Sucrose Starvation Induces Microautophagy in Plant Root Cells

New insights into plant autophagy: molecular mechanisms and roles in development and stress responses

Autophagy is an evolutionarily conserved eukaryotic intracellular degradation process. Although the molecular mechanisms of plant autophagy share similarities with those in yeast and mammals, certain unique mechanisms have been identified. Recent studies have highlighted the importance of autophagy during vegetative growth stages as well as in plant-specific developmental processes, such as seed development, germination, flowering, and somatic reprogramming. Autophagy enables plants to adapt to and manage severe environmental conditions, such as nutrient starvation, high-intensity light stress, and heat stress, leading to intracellular remodeling and physiological changes in response to stress. In the past, plant autophagy research lagged behind similar studies in yeast and mammals; however, recent advances have greatly expanded our understanding of plant-specific autophagy mechanisms and functions. This review summarizes current knowledge and latest research findings on the mechanisms and roles of plant autophagy with the objective of improving our understanding of this vital process in plants.

ATG and ESCRT control multiple modes of microautophagy

The discovery of microautophagy, the direct engulfment of cytoplasmic material by the lysosome, dates back to 1966 in a morphological study of mammalian cells by Christian de Duve. Since then, studies on microautophagy have shifted toward the elucidation of the physiological significance of the process. However, in contrast to macroautophagy, studies on the molecular mechanisms of microautophagy have been limited. Only recent studies revealed that ATG proteins involved in macroautophagy are also operative in several types of microautophagy and that ESCRT proteins, responsible for the multivesicular body pathway, play a central role in most microautophagy processes. In this review, we summarize our current knowledge on the function of ATG and ESCRT proteins in microautophagy.

Investigation of the cell structure and organelles during autolytic PCD of Antirrhinum majus "Legend White" petals

One of the classes of the plant developmental programmed cell death (PCD) is vacuolar cell death or autolysis. The results of the transmission electron microscope (TEM) studies indicated that this type of PCD occurs during the petal senescence of Antirrhinum majus "Legend White" flowers. The major hallmarks of the process related to the ultrastructure of the cells involved chloroplast degradation, vacuolation, chromatin condensation, cell wall swelling, degradation of Golgi apparatus, protoplasmic shrinkage, degradation of the endoplasmic reticulum, nuclear fragmentation, rupture of tonoplast, and plasma membrane. Macroautophagy and microautophagy processes were also clearly observed during vacuole formation. As in yeasts, in the present study, Golgi apparatus became autophagosome-like structures during degradation that had autophagy activity and then disappeared. Our results revealed a type of selective microautophagy, piecemeal microautophagy of the nucleus (PMN), in nuclear degradation during PCD of petals that has not previously been reported in plants. Moreover, vesicular structures, such as paramural and multilamellar bodies, were observed in some stages.

Overexpression of the autophagy-related gene SiATG8a from foxtail millet (Setaria italica L.) in transgenic wheat confers tolerance to phosphorus starvation

In plants, autophagy plays an important role in regulating intracellular degradation and amino acid recycling in response to nutrient starvation, senescence, and other environmental stresses. Foxtail millet (Setaria italica) shows strong resistance to various abiotic stresses; however, current understanding of the regulation network of abiotic stress resistance in foxtail millet remains limited. In this study, we aimed to determine the autophagy-related gene SiATG8a in foxtail millet. We found that SiATG8a was mainly expressed in the stem and was induced by low-phosphorus (LP) stress. Overexpression of SiATG8a in wheat (Triticum aestivum) significantly increased the grain yield and spike number per m² under LP treatment compared to those in the WT varieties S366 and S4056. There was no significant difference in the grain P content between SiATG8a-overexpressing wheat and WT wheat under normal phosphorus (NP) and LP treatments. However, the phosphorus (P) content in the roots, stems, and leaves of transgenic plants was significantly higher than that in WT plants under NP and LP conditions. Furthermore, the expression of P transporter genes, such as TaPHR1, TaPHR3, TaIPS1, and TaPT9, in SiATG8a-transgenic wheat was higher than that in WT under LP. Collectively, overexpression of SiATG8a increases the P content of roots, stems, and leaves of transgenic wheat under LP conditions by modulating the expression of P-related transporter gene, which may result in increased grain yield; thus, SiATG8a is a candidate gene for generating transgenic wheat with improved tolerance to LP stress in the field.

