8f and ATG8h in the maintenance of autophagic activity in Arabidopsis roots under phosphate starvation

Nutrient starvation-induced autophagy is a conserved process in eukaryotes. Plants defective in autophagy show hypersensitivity to carbon and nitrogen limitation. However, the role of autophagy in plant phosphate (Pi) starvation response is relatively less explored. Among the core autophagy-related (ATG) genes, ATG8 encodes a ubiquitin-like protein involved in autophagosome formation and selective cargo recruitment. The Arabidopsis thaliana ATG8 genes, AtATG8f and AtATG8h, are notably induced in roots under low Pi. In this study, we show that such upregulation correlates with their promoter activities and can be suppressed in the phosphate response 1 (phr1) mutant. Yeast one-hybrid analysis failed to attest the binding of the AtPHR1 transcription factor to the promoter regions of AtATG8f and AtATG8h. Dual luciferase reporter assays in Arabidopsis mesophyll protoplasts also indicated that AtPHR1 could not transactivate the expression of both genes. Loss of AtATG8f and AtATG8h leads to decreased root microsomal-enriched ATG8 but increased ATG8 lipidation. Moreover, atg8f/atg8h mutants exhibit reduced autophagic flux estimated by the vacuolar degradation of ATG8 in the Pi-limited root but maintain normal cellular Pi homeostasis with reduced number of lateral roots. While the expression patterns of AtATG8f and AtATG8h overlap in the root stele, AtATG8f is more strongly expressed in the root apex and root hair and remarkably at sites where lateral root primordia develop. We hypothesize that Pi starvation-induction of AtATG8f and AtATG8h may not directly contribute to Pi recycling but rely on a second wave of transcriptional activation triggered by PHR1 that fine-tunes cell type-specific autophagic activity.

CsATG101 Delays Growth and Accelerates Senescence Response to Low Nitrogen Stress in Arabidopsis thaliana

For tea plants, nitrogen (N) is a foundational element and large quantities of N are required during periods of roundly vigorous growth. However, the fluctuation of N in the tea garden could not always meet the dynamic demand of the tea plants. Autophagy, an intracellular degradation process for materials recycling in eukaryotes, plays an important role in nutrient remobilization upon stressful conditions and leaf senescence. Studies have proven that numerous autophagy-related genes (ATGs) are involved in N utilization efficiency in Arabidopsis thaliana and other species. Here, we identified an ATG gene, CsATG101, and characterized the potential functions in response to N in A. thaliana. The expression patterns of CsATG101 in four categories of aging gradient leaves among 24 tea cultivars indicated that autophagy mainly occurred in mature leaves at a relatively high level. Further, the in planta heterologous expression of CsATG101 in A. thaliana was employed to investigate the response of CsATG101 to low N stress. The results illustrated a delayed transition from vegetative to reproductive growth under normal N conditions, while premature senescence under N deficient conditions in transgenic plants vs. the wild type. The expression profiles of 12 AtATGs confirmed the autophagy process, especially in mature leaves of transgenic plants. Also, the relatively high expression levels for AtAAP1, AtLHT1, AtGLN1;1, and AtNIA1 in mature leaves illustrated that the mature leaves act as the source leaves in transgenic plants. Altogether, the findings demonstrated that CsATG101 is a candidate gene for improving annual fresh tea leaves yield under both deficient and sufficient N conditions via the autophagy process.

Linking Autophagy to Potential Agronomic Trait Improvement in Crops

Autophagy is an evolutionarily conserved catabolic process in eukaryotic cells, by which the superfluous or damaged cytoplasmic components can be delivered into vacuoles or lysosomes for degradation and recycling. Two decades of autophagy research in plants uncovers the important roles of autophagy during diverse biological processes, including development, metabolism, and various stress responses. Additionally, molecular machineries contributing to plant autophagy onset and regulation have also gradually come into people’s sights. With the advancement of our knowledge of autophagy from model plants, autophagy research has expanded to include crops in recent years, for a better understanding of autophagy engagement in crop biology and its potentials in improving agricultural performance. In this review, we summarize the current research progress of autophagy in crops and discuss the autophagy-related approaches for potential agronomic trait improvement in crop plants.

