Autophagy as a mediator of life and death in plants

Autophagy is suppressed by low temperatures and is dispensable for cold acclimation in Arabidopsis

Plants have evolved various mechanisms to adapt to the ever-changing external environment. Autophagy is one such mechanism and has been suggested to play a key role in responding to and adapting to abiotic stresses in plants. However, the role of autophagy in adaptation to cold and freezing stresses remains to be characterized in detail. Here, we investigated the role of autophagy in the low-temperature response of Arabidopsis using atg mutants. Both the atg5-1 and atg10-1 mutants exhibited normal freezing tolerance, regardless of cold acclimation. A comparison of fresh weights indicated that the difference in growth between the wild-type and atg plants under cold conditions was rather small compared with that under normal conditions. Analysis of COLD-REGULATED gene expression showed no significant differences between the atg mutants and wild type. Treatment with 3-methyladenine, an inhibitor of autophagy, did not impair the induction of COR15Apro::LUC expression upon exposure to low temperature. Evaluation of autophagic activity using transgenic plants expressing RBCS-mRFP demonstrated that autophagy was rarely induced by cold exposure, even in the dark. Taken together, these data suggest that autophagy is suppressed by low temperatures and is dispensable for cold acclimation and freezing tolerance in Arabidopsis.

The plant cell wall-dynamic, strong, and adaptable-is a natural shapeshifter

Mythology is replete with good and evil shapeshifters, who, by definition, display great adaptability and assume many different forms-with several even turning themselves into trees. Cell walls certainly fit this definition as they can undergo subtle or dramatic changes in structure, assume many shapes, and perform many functions. In this review, we cover the evolution of knowledge of the structures, biosynthesis, and functions of the 5 major cell wall polymer types that range from deceptively simple to fiendishly complex. Along the way, we recognize some of the colorful historical figures who shaped cell wall research over the past 100 years. The shapeshifter analogy emerges more clearly as we examine the evolving proposals for how cell walls are constructed to allow growth while remaining strong, the complex signaling involved in maintaining cell wall integrity and defense against disease, and the ways cell walls adapt as they progress from birth, through growth to maturation, and in the end, often function long after cell death. We predict the next century of progress will include deciphering cell type-specific wall polymers; regulation at all levels of polymer production, crosslinks, and architecture; and how walls respond to developmental and environmental signals to drive plant success in diverse environments.

Liquid-liquid phase separation in plants: Advances and perspectives from model species to crops

Membraneless biomolecular condensates play important roles in both normal biological activities and responses to environmental stimuli in living organisms. Liquid‒liquid phase separation (LLPS) is an organizational mechanism that has emerged in recent years to explain the formation of biomolecular condensates. In the past decade, advances in LLPS research have contributed to breakthroughs in disease fields. By contrast, although LLPS research in plants has progressed over the past 5 years, it has been concentrated on the model plant Arabidopsis, which has limited relevance to agricultural production. In this review, we provide an overview of recently reported advances in LLPS in plants, with a particular focus on photomorphogenesis, flowering, and abiotic and biotic stress responses. We propose that many potential LLPS proteins also exist in crops and may affect crop growth, development, and stress resistance. This possibility presents a great challenge as well as an opportunity for rigorous scientific research on the biological functions and applications of LLPS in crops.

Interplay of energy metabolism and autophagy

Macroautophagy/autophagy, is widely recognized for its crucial role in enabling cell survival and maintaining cellular energy homeostasis during starvation or energy stress. Its regulation is intricately linked to cellular energy status. In this review, covering yeast, mammals, and plants, we aim to provide a comprehensive overview of the understanding of the roles and mechanisms of carbon- or glucose-deprivation related autophagy, showing how cells effectively respond to such challenges for survival. Further investigation is needed to determine the specific degraded substrates by autophagy during glucose or energy deprivation and the diverse roles and mechanisms during varying durations of energy starvation.Abbreviations: ADP: adenosine diphosphate; AMP: adenosine monophosphate; AMPK: AMP-activated protein kinase; ATG: autophagy related; ATP: adenosine triphosphate; ER: endoplasmic reticulum; ESCRT: endosomal sorting complex required for transport; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GD: glucose deprivation; GFP: green fluorescent protein; GTPases: guanosine triphosphatases; HK2: hexokinase 2; K phaffii: Komagataella phaffii; LD: lipid droplet; MAP1LC3/LC3: microtubule-associated protein1 light chain 3; MAPK: mitogen-activated protein kinase; Mec1: mitosis entry checkpoint 1; MTOR: mechanistic target of rapamycin kinase; NAD (+): nicotinamide adenine dinucleotide; OGD: oxygen and glucose deprivation; PAS: phagophore assembly site; PCD: programmed cell death; PtdIns3K: class III phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol-3-phosphate; ROS: reactive oxygen species; S. cerevisiae: Saccharomyces cerevisiae; SIRT1: sirtuin 1; Snf1: sucrose non-fermenting 1; STK11/LKB1: serine/threonine kinase 11; TFEB: transcription factor EB; TORC1: target of rapamycin complex 1; ULK1: unc-51 like kinase 1; Vps27: vacuolar protein sorting 27; Vps4: vacuolar protein sorting 4.

Silencing GmATG7 Leads to Accelerated Senescence and Enhanced Disease Resistance in Soybean

Autophagy plays a critical role in nutrient recycling/re-utilizing under nutrient deprivation conditions. However, the role of autophagy in soybeans has not been intensively investigated. In this study, the Autophay-related gene 7 (ATG7) gene in soybeans (referred to as GmATG7) was silenced using a virus-induced gene silencing approach mediated by Bean pod mottle virus (BPMV). Our results showed that ATG8 proteins were highly accumulated in the dark-treated leaves of the GmATG7-silenced plants relative to the vector control leaves (BPMV-0), which is indicative of an impaired autophagy pathway. Consistent with the impaired autophagy, the dark-treated GmATG7-silenced leaves displayed an accelerated senescence phenotype, which was not seen on the dark-treated BPMV-0 leaves. In addition, the accumulation levels of both H2O2 and salicylic acid (SA) were significantly induced in the GmATG7-silenced plants compared with the BPMV-0 plants, indicating an activated immunity. Consistently, the GmATG7-silenced plants were more resistant against both Pseudomonas syringae pv. glycinea (Psg) and Soybean mosaic virus (SMV) compared with the BPMV-0 plants. However, the activated immunity in the GmATG7-silenced plant was not dependent upon the activation of MPK3/MPK6. Collectively, our results demonstrated that the function of GmATG7 is indispensable for autophagy in soybeans, and the activated immunity in the GmATG7-silenced plant is a result of impaired autophagy.