Intracellular phosphate recycling systems for survival during phosphate starvation in plants

Phosphorus (P) is an essential nutrient for plant growth and plants use inorganic phosphate (Pi) as their P source, but its bioavailable form, orthophosphate, is often limited in soils. Hence, plants have several mechanisms for adaptation to Pi starvation. One of the most common response strategies is "Pi recycling" in which catabolic enzymes degrade intracellular constituents, such as phosphoesters, nucleic acids and glycerophospholipids to salvage Pi. Recently, several other intracellular degradation systems have been discovered that salvage Pi from organelles. Also, one of sphingolipids has recently been identified as a degradation target for Pi recycling. So, in this mini-review we summarize the current state of knowledge, including research findings, about the targets and degradation processes for Pi recycling under Pi starvation, in order to further our knowledge of the whole mechanism of Pi recycling.

Pexophagy suppresses ROS-induced damage in leaf cells under high-intensity light

Although light is essential for photosynthesis, it has the potential to elevate intracellular levels of reactive oxygen species (ROS). Since high ROS levels are cytotoxic, plants must alleviate such damage. However, the cellular mechanism underlying ROS-induced leaf damage alleviation in peroxisomes was not fully explored. Here, we show that autophagy plays a pivotal role in the selective removal of ROS-generating peroxisomes, which protects plants from oxidative damage during photosynthesis. We present evidence that autophagy-deficient mutants show light intensity-dependent leaf damage and excess aggregation of ROS-accumulating peroxisomes. The peroxisome aggregates are specifically engulfed by pre-autophagosomal structures and vacuolar membranes in both leaf cells and isolated vacuoles, but they are not degraded in mutants. ATG18a-GFP and GFP-2×FYVE, which bind to phosphatidylinositol 3-phosphate, preferentially target the peroxisomal membranes and pre-autophagosomal structures near peroxisomes in ROS-accumulating cells under high-intensity light. Our findings provide deeper insights into the plant stress response caused by light irradiation.

Comparative changes in sugars and lipids show evidence of a critical node for regeneration in safflower seeds during aging

During seed aging, there is a critical node (CN) where the population viability drops sharply. Exploring the specific locations of the CN in different species of plants is crucial for understanding the biological storage properties of seeds and refining seed life span management. Safflower, a bulk oil crop that relies on seeds for propagation, has a short seed life. However, at present, its biological characteristics during storage are not clear, especially the changes in metabolic capability and cell structures. Such knowledge is needed to improve the management of safflower seed life span and effective preservation in gene banks. Here, the seed survival curve of oilseed safflower under the controlled deterioration conditions of 60% relative humidity and 50°C was detected. The seed population showed an inverted S shape for the fall in germination. In the first 12 days of aging, germination remained above 86%. Prior to the CN at approximately day 10 (C10), when viability was in the "plateau" interval, seed vigor reduced at the same imbibition time point. Further analysis of the changes in sugar concentration found that the sucrose content decreased slowly with aging and the content of raffinose and two monosaccharides decreased abruptly at C10. Differentially metabolized lipids, namely lysophospholipids [lyso-phosphatidylcholine (LPC) and lyso-phosphatidylethanolamines (LPE)] and PMeOH, increased at day 3 of aging (C3). Fatty acid content increased by C6, and the content of phospholipids [phosphatidylcholines (PC), phosphatidylethanolamines (PE), and phosphatidylinositols (PI) and glycolipids [digalactosyl diacylglycerol, monogalactosyl diacylglycerol, and sulphoquinovosyl diglycerides (SQDG)] decreased significantly from C10. In addition, the activities of raffinose hydrolase alpha-galactosidase and the glyoxylate key enzyme isocitrate lyase decreased with seed aging. Confocal microscopy and transmission electron microscopy revealed shrinkage of the seed plasma membrane at C10 and the later fragmentation. Seedling phenotypic indicators and 2,3,5-triphenyltetrazolium chloride activity assays also verified that there were significant changes in seeds quality at the CN. In summary, the time point C10 is a CN during seed population aging. Before the CN, sugar and lipid metabolism, especially fatty acid metabolism into sugar, can make up for the energy consumed by aging. After this point, the seeds were irreversibly damaged, and their viability was greatly and rapidly reduced as the cell structure became increasingly destroyed.