Overexpression of MdATG8i Enhances Drought Tolerance by Alleviating Oxidative Damage and Promoting Water Uptake in Transgenic Apple

Water deficit adversely affects apple (Malus domestica) productivity on the Loess Plateau. Autophagy plays a key role in plant responses to unfavorable environmental conditions. Previously, we demonstrated that a core apple autophagy-related protein, MdATG8i, was responsive to various stresses at the transcript level. Here, we investigated the function of this gene in the response of apple to severe drought and found that its overexpression (OE) significantly enhanced drought tolerance. Under drought conditions, MdATG8iOE apple plants exhibited less drought-related damage and maintained higher photosynthetic capacities compared with the wild type (WT). The accumulation of ROS (reactive oxygen species) was lower in OE plants under drought stress and was accompanied by higher activities of antioxidant enzymes. Besides, OE plants accumulated lower amounts of insoluble or oxidized proteins but greater amounts of amino acids and flavonoid under severe drought stress, probably due to their enhanced autophagic activities. Particularly, MdATG8iOE plants showed higher root hydraulic conductivity than WT plants did under drought conditions, indicating the enhanced ability of water uptake. In summary, the overexpression of MdATG8i alleviated oxidative damage, modulated amino acid metabolism and flavonoid synthesis, and improved root water uptake, ultimately contributing to enhanced drought tolerance in apple.

Overexpression of MdATG8i improves water use efficiency in transgenic apple by modulating photosynthesis, osmotic balance, and autophagic activity under moderate water deficit

Water deficit is one of the major limiting factors for apple (Malus domestica) production on the Loess Plateau, a major apple cultivation area in China. The identification of genes related to the regulation of water use efficiency (WUE) is a crucial aspect of crop breeding programs. As a conserved degradation and recycling mechanism in eukaryotes, autophagy has been reported to participate in various stress responses. However, the relationship between autophagy and WUE regulation has not been explored. We have shown that a crucial autophagy protein in apple, MdATG8i, plays a role in improving salt tolerance. Here, we explored its biological function in response to long-term moderate drought stress. The results showed that MdATG8i-overexpressing (MdATG8i-OE) apple plants exhibited higher WUE than wild-type (WT) plants under long-term moderate drought conditions. Plant WUE can be increased by improving photosynthetic efficiency. Osmoregulation plays a critical role in plant stress resistance and adaptation. Under long-term drought conditions, the photosynthetic capacity and accumulation of sugar and amino acids were higher in MdATG8i-OE plants than in WT plants. The increased photosynthetic capacity in the OE plants could be attributed to their ability to maintain optimal stomatal aperture, organized chloroplasts, and strong antioxidant activity. MdATG8i overexpression also promoted autophagic activity, which was likely related to the changes described above. In summary, our results demonstrate that MdATG8i-OE apple lines exhibited higher WUE than WT under long-term moderate drought conditions because they maintained robust photosynthesis, effective osmotic adjustment processes, and strong autophagic activity.

Autophagy mediates grain yield and nitrogen stress resistance by modulating nitrogen remobilization in rice

Autophagy, a conserved cellular process in eukaryotes, has evolved to a sophisticated process to dispose of intracellular constituents and plays important roles in plant development, metabolism, and efficient nutrients remobilization under suboptimal nutrients conditions. Here, we show that OsATG8b, an AUTOPHAGY-RELATED8 (ATG8) gene in rice, was highly induced by nitrogen (N) starvation. Elevated expression of OsATG8b significantly increased ATG8 lipidation, autophagic flux, and grain yield in rice under both sufficient and deficient N conditions. Overexpressing of OsATG8b could greatly increase the activities of enzymes related to N metabolism. Intriguingly, the 15N-labeling assay further revealed that more N was remobilized to seeds in OsATG8b-overexpressing rice, which significantly increased the N remobilization efficiency (NRE), N harvest index, N utilization efficiency (NUE), and N uptake efficiency (NUpE). Conversely, the osatg8b knock-out mutants had the opposite results on these characters. The substantial transcriptional changes of the overexpressed transgenic lines indicated the presence of complex signaling to developmental, metabolic process, and hormone, etc. Excitingly, the transgenic rice under different backgrounds all similarly be boosted in yield and NUE with OsATG8b overexpression. This work provides an excellent candidate gene for improving N remobilization, utilization, and yield in crops simultaneously.