Involvement of plant signaling network and cell metabolic homeostasis in nitrogen deficiency-induced early leaf senescence

Nitrogen (N) is a basic building block that plays an essential role in the maintenance of normal plant growth and its metabolic functions through complex regulatory networks. Such the N metabolic network comprises a series of transcription factors (TFs), with the coordinated actions of phytohormone and sugar signaling to sustain cell homeostasis. The fluctuating N concentration in plant tissues alters the sensitivity of several signaling pathways to stressful environments and regulates the senescent-associated changes in cellular structure and metabolic process. Here, we review recent advances in the interaction between N assimilation and carbon metabolism in response to N deficiency and its regulation to the nutrient remobilization from source to sink during leaf senescence. The regulatory networks of N and sugar signaling for N deficiency-induced leaf senescence is further discussed to explain the effects of N deficiency on chloroplast disassembly, reactive oxygen species (ROS) burst, asparagine metabolism, sugar transport, autophagy process, Ca2+ signaling, circadian clock response, brassinazole-resistant 1 (BZRI), and other stress cell signaling. A comprehensive understanding for the metabolic mechanism and regulatory network underlying N deficiency-induced leaf senescence may provide a theoretical guide to optimize the source-sink relationship during grain filling for the achievement of high yield by a selection of crop cultivars with the properly prolonged lifespan of functional leaves and/or by appropriate agronomic managements.

A selective autophagy receptor VISP1 induces symptom recovery by targeting viral silencing suppressors

Selective autophagy is a double-edged sword in antiviral immunity and regulated by various autophagy receptors. However, it remains unclear how to balance the opposite roles by one autophagy receptor. We previously identified a virus-induced small peptide called VISP1 as a selective autophagy receptor that facilitates virus infections by targeting components of antiviral RNA silencing. However, we show here that VISP1 can also inhibit virus infections by mediating autophagic degradation of viral suppressors of RNA silencing (VSRs). VISP1 targets the cucumber mosaic virus (CMV) 2b protein for degradation and attenuates its suppression activity on RNA silencing. Knockout and overexpression of VISP1 exhibit compromised and enhanced resistance against late infection of CMV, respectively. Consequently, VISP1 induces symptom recovery from CMV infection by triggering 2b turnover. VISP1 also targets the C2/AC2 VSRs of two geminiviruses and enhances antiviral immunity. Together, VISP1 induces symptom recovery from severe infections of plant viruses through controlling VSR accumulation.

The role of Atg16 in autophagy, anthocyanin biosynthesis, and programmed cell death in leaves of the lace plant (Aponogeton madagascariensis)

Aponogeton madagascariensis, commonly known as the lace plant, produces leaves that form perforations by programmed cell death (PCD). Leaf development is divided into several stages beginning with "pre-perforation" furled leaves enriched with red pigmentation from anthocyanins. The leaf blade is characterized by a series of grids known as areoles bounded by veins. As leaves develop into the "window stage", anthocyanins recede from the center of the areole towards the vasculature creating a gradient of pigmentation and cell death. Cells in the middle of the areole that lack anthocyanins undergo PCD (PCD cells), while cells that retain anthocyanins (non-PCD cells) maintain homeostasis and persist in the mature leaf. Autophagy has reported roles in survival or PCD promotion across different plant cell types. However, the direct involvement of autophagy in PCD and anthocyanin levels during lace plant leaf development has not been determined. Previous RNA sequencing analysis revealed the upregulation of autophagy-related gene Atg16 transcripts in pre-perforation and window stage leaves, but how Atg16 affects PCD in lace plant leaf development is unknown. In this study, we investigated the levels of Atg16 in lace plant PCD by treating whole plants with either an autophagy promoter rapamycin or inhibitors concanamycin A (ConA) or wortmannin. Following treatments, window and mature stage leaves were harvested and analyzed using microscopy, spectrophotometry, and western blotting. Western blotting showed significantly higher Atg16 levels in rapamycin-treated window leaves, coupled with lower anthocyanin levels. Wortmannin-treated leaves had significantly lower Atg16 protein and higher anthocyanin levels compared to the control. Mature leaves from rapamycin-treated plants generated significantly fewer perforations compared to control, while wortmannin had the opposite effect. However, ConA treatment did not significantly change Atg16 levels, nor the number of perforations compared to the control, but anthocyanin levels did increase significantly in window leaves. We propose autophagy plays a dual role in promoting cell survival in NPCD cells by maintaining optimal anthocyanin levels and mediating a timely cell death in PCD cells in developing lace plant leaves. How autophagy specifically affects anthocyanin levels remained unexplained.

Autophagy alleviates indium-induced programmed cell death in wheat roots

Indium released in agroecosystems is becoming an emerging plant stressor, causing cellular damage and consequently crop yield losses. Previous studies have focused on indium-induced toxicity in plants, while plant adaptive responses to such emerging metal xenobiotics are poorly understood. Here, we explored the relationship of autophagy and programmed cell death (PCD) in wheat roots under indium stress. Indium treatment significantly decreased root activity and cell viability, and suppressed the length of root epidermal cells in the elongation zones. These symptoms may be associated with indium-induced PCD, as indium-stressed wheat roots displayed condensed and granular nuclei, increased number of TUNEL-positive nuclei, enhanced nuclear DNA fragmentation and caspase-3-like protease activity compared to untreated roots. Accordingly, indium enhanced the expression levels of TaMCA1 and TaMCA4, two major metacaspase genes mediated PCD in wheat plants. The enhanced expression of autophagy genes and formation of autophagosomes indicate that autophagy could regulate metabolic adaptation and repair stress-induced damage in wheat roots. Furthermore, reinforcing autophagy by activator rapamycin significantly decreased the number of TUNEL-positive nuclei and the activity of caspase-3-like protease, whereas inhibition of autophagy by 3-methyladenine aggravated diagnostic markers for PCD. These results together suggest that autophagy suppresses indium-induced PCD in wheat roots.

Perfluorinated alkyl substances affect the growth, physiology and root proteome of hydroponically grown maize plants

Poly- and perfluorinated alkyl substances (PFAS) are a group of persistent organic pollutants causing serious global concern. Plants can accumulate PFAS but their effect on plant physiology, especially at the molecular level is not very well understood. Hence, we used hydroponically-grown maize plants treated with a combination of eleven different PFAS (each at 100 μg L^-1) to investigate their bioaccumulation and effects on the growth, physiology and their impact on the root proteome. A dose-dependent decrease in root growth parameters was evidenced with a significant reduction in the relative growth rate, fresh weight of leaves and roots and altered photosynthetic parameters in PFAS-treated plants. Higher concentration of shorter PFAS (C < 8) was detected in the leaves, while long-chain PFAS (C ≥ 8) were more retained in roots. From the root proteome analysis, we identified 75 differentially abundant proteins, mostly involved in cellular metabolic and biosynthetic processes, translation and cytoskeletal reorganization. Validating the altered protein abundance using quantitative real-time PCR, the results were further substantiated using amino acid and fatty acid profiling, thus, providing first insight into the altered metabolic state of plants exposed to PFAS from a proteomics perspective.