Enteral glucose, absorbed and metabolized, potently enhances mesenteric lymph flow in chow- and high-fat-fed rats

A portion of absorbed dietary triglycerides (TG) is retained in the intestine after the postprandial period, within intracellular and extracellular compartments. This pool of TG can be mobilized in response to several stimuli, including oral glucose. The objective of this study was to determine whether oral glucose must be absorbed and metabolized to mobilize TG in rats and whether high-fat feeding, a model of insulin resistance, alters the lipid mobilization response to glucose. Lymph flow, TG concentration, TG output, and apolipoprotein B48 (apoB48) concentration and output were assessed after an intraduodenal lipid bolus in rats exposed to the following intraduodenal administrations 5 h later: saline (placebo), glucose, 2-deoxyglucose (2-DG, absorbed but not metabolized), or glucose + phlorizin (intestinal glucose absorption inhibitor). Glucose alone, but not 2-DG or glucose + phlorizin treatments, stimulated lymph flow, TG output, and apoB48 output compared with placebo. The effects of glucose in high-fat-fed rats were similar to those in chow-fed rats. In conclusion, glucose must be both absorbed and metabolized to enhance lymph flow and intestinal lipid mobilization. This effect is qualitatively and quantitatively similar in high-fat- and chow-fed rats. The precise signaling mechanism whereby enteral glucose enhances lymph flow and mobilizes enteral lipid remains to be determined.NEW & NOTEWORTHY Glucose potently enhances mesenteric lymph flow in chow- and high-fat-fed rats. The magnitude of glucose effect on lymph flow is no different in chow- and high-fat-fed rats. Glucose must be absorbed and metabolized to enhance lymph flow and mobilize intestinal lipid.

Phytomelatonin as a signaling molecule for protein quality control via chaperone, autophagy, and ubiquitin-proteasome systems in plants

Physiological effects mediated by melatonin are attributable to its potent antioxidant activity as well as its role as a signaling molecule in inducing a vast array of melatonin-mediated genes. Here, we propose melatonin as a signaling molecule essential for protein quality control (PQC) in plants. PQC occurs by the coordinated activities of three systems: the chaperone network, autophagy, and the ubiquitin-proteasome system. With regard to the melatonin-mediated chaperone pathway, melatonin increases thermotolerance by induction of heat shock proteins and confers endoplasmic reticulum stress tolerance by increasing endoplasmic reticulum chaperone proteins. In chloroplasts, melatonin-induced chaperones, including Clps and CpHSP70s, play key roles in the PQC of chloroplast-localized proteins, such as Lhcb1, Lhcb4, and RBCL, during growth. Melatonin regulates PQC by autophagy processes, in which melatonin induces many autophagy (ATG) genes and autophagosome formation under stress conditions. Finally, melatonin-mediated plant stress tolerance is associated with up-regulation of stress-induced transcription factors, which are regulated by the ubiquitin-proteasome system. In this review, we propose that melatonin plays a pivotal role in PQC and consequently functions as a pleiotropic molecule under non-stress and adverse conditions in plants.

Image-Based Analysis Revealing the Molecular Mechanism of Peroxisome Dynamics in Plants

Peroxisomes are present in eukaryotic cells and have essential roles in various biological processes. Plant peroxisomes proliferate by de novo biosynthesis or division of pre-existing peroxisomes, degrade, or replace metabolic enzymes, in response to developmental stages, environmental changes, or external stimuli. Defects of peroxisome functions and biogenesis alter a variety of biological processes and cause aberrant plant growth. Traditionally, peroxisomal function-based screening has been employed to isolate Arabidopsis thaliana mutants that are defective in peroxisomal metabolism, such as lipid degradation and photorespiration. These analyses have revealed that the number, subcellular localization, and activity of peroxisomes are closely related to their efficient function, and the molecular mechanisms underlying peroxisome dynamics including organelle biogenesis, protein transport, and organelle interactions must be understood. Various approaches have been adopted to identify factors involved in peroxisome dynamics. With the development of imaging techniques and fluorescent proteins, peroxisome research has been accelerated. Image-based analyses provide intriguing results concerning the movement, morphology, and number of peroxisomes that were hard to obtain by other approaches. This review addresses image-based analysis of peroxisome dynamics in plants, especially A. thaliana and Marchantia polymorpha.