Genome-wide identification and expression analysis of the pear autophagy-related gene PbrATG8 and functional verification of PbrATG8c in Pyrus bretschneideri Rehd

Genome-wide identification, tissue-specific and stress expression analyses and functional characterization of PbrATG8s genes were conducted and the role of PbrATG8c in Botryosphaeria dothidea resistance was further investigated. Autophagy plays an important role in plant growth, development and stress tolerance. ATG8 has been reported to be an autophagy marker in many species. However, there is little information regarding ATG8 family members in pear (Pyrus bretschneideri Rehd). We performed a genome-wide analysis and identified nine PbrATG8 gene family members in pear. Phylogenetic analysis showed that PbrATG8 genes clustered into four major groups (Groups I-IV). Eight PbrATG8 genes were successfully mapped to 6 of the 17 chromosomes of the pear genome. The synteny results showed that two pairs are collinear. Gene expression data showed that all genes were differentially expressed in a range of pear tissues. Transcript analysis of PbrATG8 genes under dehydration, salt and pathogen infection stresses revealed that PbrATG8c responded to all test stresses. The PbrATG8c protein was localized in the nucleus and membrane. The silencing of PbrATG8c decreased the resistance to Botryosphaeria dothidea in pear. This study provides insights and rich resources for subsequent investigations of autophagy in pear.

Potential Biotechnological Applications of Autophagy for Agriculture

Autophagy is a genetically regulated, eukaryotic cellular degradation system that sequestrates cytoplasmic materials in specialised vesicles, termed autophagosomes, for delivery and breakdown in the lysosome or vacuole. In plants, autophagy plays essential roles in development (e.g., senescence) and responses to abiotic (e.g., nutrient starvation, drought and oxidative stress) and biotic stresses (e.g., hypersensitive response). Initially, autophagy was considered a non-selective bulk degradation mechanism that provides energy and building blocks for homeostatic balance during stress. Recent studies, however, reveal that autophagy may be more subtle and selectively target ubiquitylated protein aggregates, protein complexes and even organelles for degradation to regulate vital cellular processes even during favourable conditions. The selective nature of autophagy lends itself to potential manipulation and exploitation as part of designer protein turnover machinery for the development of stress-tolerant and disease-resistant crops, crops with increased yield potential and agricultural efficiency and reduced post-harvest losses. Here, we discuss our current understanding of autophagy and speculate its potential manipulation for improved agricultural performance.

Signaling Responses to N Starvation: Focusing on Wheat and Filling the Putative Gaps With Findings Obtained in Other Plants. A Review

Wheat is one of the most important food crops worldwide. In recent decades, fertilizers, especially nitrogen (N), have been increasingly utilized to maximize wheat productivity. However, a large proportion of N is not used by plants and is in fact lost into the environment and causes serious environmental pollution. Therefore, achieving a low N optimum via efficient physiological and biochemical processes in wheat grown under low-N conditions is highly important for agricultural sustainability. Although N stress-related N capture in wheat has become a heavily researched subject, how this plant adapts and responds to N starvation has not been fully elucidated. This review summarizes the current knowledge on the signaling mechanisms activated in wheat plants in response to N starvation. Furthermore, we filled the putative gaps on this subject with findings obtained in other plants, primarily rice, maize, and Arabidopsis. Phytohormones have been determined to play essential roles in sensing environmental N starvation and transducing this signal into an adjustment of N transporters and phenotypic adaptation. The critical roles played by protein kinases and critical kinases and phosphatases, such as MAPK and PP2C, as well as the multifaceted functions of transcription factors, such as NF-Y, MYB, DOF, and WRKY, in regulating the expression levels of their target genes (proteins) for low-N tolerance are also discussed. Optimization of root system architecture (RSA) via root branching and thinning, improvement of N acquisition and assimilation, and fine-tuned autophagy are pivotal strategies by which plants respond to N starvation. In light of these findings, we attempted to construct regulatory networks for RSA modification and N uptake, transport, assimilation, and remobilization.

Genome-Wide Identification of CsATGs in Tea Plant and the Involvement of CsATG8e in Nitrogen Utilization

Nitrogen (N) is a macroelement with an indispensable role in the growth and development of plants, and tea plant (Camellia sinensis) is an evergreen perennial woody species with young shoots for harvest. During senescence or upon N stress, autophagy has been shown to be induced in leaves, involving a variety of autophagy-related genes (ATGs), which have not been characterized in tea plant yet. In this study, a genome-wide survey in tea plant genome identified a total of 80 Camellia Sinensis autophagy-related genes, CsATGs. The expression of CsATG8s in the tea plant showed an obvious increase from S1 (stage 1) to S4 (stage 4), especially for CsATG8e. The expression levels of AtATGs (Arabidopsis thaliana) and genes involved in N transport and assimilation were greatly improved in CsATG8e-overexpressed Arabidopsis. Compared with wild type, the overexpression plants showed earlier bolting, an increase in amino N content, as well as a decrease in biomass and the levels of N, phosphorus and potassium. However, the N level was found significantly higher in APER (aerial part excluding rosette) in the overexpression plants relative to wild type. All these results demonstrated a convincing function of CsATG8e in N remobilization and plant development, indicating CsATG8e as a potential gene for modifying plant nutrient utilization.