The Metacaspase TaMCA-Id Negatively Regulates Salt-Induced Programmed Cell Death and Functionally Links With Autophagy in Wheat

Metacaspases (MCAs), a family of caspase-like proteins, are important regulators of programmed cell death (PCD) in plant defense response. Autophagy is an important regulator of PCD. This study explored the underlying mechanism of the interaction among PCD, MCAs, and autophagy and their impact on wheat response to salt stress. In this study, the wheat salt-responsive gene TaMCA-Id was identified. The open reading frame (ORF) of TaMCA-Id was 1,071 bp, coding 356 amino acids. The predicted molecular weight and isoelectric point were 38,337.03 Da and 8.45, respectively. TaMCA-Id had classic characteristics of type I MCAs domains, a typical N-terminal pro-domain rich in proline. TaMCA-Id was mainly localized in the chloroplast and exhibited nucleocytoplasmictrafficking under NaCl treatment. Increased expression of TaMCA-Id in wheat seedling roots and leaves was triggered by 150 mM NaCl treatment. Silencing of TaMCA-Id enhanced sensitivity of wheat seedlings to NaCl stress. Under NaCl stress, TaMCA-Id-silenced seedlings exhibited a reduction in activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), higher accumulation of H₂O₂ and O 2 . - , more serious injury to photosystem II (PSII), increase in PCD level, and autophagy activity in leaves of wheat seedlings. These results indicated that TaMCA-Id functioned in PCD through interacting with autophagy under NaCl stress, which could be used to improve the salt tolerance of crop plants.

Hydrogen sulfide action in the regulation of plant autophagy

Hydrogen sulfide is a signaling molecule with a well-established impact on both plant and animal physiology. Intense investigation into the regulation of autophagy by sulfide in Arabidopsis thaliana has revealed that the post-translational modification of persulfidation/S-sulfhydration plays a key role. In this review focused on plants, we discuss the nature of the sulfide molecule involved in the regulation of autophagy, the final outcome of this modification, and the persulfidated autophagy proteins identified so far. A detailed outline of the actual knowledge of the regulation mechanism of the autophagy-related proteins ATG4a and ATG18a from Arabidopsis by sulfide is also included. This information will be instrumental for furthering research on the regulation of autophagy by sulfide.

Cargo receptors and adaptors for selective autophagy in plant cells

Plant selective (macro)autophagy is a highly regulated process whereby eukaryotic cells spatiotemporally degrade some of their constituents that have become superfluous or harmful. The identification and characterization of the factors determining this selectivity make it possible to integrate selective (macro)autophagy into plant cell physiology and homeostasis. The specific cargo receptors and/or scaffold proteins involved in this pathway are generally not structurally conserved, as are the biochemical mechanisms underlying recognition and integration of a given cargo into the autophagosome in different cell types. This review discusses the few specific cargo receptors described in plant cells to highlight key features of selective autophagy in the plant kingdom and its integration with plant physiology, so as to identify evolutionary convergence and knowledge gaps to be filled by future research.

A bacterial effector counteracts host autophagy by promoting degradation of an autophagy component

Beyond its role in cellular homeostasis, autophagy plays anti‐ and promicrobial roles in host–microbe interactions, both in animals and plants. One prominent role of antimicrobial autophagy is to degrade intracellular pathogens or microbial molecules, in a process termed xenophagy. Consequently, microbes evolved mechanisms to hijack or modulate autophagy to escape elimination. Although well‐described in animals, the extent to which xenophagy contributes to plant–bacteria interactions remains unknown. Here, we provide evidence that Xanthomonas campestris pv. vesicatoria (Xcv) suppresses host autophagy by utilizing type‐III effector XopL. XopL interacts with and degrades the autophagy component SH3P2 via its E3 ligase activity to promote infection. Intriguingly, XopL is targeted for degradation by defense‐related selective autophagy mediated by NBR1/Joka2, revealing a complex antagonistic interplay between XopL and the host autophagy machinery. Our results implicate plant antimicrobial autophagy in the depletion of a bacterial virulence factor and unravel an unprecedented pathogen strategy to counteract defense‐related autophagy in plant–bacteria interactions.

CRISPR/Cas9-Mediated Editing of Autophagy Gene 6 in Petunia Decreases Flower Longevity, Seed Yield, and Phosphorus Remobilization by Accelerating Ethylene Production and Senescence-Related Gene Expression

Developmental petal senescence is a type of programmed cell death (PCD), during which the production of ethylene is induced, the expression of PCD-related genes is upregulated, and nutrients are recycled. Autophagy is an intracellular mechanism involved in PCD modulation and nutrient cycling. As a central component of the autophagy pathway, Autophagy Gene 6 (ATG6) was previously shown as a negative regulator of petal senescence. To better understand the role of autophagy in ethylene biosynthesis and nutrient remobilization during petal senescence, we generated and characterized the knockout (KO) mutants of PhATG6 using CRISPR/Cas9 in Petunia × hybrida 'Mitchell Diploid.' PhATG6-KO lines exhibited decreased flower longevity when compared to the flowers of the wild-type or a non-mutated regenerative line (controls), confirming the negative regulatory role of ATG6 in petal senescence. Smaller capsules and fewer seeds per capsule were produced in the KO plants, indicating the crucial function of autophagy in seed production. Ethylene production and ethylene biosynthesis genes were upregulated earlier in the KO lines than the controls, indicating that autophagy affects flower longevity through ethylene. The transcript levels of petal PCD-related genes, including PhATG6, PhATG8d, PhPI3K (Phosphatidylinositol 3-Kinase), and a metacaspase gene PhMC1, were upregulated earlier in the corollas of PhATG6-KO lines, which supported the accelerated PCD in the KO plants. The remobilization of phosphorus was reduced in the KO lines, showing that nutrient recycling was compromised. Our study demonstrated the important role of autophagy in flower lifespan and seed production and supported the interactions between autophagy and various regulatory factors during developmental petal senescence.

Interaction of ToLCNDV TrAP with SlATG8f marks it susceptible to degradation by autophagy

Tomato leaf curl New Delhi virus (ToLCNDV) is a devastating plant pathogen which causes significant losses in tomato yield. According to previous reports, proteins of geminiviruses like βC1 of Cotton leaf curl Multan virus and C1 of Tomato leaf curl Yunnan virus are degraded by the autophagy pathway. There are no reports on the role of autophagy in ToLCNDV pathogenesis. In this study, we have shown that SlATG8f interacts with the ToLCNDV Transcription activator protein (TrAP; AC2) to mediate its degradation by the autophagy pathway. Silencing of SlATG8f in a ToLCNDV tolerant tomato cultivar; H-88-78-1 resulted in enhanced viral symptoms and ToLCNDV accumulation suggesting an anti-viral role for SlATG8f against ToLCNDV. TrAP is a nucleus localized protein, but it interacts with SlATG8f in and outside the nucleus indicating its nuclear export. This export might be mediated by Exportin1 as treatment with Exportin1 inhibitor inhibits TrAP export outside the nucleus. ToLCNDV TrAP is known to possess host RNA silencing suppression (RSS) activity. Degradation of TrAP results in the attenuation of its RSS activity. To the best of our knowledge, we have shown for the first time that SlATG8f-TrAP interaction leads to TrAP degradation providing defence against ToLCNDV.