Serine Hydroxymethyltransferase 1 Is Essential for Primary-Root Growth at Low-Sucrose Conditions

Plant roots are essential organs for absorbing nutrients from the soil or medium. Sucrose functions as a vital carbon source in root development, and sucrose starvation interferes with the redox state of plant cells. However, the mechanism of root growth at sucrose starvation remains unclear. Here, we report that SHMT1 (serine hydroxymethyltransferase 1) plays a crucial role in primary-root growth. SHMT1 mutation caused decreased sugar levels, excessive H₂O₂ accumulation, and severe root-growth arrest at sucrose-free conditions, whereas plants with SHMT1 overexpression had increased sugar and decreased H₂O₂ levels, and longer primary roots. Sucrose supply fully restored root growth of shm1-2, but CO₂ alone could not, and SHMT1 is much more stable in roots than shoots at sucrose conditions, suggesting that SHMT1 accumulation in roots is critical for sucrose accumulation and root growth. Further ROS scavenging by GSH application or ROS synthesis inhibition by apocynin application or RBOHD mutation reduced H₂O₂ levels and partially restored the root-growth arrest phenotype of shm1-2 at low-sucrose conditions, suggesting that SHMT1 modulates root growth via sucrose-mediated ROS accumulation. Our findings demonstrated the role of SHMT1 in primary-root growth by regulating sucrose accumulation and ROS homeostasis in roots.

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.

Current progress in plant V-ATPase: From biochemical properties to physiological functions

Vacuolar-type adenosine triphosphatase (V-ATPase, VHA) is a highly conserved, ATP-driven multisubunit proton pump that is widely distributed in all eukaryotic cells. V-ATPase consists of two domains formed by at least 13 different subunits, the membrane peripheral V₁ domain responsible for ATP hydrolysis, and the membrane-integral V₀ domain responsible for proton translocation. V-ATPase plays an essential role in energizing secondary active transport and is indispensable to plants. In addition to multiple stress responses, plant V-ATPase is also implicated in physiological processes such as growth, development, and morphogenesis. Based on the identification of distinct V-ATPase mutants and advances in luminal pH measurements in vivo, it has been revealed that this holoenzyme complex plays a pivotal role in pH homeostasis of the plant endomembrane system and endocytic and secretory trafficking. Here, we review recent progress in comprehending the biochemical properties and physiological functions of plant V-ATPase and explore the topics that require further elucidation.

Selective elimination of host cells harboring replication-competent human immunodeficiency virus reservoirs: a promising therapeutic strategy for HIV cure

ABSTRACT

Many seminal advances have been made in human immunodeficiency virus (HIV)/AIDS research over the past four decades. Treatment strategies, such as gene therapy and immunotherapy, are yielding promising results to effectively control HIV infection. Despite this, a cure for HIV/AIDS is not envisioned in the near future. A recently published academic study has raised awareness regarding a promising alternative therapeutic option for HIV/AIDS, referred to as "selective elimination of host cells capable of producing HIV" (SECH). Similar to the "shock and kill strategy," the SECH approach requires the simultaneous administration of drugs targeting key mechanisms in specific cells to efficiently eliminate HIV replication-competent cellular reservoirs. Herein, we comprehensively review the specific mechanisms targeted by the SECH strategy. Briefly, the suggested cocktail of drugs should contain (i) latency reversal agents to promote the latency reversal process in replication-competent reservoir cells, (ii) pro-apoptotic and anti-autophagy drugs to induce death of infected cells through various pathways, and finally (iii) drugs that eliminate new cycles of infection by prevention of HIV attachment to host cells, and by HIV integrase inhibitor drugs. Finally, we discuss three major challenges that are likely to restrict the application of the SECH strategy in HIV/AIDS patients.