Transcriptional Plasticity of Autophagy-Related Genes Correlates with the Genetic Response to Nitrate Starvation in Arabidopsis Thaliana

In eukaryotes, autophagy, a catabolic mechanism for macromolecule and protein recycling, allows the maintenance of amino acid pools and nutrient remobilization. For a better understanding of the relationship between autophagy and nitrogen metabolism, we studied the transcriptional plasticity of autophagy genes (ATG) in nine Arabidopsis accessions grown under normal and nitrate starvation conditions. The status of the N metabolism in accessions was monitored by measuring the relative expression of 11 genes related to N metabolism in rosette leaves. The transcriptional variation of the genes coding for enzymes involved in ammonium assimilation characterize the genetic diversity of the response to nitrate starvation. Starvation enhanced the expression of most of the autophagy genes tested, suggesting a control of autophagy at transcriptomic level by nitrogen. The diversity of the gene responses among natural accessions revealed the genetic variation existing for autophagy independently of the nutritive condition, and the degree of response to nitrate starvation. We showed here that the genetic diversity of the expression of N metabolism genes correlates with that of the ATG genes in the two nutritive conditions, suggesting that the basal autophagy activity is part of the integral response of the N metabolism to nitrate availability.

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

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.

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

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 ATG9 homologs gene in apple, MdATG9, from Malus domestica. The analysis of its sequence, subcellular localization, promoter cis-elements, and expression patterns revealed the potential function of MdATG9 in response to abiotic stressors. Overexpression of MdATG9 in apple callus conferred enhanced tolerance to nitrogen depletion stress. During the treatment, other important MdATGs 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 MdATG9-overexpression apple callus compared with the wild type in response to nitrogen starvation, and the expression levels of MdNRT1.1, MdNRT2.5, MdNIA1, and MdNIA2 were all increased higher in transgenic lines. These data suggest that, as an important autophagy gene, MdATG9 plays an important role in the maintenance of amino acids and sugars in response to nutrient starvation in apple.

Overexpression of Banana ATG8f Modulates Drought Stress Resistance in Arabidopsis

Autophagy is essential for plant growth, development, and stress resistance. However, the involvement of banana autophagy-related genes in drought stress response and the underlying mechanism remain elusive. In this study, we found that the transcripts of 10 banana ATG8s responded to drought stress in different ways, and MaATG8f with the highest transcript in response to drought stress among them was chosen for functional analysis. Overexpression of MaATG8f improved drought stress resistance in Arabidopsis, with lower malonaldehyde level and higher level of assimilation rate. On the one hand, overexpression of MaATG8f activated the activities of superoxide dismutase, catalase, and peroxidase under drought stress conditions, so as to regulate reactive oxygen species accumulation. On the other hand, MaATG8f-overexpressing lines exhibited higher endogenous abscisic acid (ABA) level and more sensitivity to abscisic acid. Notably, the autophagosomes as visualized by CaMV35S::GFP–MaATG8f was activated after ABA treatment. Taken together, overexpression of MaATG8f positively regulated plant drought stress resistance through modulating reactive oxygen species metabolism, abscisic acid biosynthesis, and autophagic activity.

Autophagy and Nutrients Management in Plants

Nutrient recycling and mobilization from organ to organ all along the plant lifespan is essential for plant survival under changing environments. Nutrient remobilization to the seeds is also essential for good seed production. In this review, we summarize the recent advances made to understand how plants manage nutrient remobilization from senescing organs to sink tissues and what is the contribution of autophagy in this process. Plant engineering manipulating autophagy for better yield and plant tolerance to stresses will be presented.

OsATG8c-Mediated Increased Autophagy Regulates the Yield and Nitrogen Use Efficiency in Rice

Autophagy, a conserved pathway in eukaryotes, degrades and recycles cellular components, thus playing an important role in nitrogen (N) remobilization. N plays an important role in the growth and development of plants, which also affects plant yield and quality. In this research, it was found that the transcriptional level of a core autophagy gene of rice (Oryza sativa), OsATG8c, was increased during N starvation conditions. It was found that the overexpression of OsATG8c significantly enhanced the activity of autophagy and that the number of autophagosomes, dwarfed the plant height and increased the effective tillers’ number and yield. The nitrogen uptake efficiency (NUpE) and nitrogen use efficiency (NUE) significantly increased in the transgenic rice under both optimal and suboptimal N conditions. Based on our results, OsATG8c is considered to be a good candidate gene for increasing NUE, especially under suboptimal field conditions.