Autophagy promotes programmed cell death and corpse clearance in specific cell types of the Arabidopsis root cap

Autophagy is a conserved quality control pathway that mediates the degradation of cellular components by targeting them to the lysosomes or vacuoles.¹ Autophagy has been implicated in the regulation of some regulated cell death processes in animal systems.² However, its function in developmentally controlled programmed cell death (dPCD) in plants remains little studied and controversial.³ Some studies have reported autophagy pro-survival roles,^4,5 while others have suggested pro-death functions for autophagy,^6,7 calling for further detailed investigations. Here, we investigated the role of autophagy in dPCD using the Arabidopsis root cap as an accessible and genetically tractable model system.⁸ In Arabidopsis, dPCD is an integral part of root cap differentiation, restricting root cap organ size to the root meristem.⁹ The root cap consists of two distinct tissues: the proximally positioned columella that is located at the very root tip and the lateral root cap (LRC) that flanks the root meristem up to its distal end at the start of the root elongation zone.¹⁰ We show that autophagic flux strongly increased prior to dPCD execution in both root cap tissues and depends on the key autophagy genes ATG2, ATG5, and ATG7. Systemic and organ-specific mutation of these genes shows delayed PCD execution and lack of postmortem corpse clearance in the columella but no defects in dPCD execution or corpse clearance in the distal LRC. Our results reveal a high degree of cell-type specificity in autophagy functions and suggest that autophagy roles in dPCD can considerably diverge between different cell types of the same plant organ.

Hydrogen Sulfide: A Key Role in Autophagy Regulation from Plants to Mammalians

Autophagy is a degradative conserved process in eukaryotes to recycle unwanted cellular protein aggregates and damaged organelles. Autophagy plays an important role under normal physiological conditions in multiple biological processes, but it is induced under cellular stress. Therefore, it needs to be tightly regulated to respond to different cellular stimuli. In this review, the regulation of autophagy by hydrogen sulfide is described in both animal and plant systems. The underlying mechanism of action of sulfide is deciphered as the persulfidation of specific targets, regulating the pro- or anti-autophagic role of sulfide with a cell survival outcome. This review aims to highlight the importance of sulfide and persulfidation in autophagy regulation comparing the knowledge available in mammals and plants.

Programmed Degradation of Pericarp Cells in Wheat Grains Depends on Autophagy

Wheat is one of the most important food crops in the world, with development of the grains directly determining yield and quality. Understanding grain development and the underlying regulatory mechanisms is therefore essential in improving the yield and quality of wheat. In this study, the developmental characteristics of the pericarp was examined in developing wheat grains of the new variety Jimai 70. As a result, pericarp thickness was found to be thinnest in grains at the top of the spike, followed by those in the middle and thickest at the bottom. Moreover, this difference corresponded to the number of cell layers in the pericarp, which decreased as a result of programmed cell death (PCD). A number of autophagy-related genes (ATGs) are involved in the process of PCD in the pericarp, and in this study, an increase in ATG8-PE expression was observed followed by the appearance of autophagy structures. Meanwhile, following interference of the key autophagy gene ATG8, PCD was inhibited and the thickness of the pericarp increased, resulting in small premature grains. These findings suggest that autophagy and PCD coexist in the pericarp during early development of wheat grains, with both processes increasing from the bottom to the top of the spike. Moreover, PCD was also found to rely on ATG8-mediated autophagy. The results of this study therefore provide a theoretical basis for in-depth studies of the regulatory mechanisms of wheat grain development.

Initiation and Execution of Programmed Cell Death and Regulation of Reactive Oxygen Species in Plants

Programmed cell death (PCD) plays crucial roles in plant development and defence response. Reactive oxygen species (ROS) are produced during normal plant growth, and high ROS concentrations can change the antioxidant status of cells, leading to spontaneous cell death. In addition, ROS function as signalling molecules to improve plant stress tolerance, and they induce PCD under different conditions. This review describes the mechanisms underlying plant PCD, the key functions of mitochondria and chloroplasts in PCD, and the relationship between mitochondria and chloroplasts during PCD. Additionally, the review discusses the factors that regulate PCD. Most importantly, in this review, we summarise the sites of production of ROS and discuss the roles of ROS that not only trigger multiple signalling pathways leading to PCD but also participate in the execution of PCD, highlighting the importance of ROS in PCD.

Autophagy Is Involved in the Viability of Overexpressing Thioredoxin o1 Tobacco BY-2 Cells under Oxidative Conditions

Autophagy is an essential process for the degradation of non-useful components, although the mechanism involved in its regulation is less known in plants than in animal systems. Redox regulation of autophagy components is emerging as a possible key mechanism with thioredoxins (TRXs) proposed as involved candidates. In this work, using overexpressing PsTRXo1 tobacco cells (OEX), which present higher viability than non-overexpressing cells after H₂O₂ treatment, we examine the functional interaction of autophagy and PsTRXo1 in a collaborative response. OEX cells present higher gene expression of the ATG (Autophagy related) marker ATG4 and higher protein content of ATG4, ATG8, and lipidated ATG8 as well as higher ATG4 activity than control cells, supporting the involvement of autophagy in their response to H₂O₂. In this oxidative situation, autophagy occurs in OEX cells as is evident from an accumulation of autolysosomes and ATG8 immunolocalization when the E-64d autophagy inhibitor is used. Interestingly, cell viability decreases in the presence of the inhibitor, pointing to autophagy as being involved in cell survival. The in vitro interaction of ATG4 and PsTRXo1 proteins is confirmed by dot-blot and co-immunoprecipitation assays as well as the redox regulation of ATG4 activity by PsTRXo1. These findings extend the role of TRXs in mediating the redox regulation of the autophagy process in plant cells.

Silencing Autophagy-Related Gene 2 (ATG2) Results in Accelerated Senescence and Enhanced Immunity in Soybean

Autophagy plays a critical role in nutrient recycling and stress adaptations. However, the role of autophagy has not been extensively investigated in crop plants. In this study, soybean autophagy-related gene 2 (GmATG2) was silenced, using virus-induced silencing (VIGS) mediated by Bean pod mottle virus (BPMV). An accelerated senescence phenotype was exclusively observed for the GmATG2-silenced plants under dark conditions. In addition, significantly increased accumulation of both ROS and SA as well as a significantly induced expression of the pathogenesis-related gene 1 (PR1) were also observed on the leaves of the GmATG2-silenced plants, indicating an activated immune response. Consistent with this, GmATG2-silenced plants exhibited a significantly enhanced resistance to Pseudomonas syringae pv. glycinea (Psg) relative to empty vector control plants (BPMV-0). Notably, the activated immunity of the GmATG2-silenced plants was independent of the MAPK signaling pathway. The fact that the accumulation levels of ATG8 protein and poly-ubiquitinated proteins were significantly increased in the dark-treated GmATG2-silenced plants relative to the BPMV-0 plants indicated that the autophagic degradation is compromised in the GmATG2-silenced plants. Together, our results indicated that silencing GmATG2 compromises the autophagy pathway, and the autophagy pathway is conserved in different plant species.

Research Advances in Potyviruses: From the Laboratory Bench to the Field

Potyviruses (viruses in the genus Potyvirus, family Potyviridae) constitute the largest group of known plant-infecting RNA viruses and include many agriculturally important viruses that cause devastating epidemics and significant yield losses in many crops worldwide. Several potyviruses are recognized as the most economically important viral pathogens. Therefore, potyviruses are more studied than other groups of plant viruses. In the past decade, a large amount of knowledge has been generated to better understand potyviruses and their infection process. In this review, we list the top 10 economically important potyviruses and present a brief profile of each. We highlight recent exciting findings on the novel genome expression strategy and the biological functions of potyviral proteins and discuss recent advances in molecular plant-potyvirus interactions, particularly regarding the coevolutionary arms race. Finally, we summarize current disease control strategies, with a focus on biotechnology-based genetic resistance, and point out future research directions.