A proposed role for endomembrane trafficking processes in regulating tonoplast content and vacuole dynamics under ammonium stress conditions in Arabidopsis root cells

Ammonium (NH₄⁺) stress has multiple effects on plant physiology, therefore, plant responses are complex, and multiple mechanisms are involved in NH₄⁺ sensitivity and tolerance in plants. Root growth inhibition is an important quantitative readout of the effects of NH₄⁺ stress on plant physiology, and cell elongation appear as the principal growth inhibition target. We recently proposed autophagy as a relevant physiological mechanisms underlying NH₄⁺ sensitivity response in Arabidopsis. In a brief overview, the impaired macro-autophagic flux observed under NH₄⁺ stress conditions has a detrimental impact on the cellular energetic balance, and therefore on the energy-demanding plant growth. In contrast to its inhibitory effect on the autophagosomes flux to vacuole, NH₄⁺ toxicity induced a micro-autophagy-like process. Consistent with the reduced membrane flux to the vacuole related to macro-autophagy inhibition and the increased tonoplast degradation due to enhanced micro-autophagy, the vacuoles of the root cells of the NH₄⁺-stressed plants showed lower tonoplast content and a decreased perimeter/area ratio. As the endosome-to-vacuole trafficking is another important process that contributes to membrane flux toward the vacuole, we evaluated the effects of NH₄⁺ stress on this process. This allows us to propose that autophagy could contribute to vacuole development as well as possible avenues to follow for future studies.

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.

Diurnal variation of transitory starch metabolism is regulated by plastid proteins WXR1/WXR3 in Arabidopsis young seedlings

Transitory starch is the portion of starch that is synthesized during the day in the chloroplast and usually used for plant growth overnight. Here, we report altered metabolism of transitory starch in the wxr1/wxr3 (weak auxin response 1/3) mutants of Arabidopsis. WXR1/WXR3 were previously reported to regulate root growth of young seedlings and affect the auxin response mediated by auxin polar transport in Arabidopsis. In this study the wxr1/wxr3 mutants accumulated transitory starch in cotyledon, young leaf, and hypocotyl at the end of night. WXR1/WXR3 expression showed diurnal variation. Grafting experiments indicated that the WXRs in root were necessary for proper starch metabolism and plant growth. We also found that photosynthesis was inhibited and the transcription level of DIN1/DIN6 (Dark-Inducible 1/6) was reduced in wxr1/wxr3. The mutants also showed a defect in the ionic equilibrium of Na+ and K+, consistent with our bioinformatics data that genes related to ionic equilibrium were misregulated in wxr1. Loss of function of WXR1 also resulted in abnormal trafficking of membrane lipids and proteins. This study reveals that the plastid proteins WXR1/WXR3 play important roles in promoting transitory starch degradation for plant growth over night, possibly through regulating ionic equilibrium in the root.

Ammonium stress increases microautophagic activity while impairing macroautophagic flux in Arabidopsis roots

Plant responses to NH₄⁺ stress are complex, and multiple mechanisms underlying NH₄⁺ sensitivity and tolerance in plants may be involved. Here, we demonstrate that macro- and microautophagic activities are oppositely affected in plants grown under NH₄⁺ toxicity conditions. When grown under NH₄⁺ stress conditions, macroautophagic activity was impaired in roots. Root cells accumulated autophagosomes in the cytoplasm, but showed less autophagic flux, indicating that late steps of the macroautophagy process are affected under NH₄⁺ stress conditions. Under this scenario, we also found that the CCZ1-MON1 complex, a critical factor for vacuole delivery pathways, functions in the late step of the macroautophagic pathway in Arabidopsis. In contrast, an accumulation of tonoplast-derived vesicles was observed in vacuolar lumens of root cells of NH₄⁺ -stressed plants, suggesting the induction of a microautophagy-like process. In this sense, some SYP22-, but mainly VAMP711-positive vesicles were observed inside vacuole in roots of NH₄⁺ -stressed plants. Consistent with the increased tonoplast degradation and the reduced membrane flow to the vacuole due to the impaired macroautophagic flux, the vacuoles of root cells of NH₄⁺ -stressed plants showed a simplified structure and lower tonoplast content. Taken together, this study presents evidence that postulates late steps of the macroautophagic process as a relevant physiological mechanism underlying the NH₄⁺ sensitivity response in Arabidopsis, and additionally provides insights into the molecular tools for studying microautophagy in plants.