Overexpression of rice gene OsATG8b confers tolerance to nitrogen starvation and increases yield and nitrogen use efficiency (NUE) in Arabidopsis

Nitrogen (N) is an important element required for plant growth and development, which also affects plant yield and quality. Autophagy, a conserved pathway in eukaryotes, degrades and recycles cellular components, thus playing an important role in N remobilization. However, only a few autophagy genes related to N remobilization in rice (Oryza sativa) have been reported. Here, we identified a core autophagy gene in rice, OsATG8b, with increased expression levels under N starvation conditions. It was investigated the function of OsATG8b by generating three independent homozygous 35S-OsATG8b transgenic Arabidopsis thaliana lines. The overexpression of OsATG8b significantly enhanced autophagic flux in the transgenic Arabidopsis plants. It was also showed that over-expressing OsATG8b promoted growth and development of Arabidopsis, in which the rosette leaves were larger than those of the wild type (WT), and the yield increased significantly by 25.25%. In addition, the transgenic lines accumulated more N in seeds than in the rosette leaves. Further examination revealed that overexpression of OsATG8b could effectively alleviate the growth inhibition of transgenic Arabidopsis under nitrogen (N) stress. N partitioning studies revealed that nitrogen-harvest index (NHI) and nitrogen use efficiency (NUE) were significantly increased in the transgenic Arabidopsis, as well as the ¹⁵N-tracer experiments revealing that the remobilization of N to seeds in the OsATG8b-overexpressing transgenic Arabidopsis was high and more than WT. Based on our findings, we consider OsATG8b to be a great candidate gene to increase NUE and yield, especially under suboptimal field conditions.

Increased Autophagy of Rice Can Increase Yield and Nitrogen Use Efficiency (NUE)

Autophagy (self-eating), a conserved pathway in eukaryotes, which is designed to handle cytoplasmic material in bulk and plays an important role in the remobilization of nutrient, such as nitrogen (N) under suboptimal nutrient conditions. Here, we identified a core component of an autophagy gene in rice (Oryza sativa), OsATG8a, with increased expression levels under N starvation conditions. Overexpression of OsATG8a significantly enhanced the level of autophagy and the number of effective tillers in the transgenic rice. In addition, the transgenic lines accumulated more N in grains than in the dry remains and the yield was significantly increased under normal N conditions. Further N allocation studies revealed that the nitrogen uptake efficiency (NUpE) and nitrogen use efficiency (NUE) significantly increased. Otherwise, under suboptimal N conditions, overexpression of OsATG8a did not seem to have any effect on yield and NUE, but NUpE was still improved significantly. Based on our findings, we consider OsATG8a to be a great candidate gene to increase NUE and yield.

Autophagy in crop plants: what's new beyond Arabidopsis?

Autophagy is a major degradation and recycling pathway in plants. It functions to maintain cellular homeostasis and is induced by environmental cues and developmental stimuli. Over the past decade, the study of autophagy has expanded from model plants to crop species. Many features of the core machinery and physiological functions of autophagy are conserved among diverse organisms. However, several novel functions and regulators of autophagy have been characterized in individual plant species. In light of its critical role in development and stress responses, a better understanding of autophagy in crop plants may eventually lead to beneficial agricultural applications. Here, we review recent progress on understanding autophagy in crops and discuss potential future research directions.

Genome-wide analysis of autophagy-related genes (ARGs) in grapevine and plant tolerance to copper stress

Grapevine autophagy-related genes (ARGs) include 35 members that have unique evolutionary backgrounds and expression patterns, with some of them responding to abiotic stresses, including copper stress. Autophagy is one of the most crucial self-regulating phenomena in livings organisms, including animals, plants, yeasts, etc. In the genomes of plants, like Arabidopsis, rice, tobacco, and barley, more than 30 autophagy-related genes (ARGs) have been found. These ARGs are involved in plant development, programed cell death, and the stress response process. In plants, and particularly in grapevine, high copper stress results from the application of the Bordeaux mixture, a widely used fungicide. However, the function of autophagy in plant tolerance to copper stress is unknown. Accordingly, in this study, a genome-wide analysis was performed to identify Vitis vinifera ARGs (VvARGs), and 35 VvARGs were detected. A gene family analysis revealed that the tandem and segmental duplication events played significant roles in the VvARG gene family expansion. Moreover, there was more intense signature of purifying selection for the comparison between grape and rice than between grape and Arabidopsis. In response to copper treatment, both the autophagosome number and malondialdehyde concentration increased during the initial 4 h post-treatment, and reached maximal values at 24 h. An expression analysis indicated that most VvARGs responded to copper stress at 4 h post-treatment, and some VvARGs (e.g., VvATG6, VvATG8i, and VvATG18h) exhibited responses to most abiotic stresses. These results provide a detailed overview of the ARGs in grapevine and indicate multiple functions of autophagy in fruit development and abiotic stresses in grapevine. The key ARG (e.g., ATG8i) should be investigated in more detail in grapevine and other plant species.