Short-term waterlogging-induced autophagy in root cells of wheat can inhibit programmed cell death

Autophagy is a pathway for the degradation of cytoplasmic components in eukaryotes. In wheat, the mechanism by which autophagy regulates programmed cell death (PCD) is unknown. Here, we demonstrated that short-term waterlogging-induced autophagy inhibited PCD in root cells of wheat. The waterlogging-tolerant wheat cultivar Huamai 8 and the waterlogging-sensitive wheat cultivar Huamai 9 were used as experimental materials, and their roots were waterlogged for 0-48 h. Waterlogging stress increased the number of autophagic structures, the expression levels of autophagy-related genes (TaATG), and the occurrence of PCD in root cells. PCD manifested as morphological changes in the cell nucleus, significant enhancement of DNA laddering bands, and increases in caspase-like protease activity and the expression levels of metacaspase genes. The autophagy promoter rapamycin (RAPA) reduced PCD levels, whereas the autophagy inhibitor 3-methyladenine (3-MA) enhanced them. The expression levels of TaATG genes and the number of autophagic structures were lower in cortex cells than in stele cells, but the levels of PCD were higher in cortex cells. The number of autophagic structures was greater in Huamai 8 than in Huamai 9, but the levels of PCD were lower. In summary, our results showed that short-term waterlogging induced autophagy which could inhibit PCD. Mechanisms of response to waterlogging stress differed between cortex and stele cells and between two wheat cultivars of contrasting waterlogging tolerance.

Bacterial effectors mimicking ubiquitin-proteasome pathway tweak plant immunity

Plant pathogenic Gram-negative bacteria evade the host plant immune system by secreting Type III (T3E) and Type IV effector (T4E) proteins into the plant cytoplasm. Mostly T3Es are secreted into the plant cells to establish pathogenicity by affecting the vital plant process viz. metabolic pathways, signal transduction and hormonal regulation. Ubiquitin-26S proteasome system (UPS) exists as one of the important pathways in plants to control plant immunity and various cellular processes by employing several enzymes and enzyme components. Pathogenic and non-pathogenic bacteria are found to secrete effectors into plants with structural and/or functional similarity to UPS pathway components like ubiquitin E3 ligases, F-box domains, cysteine proteases, inhibitor of host UPS or its components, etc. The bacterial effectors mimic UPS components and target plant resistance proteins for degradation by proteasomes, thereby taking control over the host cellular activities as a strategy to exert virulence. Thus, the bacterial effectors circumvent plant cellular pathways leading to infection and disease development. This review highlights known bacterial T3E and T4E proteins that function and interfere with the ubiquitination pathway to regulate the immune system of plants.

Autophagy-an underestimated coordinator of construction and destruction during plant root ontogeny

MAIN CONCLUSION

Autophagy is a key but undervalued process in root ontogeny, ensuring both the proper development of root tissues as well as the senescence of the entire organ. Autophagy is a process which occurs during plant adaptation to changing environmental conditions as well as during plant ontogeny. Autophagy is also engaged in plant root development, however, the limitations of belowground studies make it challenging to understand the entirety of the developmental processes. We summarize and discuss the current data pertaining to autophagy in the roots of higher plants during their formation and degradation, from the beginning of root tissue differentiation and maturation; all the way to the aging of the entire organ. During root growth, autophagy participates in the processes of central vacuole formation in cortical tissue development, as well as vascular tissue differentiation and root senescence. At present, several key issues are still not entirely understood and remain to be addressed in future studies. The major challenge lies in the portrayal of the mechanisms of autophagy on subcellular events in belowground plant organs during the programmed control of cellular degradation pathways in roots. Given the wide range of technical areas of inquiry where root-related research can be applied, including cutting-edge cell biological methods to track, sort and screen cells from different root tissues and zones of growth, the identification of several lines of evidence pertaining to autophagy during root developmental processes is the most urgent challenge. Consequently, a substantial effort must be made to ensure whether the analyzed process is autophagy-dependent or not.

Arabidopsis FAR-RED ELONGATED HYPOCOTYL3 negatively regulates carbon starvation responses

Light is one of the most important environmental factors that affects various cellular processes in plant growth and development; it is also crucial for the metabolism of carbohydrates as it provides the energy source for photosynthesis. Under extended darkness conditions, carbon starvation responses are triggered by depletion of stored energy. Although light rapidly inhibits starvation responses, the molecular mechanisms by which light signalling affects this process remain largely unknown. In this study, we showed that the Arabidopsis thaliana light signalling protein FAR-RED ELONGATED HYPOCOTYL3 (FHY3) and its homolog FAR-RED IMPAIRED RESPONSE1 (FAR1) are essential for plant survival after extended darkness treatment at both seedling and adult stages. Transmission electron microscopy analyses revealed that disruption of both FHY3 and FAR1 resulted in destruction of chloroplast envelopes and thylakoid membranes under extended darkness conditions. Furthermore, treatment with sucrose, but not glucose, completely rescued carbon starvation-induced cell death in the rosette leaves and arrested early seedling establishment in the fhy3 far1 plants. We thus concluded that the light signalling proteins FHY3 and FAR1 negatively regulate carbon starvation responses in Arabidopsis.

Potential interaction between autophagy and auxin during maize leaf senescence

Leaf senescence is important for crop yield as delaying it can increase the average yield. In this study, population genetics and transcriptomic profiling were combined to dissect its genetic basis in maize. To do this, the progenies of an elite maize hybrid Jidan27 and its parental lines Si-287 (early senescence) and Si-144 (stay-green), as well as 173 maize inbred lines were used. We identified two novel loci and their candidate genes, Stg3 (ZmATG18b) and Stg7 (ZmGH3.8), which are predicted to be members of autophagy and auxin pathways, respectively. Genomic variations in the promoter regions of these two genes were detected, and four allelic combinations existed in the examined maize inbred lines. The Stg3Si-144/Stg7Si-144 allelic combination with lower ZmATG18b expression and higher ZmGH3.8 expression could distinctively delay leaf senescence, increase ear weight and the improved hybrid of NIL-Stg3Si-144/Stg7Si-144 × Si-144 significantly reduced ear weight loss under drought stress, while opposite effects were observed in the Stg3Si-287/Stg7Si-287 combination with a higher ZmATG18b expression and lower ZmGH3.8 expression. Thus, we identify a potential interaction between autophagy and auxin which could modulate the timing of maize leaf senescence.