Autophagy in plants: Physiological roles and post-translational regulation

In eukaryotes, autophagy helps maintain cellular homeostasis by degrading and recycling cytoplasmic materials via a tightly regulated pathway. Over the past few decades, significant progress has been made towards understanding the physiological functions and molecular regulation of autophagy in plant cells. Increasing evidence indicates that autophagy is essential for plant responses to several developmental and environmental cues, functioning in diverse processes such as senescence, male fertility, root meristem maintenance, responses to nutrient starvation, and biotic and abiotic stress. Recent studies have demonstrated that, similar to nonplant systems, the modulation of core proteins in the plant autophagy machinery by posttranslational modifications such as phosphorylation, ubiquitination, lipidation, S-sulfhydration, S-nitrosylation, and acetylation is widely involved in the initiation and progression of autophagy. Here, we provide an overview of the physiological roles and posttranslational regulation of autophagy in plants.

Neurodegeneration-related beta-amyloid as autocatabolism-attenuator in a micro-in vivo system

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Bdelloids are adaptive models for studying aggregate-metabolism interactions.

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Starvation causes reversible organ shrinkage in bdelloids.

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The organ shrinkage is in connection with autocatabolic processes.

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Beta-amyloid attenuates the starvation-induced germovitellaria shrinkage.

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Human-type amyloid-aggregates are metabolism-regulators in two bdelloid species.

Investigation of human neurodegeneration-related aggregates of beta-amyloid 1–42 (Aβ42) on bdelloid rotifers is a novel interdisciplinary approach in life sciences. We reapplied an organ size-based in vivo monitoring system, exploring the autocatabolism-related alterations evoked by Aβ42, in a glucose-supplemented starvation model. The experientially easy-to-follow size reduction of the bilateral reproductive organ (germovitellaria) in fasted rotifers was rescued by Aβ42, serving as a nutrient source- and peptide sequence-specific attenuator of the organ shrinkage phase and enhancer of the regenerative one including egg reproduction. Recovery of the germovitellaria was significant in comparison with the greatly shrunken form. In contrast to the well-known neurotoxic Aβ42 (except the bdelloids) with specific regulatory roles, the artificially designed scrambled version (random order of amino acids) was inefficient in autocatabolism attenuation, behaving as negative control. This native Aβ42-related modulation of the ‘functionally reversible organ shrinkage’ can be a potential experiential and supramolecular marker of autocatabolism in vivo.

Autophagy Plays Prominent Roles in Amino Acid, Nucleotide, and Carbohydrate Metabolism during Fixed-Carbon Starvation in Maize

Autophagic recycling of proteins, lipids, nucleic acids, carbohydrates, and organelles is essential for cellular homeostasis and optimal health, especially under nutrient-limiting conditions. To better understand how this turnover affects plant growth, development, and survival upon nutrient stress, we applied an integrated multiomics approach to study maize (Zea mays) autophagy mutants subjected to fixed-carbon starvation induced by darkness. Broad metabolic alterations were evident in leaves missing the core autophagy component ATG12 under normal growth conditions (e.g., lipids and secondary metabolism), while changes in amino acid-, carbohydrate-, and nucleotide-related metabolites selectively emerged during fixed-carbon starvation. Through combined proteomic and transcriptomic analyses, we identified numerous autophagy-responsive proteins, which revealed processes underpinning the various metabolic changes seen during carbon stress as well as potential autophagic cargo. Strikingly, a strong upregulation of various catabolic processes was observed in the absence of autophagy, including increases in simple carbohydrate levels with a commensurate drop in starch levels, elevated free amino acid levels with a corresponding reduction in intact protein levels, and a strong increase in the abundance of several nitrogen-rich nucleotide catabolites. Altogether, this analysis showed that fixed-carbon starvation in the absence of autophagy adjusts the choice of respiratory substrates, promotes the transition of peroxisomes to glyoxysomes, and enhances the retention of assimilated nitrogen.

Microautophagy in Plants: Consideration of Its Molecular Mechanism

Microautophagy is a type of autophagy. It is characterized by direct enclosing with the vacuolar/lysosomal membrane, which completes the isolation and uptake of cell components in the vacuole. Several publications present evidence that plants exhibit microautophagy. Plant microautophagy is involved in anthocyanin accumulation in the vacuole, eliminating damaged chloroplasts and degrading cellular components during starvation. However, information on the molecular mechanism of microautophagy is less available than that on the general macroautophagy, because the research focusing on microautophagy has not been widely reported. In yeast and animals, it is suggested that microautophagy can be classified into several types depending on morphology and the requirements of autophagy-related (ATG) genes. This review summarizes the studies on plant microautophagy and discusses possible techniques for a future study in this field while taking into account the information on microautophagy obtained from yeast and animals.