Detection of autophagy processes during the development of nonarticulated laticifers in Euphorbia kansui Liou

Autophagy is involved in cytoplasmic degradation through directly engulfing cytosol and organelles by autophagosomes and then fusing with lysosome-like vesicles during the development of nonarticulated laticifers in Euphorbia kansui Liou. Autophagy has been reported to play an important role in a wide range of eukaryotic organisms during responses to various abiotic and biotic stresses. However, until recently, the functions of autophagy in normal plant differentiation and development were still in their infancy. Nonarticulated laticifers, a type of secretory tissue in plants, undergo the degradation of cytosol and organelles during their development. However, little evidence of autophagy in laticifer differentiation has been provided. In the present study, using anti-ATG8 antibody-Alexa Fluor 488, Lyso-Tracker Red (LTR) and monodansylcadaverine (MDC) as markers for detecting autophagosomes, as well as autophagy-related structures, we observed that the green fluorescence of ATG8a largely colocalized with the red fluorescence of LTR and purple fluorescence of MDC and the quantity of autophagosomes experienced a trend from less to more to less during laticifer development. Additionally, we described the autophagy process during the development of nonarticulated laticifers in Euphorbia kansui Liou at the ultrastructural level in detail. In addition, further immunogold TEM studies also verified the presence of autophagosomes, autolysosomes and lysosome-like structures in laticifers. Taken together, these results suggest that autophagy contributes to the development of the nonarticulated laticifers in E. kansui Liou and that autophagosomes fuse with lysosome-like structures for degradation. These results will lay an important foundation for further studies on laticifer regulation.

Overexpression of MdATG18a in apple improves resistance to Diplocarpon mali infection by enhancing antioxidant activity and salicylic acid levels

Marssonina apple blotch, caused by Diplocarpon mali, is one of the most serious diseases of apple. Autophagy plays a key role in pathogen resistance. We previously showed that MdATG18a has a positive influence on drought tolerance. Herein, we describe how overexpression (OE) of MdATG18a enhances resistance to D. mali infection, probably because less H₂O₂ but more salicylic acid (SA) is accumulated in the leaves of OE apple plants. Expression of chitinase, β-1,3-glucanase, and SA-related marker genes was induced more strongly by D. mali in OE lines. Transcript levels of other important MdATG genes were also drastically increased by D. mali in OE plants, which indicated increased autophagy activities. Taken together, these results demonstrate that OE of MdATG18a enhances resistance to infection by D. mali and plays positive roles in H₂O₂-scavenging and SA accumulations. Our findings provide important information for designing strategies which could induce autophagy to minimize the impact of this disease on apple production.

Endocytosis of AtRGS1 Is Regulated by the Autophagy Pathway after D-Glucose Stimulation

Sugar, as a signal molecule, has significant functions in signal transduction in which the seven-transmembrane regulator of G-protein signaling (RGS1) protein participates. D-Glucose causes endocytosis of the AtRGS1, leading to the physical uncoupling of AtRGS1 from AtGPA1 and thus a release of the GAP activity and concomitant sustained activation of G-protein signaling. Autophagy involves in massive degradation and recycling of cytoplasmic components to survive environmental stresses. The function of autophagy in AtRGS1 endocytosis during D-glucose stimulation has not been elucidated. In this study, we investigate the relationship between autophagy and AtRGS1 in response to D-glucose. Our findings demonstrated that AtRGS1 mediated the activation of autophagy by affecting the activities of the five functional groups of protein complexes and promoted the formation of autophagosomes under D-glucose application. When the autophagy pathway was interrupted, AtRGS1 recovery increased and endocytosis of ATRGS1 was inhibited, indicating that autophagy pathway plays an important role in regulating the endocytosis and recovery of AtRGS1 after D-glucose stimulation.