Molecular characterization of SlATG18f in response to Tomato leaf curl New Delhi virus infection in tomato and development of a CAPS marker for leaf curl disease tolerance

Analysis of autophagy-related genes in tomato shows the involvement of SlATG18f in leaf curl disease tolerance and a CAPS marker developed from this gene demonstrates its usefulness in marker-assisted selection. Autophagy is a highly conserved catabolic process regulating cellular homeostasis and adaptation to different biotic and abiotic stress. Several autophagy-related proteins (ATGs) are reported to be involved in autophagic processes, and considering their importance in regulating growth and stress adaptation, these proteins have been identified and characterized in several plant species. However, there is no information available on the role of autophagy-related proteins regulating the tolerance of tomato to tomato leaf curl disease (ToLCD). Given this, the present genome-wide study identified thirty ATG-encoding genes (SlATG) in tomato, followed by their functional characterization. Expression profiling of the SlATG genes in contrasting tomato cultivars subjected to virus infection showed a 4.5-fold upregulation of SlATG18f in the tolerant cultivar. Further, virus-induced gene silencing of SlATG18f in the tolerant cultivar conferred disease susceptibility, which suggested the role of this gene in Tomato leaf curl New Delhi virus tolerance. Comparison of the gene sequence of both tolerant and susceptible cultivars along with the 5' upstream regions identified an SNP (A/T) at -2916 upstream of the start codon. A cleaved amplified polymorphic sequence (CAPS) marker was developed targeting this region, which showed a significant association with the tolerance characteristics in the tomato germplasm (R² = 0.1787). Altogether, the study identified a potential gene that could be used to develop ToLCNDV tolerant tomato cultivars using transgene-based or marker-assisted breeding-based approaches.

Autophagy in Plant Abiotic Stress Management

Plants can be considered an open system. Throughout their life cycle, plants need to exchange material, energy and information with the outside world. To improve their survival and complete their life cycle, plants have developed sophisticated mechanisms to maintain cellular homeostasis during development and in response to environmental changes. Autophagy is an evolutionarily conserved self-degradative process that occurs ubiquitously in all eukaryotic cells and plays many physiological roles in maintaining cellular homeostasis. In recent years, an increasing number of studies have shown that autophagy can be induced not only by starvation but also as a cellular response to various abiotic stresses, including oxidative, salt, drought, cold and heat stresses. This review focuses mainly on the role of autophagy in plant abiotic stress management.

The ATG4 protease integrates redox and stress signals to regulate autophagy

Autophagy is a highly conserved degradative pathway that ensures cellular homeostasis through the removal of damaged or useless intracellular components including proteins, membranes, or even entire organelles. A main hallmark of autophagy is the biogenesis of autophagosomes, double-membrane vesicles that engulf and transport to the vacuole the material to be degraded and recycled. The formation of autophagosomes responds to integrated signals produced as a consequence of metabolic reactions or different types of stress and is mediated by the coordinated action of core autophagy-related (ATG) proteins. ATG4 is a key Cys-protease with a dual function in both ATG8 lipidation and free ATG8 recycling whose balance is crucial for proper biogenesis of the autophagosome. ATG4 is conserved in the green lineage, and its regulation by different post-translational modifications has been reported in the model systems Chlamydomonas reinhardtii and Arabidopsis. In this review, we discuss the major role of ATG4 in the integration of stress and redox signals that regulate autophagy in algae and plants.

Suppression of Metacaspase- and Autophagy-Dependent Cell Death Improves Stress-Induced Microspore Embryogenesis in Brassica napus

Microspore embryogenesis is a biotechnological process that allows us to rapidly obtain doubled-haploid plants for breeding programs. The process is initiated by the application of stress treatment, which reprograms microspores to embark on embryonic development. Typically, a part of the microspores undergoes cell death that reduces the efficiency of the process. Metacaspases (MCAs), a phylogenetically broad group of cysteine proteases, and autophagy, the major catabolic process in eukaryotes, are critical regulators of the balance between cell death and survival in various organisms. In this study, we analyzed the role of MCAs and autophagy in cell death during stress-induced microspore embryogenesis in Brassica napus. We demonstrate that this cell death is accompanied by the transcriptional upregulation of three BnMCA genes (BnMCA-Ia, BnMCA-IIa and BnMCA-IIi), an increase in MCA proteolytic activity and the activation of autophagy. Accordingly, inhibition of autophagy and MCA activity, either individually or in combination, suppressed cell death and increased the number of proembryos, indicating that both components play a pro-cell death role and account for decreased efficiency of early embryonic development. Therefore, MCAs and/or autophagy can be used as new biotechnological targets to improve in vitro embryogenesis in Brassica species and doubled-haploid plant production in crop breeding and propagation programs.

A cell surface-exposed protein complex with an essential virulence function in Ustilago maydis

Plant pathogenic fungi colonizing living plant tissue secrete a cocktail of effector proteins to suppress plant immunity and reprogramme host cells. Although many of these effectors function inside host cells, delivery systems used by pathogenic bacteria to translocate effectors into host cells have not been detected in fungi. Here, we show that five unrelated effectors and two membrane proteins from Ustilago maydis, a biotrophic fungus causing smut disease in corn, form a stable protein complex. All seven genes appear co-regulated and are only expressed during colonization. Single mutants arrest in the epidermal layer, fail to suppress host defence responses and fail to induce non-host resistance, two reactions that likely depend on translocated effectors. The complex is anchored in the fungal membrane, protrudes into host cells and likely contacts channel-forming plant plasma membrane proteins. Constitutive expression of all seven complex members resulted in a surface-exposed form in cultured U. maydis cells. As orthologues of the complex-forming proteins are conserved in smut fungi, the complex may become an interesting fungicide target.

Silencing ATG6 and PI3K accelerates petal senescence and reduces flower number and shoot biomass in petunia

Petal senescence is a form of developmental programmed cell death (PCD) that is regulated by internal and environmental signals. Autophagy, a metabolic pathway that regulates intercellular nutrient recycling, is thought to play an important role in the regulation of petal senescence-associated PCD. To characterize the function of two central autophagy genes in petal senescence, we down-regulated Autophagy Gene 6 (PhATG6) and Phosphoinositide 3-Kinase (PhPI3K) using Virus-Induced Gene Silencing (VIGS) in Petunia × hybrida. The silencing of PhATG6 and PhPI3K accelerated petal senescence, thereby reducing flower longevity. Both PhATG6- and PhPI3K-silenced petunias had reduced flower numbers, flower biomass, and vegetative shoot biomass. These phenotypes were intensified when plants were grown under low nutrient conditions. Additionally, two important regulators of senescence, an ethylene biosynthesis gene (PhACS) and a type I metacaspase gene (PhMC1), were suppressed in senescing petals of PhATG6- and PhPI3K-silenced plants. In conclusion, our study identified PhATG6 and PhPI3K as negative regulators of flower senescence and demonstrated the influence of nutrient limitation on the function of autophagy during petal senescence. Our study also found that autophagy genes potentially influence the transcriptional regulation of metacaspases and ethylene biosynthetic genes during petal senescence. The results of this project will be fundamental for future studies of petal senescence and will provide genetic information for future crop improvement.

Vacuolar processing enzyme translocates to the vacuole through the autophagy pathway to induce programmed cell death

The caspase-like vacuolar processing enzyme (VPE) is a key factor in programmed cell death (PCD) associated with plant stress responses. Growth medium lacking a carbon source and dark conditions caused punctate labeling of 35S::VPE1-GFP (StVPE1-GFP) in potato leaves. Under conditions of carbon starvation, VPE activity and PCD symptoms strongly increased in BY-2 cells, but to a much lesser extent in VPE-RNAi BY-2 cells. During extended exposure to carbon starvation, VPE expression and activity levels peaked, with a gradual increase in BY-2 cell death. Histological analysis of StVPE1-GFP in BY-2 cells showed that carbon starvation induces its translocation from the endoplasmic reticulum to the central vacuole through tonoplast engulfment. Exposure of BY-2 culture to the macroautophagy/autophagy inhibitor concanamycin A led to, along with an accumulation of autophagic bodies, accumulation of StVPE1-GFP in the cell vacuole. This accumulation did not occur in the presence of 3-methyladenine, an inhibitor of early-stage autophagy. BY-2 cells constitutively expressing RFP-StATG8IL, an autophagosome marker, showed colocalization with the StVPE1-GFP protein in the cytoplasm and vacuole. RNAi silencing of the core autophagy component ATG4 in BY-2 cells reduced VPE activity and cell death. These results are the first to suggest that VPE translocates to the cell vacuole through the autophagy pathway, leading to PCD.Abbreviations: ATG: autophagy related; CLP: caspase-like protease; HR: hypersensitive response; PCD: programmed cell death; St: Solanum tuberosum; VPE: vacuolar processing enzyme.

Knowing When to Self-Eat - Fine-Tuning Autophagy Through ATG8 Iso-forms in Plants

Autophagy is a catabolic process that takes place under both normal and adverse conditions and is important for the degradation of various organelles and proteins that are no longer needed. Thus, it can be viewed as both a constitutive recycling machinery and an adaptation mechanism. Increase in the activity of autophagy can be caused by multiple biotic and abiotic stress factors. Though intensive research in the past decade has elucidated many molecular details of plant autophagy, the mechanisms of induction and regulation of the process remain understudied. Here, we discuss the role of ATG8 proteins in autophagic signaling and regulation with an emphasis on the significance of ATG8 diversification for adapting autophagy to the changing needs of plants.

Transcriptomic and functional analyses reveal an antiviral role of autophagy during pepper mild mottle virus infection

BACKGROUND

Pepper mild mottle virus (PMMoV) is a member in the genus Tobamovirus and infects mainly solanaceous plants. However, the mechanism of virus-host interactions remains unclear. To explore the responses of pepper plants to PMMoV infection, we analyzed the transcriptomic changes in pepper plants after PMMoV infection using a high-throughput RNA sequencing approach and explored the roles of host autophagy in regulating PMMoV infection.

RESULTS

A total of 197 differentially expressed genes (DEGs) were obtained after PMMoV infection, including 172 significantly up-regulated genes and 25 down-regulated genes. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses revealed that most up-regulated DEGs were involved in plant abiotic and biotic stresses. Further analyses showed the expressions of multiple autophagy-related genes (ATGs) were increased after PMMoV infection in pepper and Nicotiana benthamiana plants. Through confocal microscopy and transmission electron microscopy, we have found that PMMoV infection in plant can induce autophagy, evidenced by the increased number of GFP-ATG8a fluorescent punctate and the appearance of double membrane autophagic structures in cells of N. benthamiana. Additionally, inhibition of autophagy significantly increased PMMoV RNA accumulation and aggravated systemic PMMoV symptoms through autophagy inhibitor (3-MA and E64d) treatment and silencing of NbATG expressions by a Tobacco rattle virus-induced gene silencing assays. These results indicated that autophagy played a positive role in plant resistance to PMMoV infection.

CONCLUSIONS

Taken together, our results provide a transcriptomic insight into pepper responding to PMMoV infection and reveal that autophagy induced by PMMoV infection has an antiviral role in regulating PMMoV infection. These results also help us to better understand the mechanism controlling PMMoV infection in plants and to develop better strategies for breeding projects for virus-resistant crops.

Comprehensive analysis of the Ppatg3 mutant reveals that autophagy plays important roles in gametophore senescence in Physcomitrella patens

BACKGROUND

Autophagy is an evolutionarily conserved system for the degradation of intracellular components in eukaryotic organisms. Autophagy plays essential roles in preventing premature senescence and extending the longevity of vascular plants. However, the mechanisms and physiological roles of autophagy in preventing senescence in basal land plants are still obscure.

RESULTS

Here, we investigated the functional roles of the autophagy-related gene PpATG3 from Physcomitrella patens and demonstrated that its deletion prevents autophagy. In addition, Ppatg3 mutant showed premature gametophore senescence and reduced protonema formation compared to wild-type (WT) plants under normal growth conditions. The abundance of nitrogen (N) but not carbon (C) differed significantly between Ppatg3 mutant and WT plants, as did relative fatty acid levels. In vivo protein localization indicated that PpATG3 localizes to the cytoplasm, and in vitro Y2H assays confirmed that PpATG3 interacts with PpATG7 and PpATG12. Plastoglobuli (PGs) accumulated in Ppatg3, indicating that the process that degrades damaged chloroplasts in senescent gametophore cells was impaired in this mutant. RNA-Seq uncovered a detailed, comprehensive set of regulatory pathways that were affected by the autophagy mutation.

CONCLUSIONS

The autophagy-related gene PpATG3 is essential for autophagosome formation in P. patens. Our findings provide evidence that autophagy functions in N utilization, fatty acid metabolism and damaged chloroplast degradation under non-stress conditions. We identified differentially expressed genes in Ppatg3 involved in numerous biosynthetic and metabolic pathways, such as chlorophyll biosynthesis, lipid metabolism, reactive oxygen species removal and the recycling of unnecessary proteins that might have led to the premature senescence of this mutant due to defective autophagy. Our study provides new insights into the role of autophagy in preventing senescence to increase longevity in basal land plants.

Emerging Roles of the Selective Autophagy in Plant Immunity and Stress Tolerance

Autophagy is a conserved recycling system required for cellular homeostasis. Identifications of diverse selective receptors/adaptors that recruit appropriate autophagic cargoes have revealed critical roles of selective autophagy in different biological processes in plants. In this review, we summarize the emerging roles of selective autophagy in both biotic and abiotic stress tolerance and highlight the new features of selective receptors/adaptors and their interactions with both the cargoes and Autophagy-related gene 8s (ATG8s). In addition, we review how the two major degradation systems, namely the ubiquitin-proteasome system (UPS) and selective autophagy, are coordinated to cope with stress in plants. We especially emphasize how plants develop the selective autophagy as a weapon to fight against pathogens and how adapted pathogens have evolved the strategies to counter and/or subvert the immunity mediated by selective autophagy.

Formation of NPR1 Condensates Promotes Cell Survival during the Plant Immune Response

In plants, pathogen effector-triggered immunity (ETI) often leads to programmed cell death, which is restricted by NPR1, an activator of systemic acquired resistance. However, the biochemical activities of NPR1 enabling it to promote defense and restrict cell death remain unclear. Here we show that NPR1 promotes cell survival by targeting substrates for ubiquitination and degradation through formation of salicylic acid-induced NPR1 condensates (SINCs). SINCs are enriched with stress response proteins, including nucleotide-binding leucine-rich repeat immune receptors, oxidative and DNA damage response proteins, and protein quality control machineries. Transition of NPR1 into condensates is required for formation of the NPR1-Cullin 3 E3 ligase complex to ubiquitinate SINC-localized substrates, such as EDS1 and specific WRKY transcription factors, and promote cell survival during ETI. Our analysis of SINCs suggests that NPR1 is centrally integrated into the cell death or survival decisions in plant immunity by modulating multiple stress-responsive processes in this quasi-organelle.

Valproic Acid Sensitizes Glioma Cells to Luteolin Through Induction of Apoptosis and Autophagy via Akt Signaling

Glioma is a highly malignant type of intracranial tumor with a poor prognosis resulting from traditional chemo-resistance with temozolomide (TMZ). Luteolin has been detected to exert limited anti-tumor effects on gliomas, while valproic acid (VPA) is a common chemotherapy sensitizer in the treatment of tumors. In this study, three glioma cell lines including U251, LN229 and SNB19 were selected for evaluation of combined anti-tumor effects of VPA and luteolin via Cell Counting Kit-8 (CCK-8) assay, colony formation assay, wound-healing assay, flow cytometry and western blot assay. The results disclosed that VPA sensitized glioma cells to luteolin by repressing cell viability, colony formation and migration. Mechanically, VPA boosted cellular apoptosis and cell-cycle arrest by increased level of cleaved caspase-3/caspase-3, cleaved PARP/PARP and Bax/Bcl-2. In addition, VPA also facilitated cellular autophagy via the decline of p62, p-Akt/Akt and the accumulation of LC3-II. These findings suggested that VPA enhanced the anticancer effects of luteolin by strengthening apoptosis and autophagy via Akt signaling, which could be adopted as a novel therapy for glioma.

Microbial Effector Proteins - A Journey through the Proteolytic Landscape

In the evolutionary arms race between pathogens and plants, pathogens evolved effector molecules that they secrete into the host to subvert plant cellular responses in a process termed the effector-targeted pathway (ETP). During recent years the repertoire of ETPs has increased and mounting evidence indicates that the proteasome and autophagy pathways are central hubs of microbial effectors. Both degradation pathways are implicated in a broad array of cellular responses and thus constitute an attractive target for effector proteins to have a broader impact on the host. In this article we first summarize recent findings on how effectors from various pathogens modulate proteolytic pathways and then provide a network analysis of established effector targets implicated in proteolytic degradation machineries. With this network we emphasize the idea that effectors targeting proteolytic degradation pathways will affect the protein synthesis-transport and degradation triangle. We put in perspective that, in utilizing the effector diversity of microbes, we produce excellent tools to study diverse cellular pathways and their possible interplay with each other.

Disruption of Arabidopsis neutral ceramidases 1 and 2 results in specific sphingolipid imbalances triggering different phytohormone-dependent plant cell death programmes

Sphingolipids act as regulators of programmed cell death (PCD) and the plant defence response. The homeostasis between long-chain base (LCB) and ceramide (Cer) seems to play an important role in executions of PCD. Therefore, deciphering the role of neutral ceramidases (NCER) is crucial to identify the sphingolipid compounds that trigger and execute PCD. We performed comprehensive sphingolipid and phytohormone analyses of Arabidopsis ncer mutants, combined with gene expression profiling and microscopic analyses. While ncer1 exhibited early leaf senescence (developmentally controlled PCD - dPCD) and an increase in hydroxyceramides, ncer2 showed spontaneous cell death (pathogen-triggered PCD-like - pPCD) accompanied by an increase in LCB t18:0 at 35 d, respectively. Loss of NCER1 function resulted in accumulation of jasmonoyl-isoleucine (JA-Ile) in the leaves, whereas disruption of NCER2 was accompanied by higher levels of salicylic acid (SA) and increased sensitivity to Fumonisin B₁ (FB₁ ). All mutants were also found to activate plant defence pathways. These data strongly suggest that NCER1 hydrolyses ceramides whereas NCER2 functions as a ceramide synthase. Our results reveal an important role of NCER in the regulation of both dPCD and pPCD via a tight connection between the phytohormone and sphingolipid levels in these two processes.

Autophagy-Independent Functions of the Autophagy Machinery

Macroautophagy (herein referred to as autophagy) is an evolutionary ancient mechanism that culminates with the lysosomal degradation of superfluous or potentially dangerous cytosolic entities. Over the past 2 decades, the molecular mechanisms underlying several variants of autophagy have been characterized in detail. Accumulating evidence suggests that most, if not all, components of the molecular machinery for autophagy also mediate autophagy-independent functions. Here, we discuss emerging data on the non-autophagic functions of autophagy-relevant proteins.

Nuclear autophagy degrades a geminivirus nuclear protein to restrict viral infection in solanaceous plants

Autophagy is an evolutionarily conserved degradation pathway in the cytoplasm and has emerged as a key defense mechanism against invading pathogens. However, there is no evidence showing nuclear autophagy in plants. Here, we show that a geminivirus nuclear protein, C1 of tomato leaf curl Yunnan virus (TLCYnV) induces autophagy and interacts directly with the core autophagy-related protein ATG8h. The interaction between ATG8h and C1 leads to the translocation of the C1 protein from the nucleus to the cytoplasm and the decreased protein accumulation of C1, which is dependent on the exportin1-mediated nuclear export pathway. The degradation of C1 is blocked by autophagy inhibitors and compromised when the autophagy-related genes (ATGs) ATG8h, ATG5, or ATG7 are knocked down. Similarly, silencing of these ATGs also promotes TLCYnV infection in Nicotiana benthamiana and Solanum lycopersicum plants. The mutation of a potential ATG8 interacting motif (AIM) in C1 abolishes its interaction with ATG8h in the cytoplasm but favors its interaction with Fibrillarin1 in the nucleolus. TLCYnV carrying the AIM mutation displays enhanced pathogenicity in solanaceous plants. Taken together, these data suggest that a new type of nuclear autophagy-mediated degradation of viral proteins through an exportin1-dependent nuclear export pathway restricts virus infection in plants.

Genomic Characterization and Expressional Profiles of Autophagy-Related Genes (ATGs) in Oilseed Crop Castor Bean (Ricinus communis L.)

Cellular autophagy is a widely-occurring conserved process for turning over damaged organelles or recycling cytoplasmic contents in cells. Although autophagy-related genes (ATGs) have been broadly identified from many plants, little is known about the potential function of autophagy in mediating plant growth and development, particularly in recycling cytoplasmic contents during seed development and germination. Castor bean (Ricinus communis) is one of the most important inedible oilseed crops. Its mature seed has a persistent and large endosperm with a hard and lignified seed coat, and is considered a model system for studying seed biology. Here, a total of 34 RcATG genes were identified in the castor bean genome and their sequence structures were characterized. The expressional profiles of these RcATGs were examined using RNA-seq and real-time PCR in a variety of tissues. In particular, we found that most RcATGs were significantly up-regulated in the later stage of seed coat development, tightly associated with the lignification of cell wall tissues. During seed germination, the expression patterns of most RcATGs were associated with the decomposition of storage oils. Furthermore, we observed by electron microscopy that the lipid droplets were directly swallowed by the vacuoles, suggesting that autophagy directly participates in mediating the decomposition of lipid droplets via the microlipophagy pathway in germinating castor bean seeds. This study provides novel insights into understanding the potential function of autophagy in mediating seed development and germination.