Involvement of autophagy in the direct ER to vacuole protein trafficking route in plants

Correlative microscopy - illuminating the endomembrane system of plant seeds

The endomembrane system of cereal seed endosperm is a highly plastic and dynamic system reflecting the high degree of specialization of this tissue. It is capable of coping with high levels of storage protein synthesis and undergoes rapid changes to accommodate these storage proteins in newly formed storage organelles such as endoplasmic reticulum-derived protein bodies or protein storage vacuoles. The study of endomembrane morphology in cereal endosperm is challenging due to the amount of starch that cereal seeds accumulate and the progressive desiccation of the tissue. Here, we present a comprehensive study of the endomembrane system of developing barley endosperm cells, complemented by correlative light and electron microscopy (CLEM) imaging. The use of genetically fused fluorescent protein tags in combination with the high resolution of electron microscopy brings ultrastructural research to a new level and can be used to generate novel insights in cell biology in general and in cereal seed research in particular.

Superoxide Dismutase Premodulates Oxidative Stress in Plastids for Protection of Tobacco Plants from Cold Damage Ultrastructure Damage

ROS-dependent induction of oxidative damage can be used as a trigger initiating genetically determined non-specific protection in plant cells and tissues. Plants are potentially able to withstand various specific (toxic, osmotic) factors of abiotic effects, but do not have sufficient or specific sensitivity to form an adequate effective response. In this work, we demonstrate one of the possible approaches for successful cold acclimation through the formation of effective protection of photosynthetic structures due to the insertion of the heterologous FeSOD gene into the tobacco genome under the control of the constitutive promoter and equipped with a signal sequence targeting the protein to plastid. The increased enzymatic activity of superoxide dismutase in the plastid compartment of transgenic tobacco plants enables them to tolerate the oxidative factor of environmental stresses scavenging ROS. On the other hand, the cost of such resistance is quite high and, when grown under normal conditions, disturbs the arrangement of the intrachloroplastic subdomains leading to the modification of stromal thylakoids, probably significantly affecting the photosynthesis processes that regulate the efficiency of photosystem II. This is partially compensated for by the fact that, at the same time, under normal conditions, the production of peroxide induces the activation of ROS detoxification enzymes. However, a violation of a number of processes, such as the metabolism of accumulation, and utilization and transportation of sugars and starch, is significantly altered, which leads to a shift in metabolic chains. The expected step for further improvement of the applied technology could be both the use of inducible promoters in the expression cassette, and the addition of other genes encoding for hydrogen peroxide-scavenging enzymes in the genetic construct that are downstream in the metabolic chain.

Vacuolar Processing Enzymes in Plant Programmed Cell Death and Autophagy

Vacuolar processing enzymes (VPEs) are plant cysteine proteases that are subjected to autoactivation in an acidic pH. It is presumed that VPEs, by activating other vacuolar hydrolases, are in control of tonoplast rupture during programmed cell death (PCD). Involvement of VPEs has been indicated in various types of plant PCD related to development, senescence, and environmental stress responses. Another pathway induced during such processes is autophagy, which leads to the degradation of cellular components and metabolite salvage, and it is presumed that VPEs may be involved in the degradation of autophagic bodies during plant autophagy. As both PCD and autophagy occur under similar conditions, research on the relationship between them is needed, and VPEs, as key vacuolar proteases, seem to be an important factor to consider. They may even constitute a potential point of crosstalk between cell death and autophagy in plant cells. This review describes new insights into the role of VPEs in plant PCD, with an emphasis on evidence and hypotheses on the interconnections between autophagy and cell death, and indicates several new research opportunities.

Multiscale imaging reveals the presence of autophagic vacuoles in developing maize endosperm

Cereal endosperm is solely devoted to the storage of proteins and starch that will be used by the embryo upon germination. The high degree of specialization of this tissue is reflected in its endomembrane system, in which ER derived protein bodies and protein storage vacuoles (PSVs) are of particular interest. In maize seeds, the main storage proteins are zeins, that form transport incompetent aggregates within the ER lumen and finally build protein bodies that bud from the ER. In contrast to the zeins, the maize globulins are not very abundant and the vacuolar storage compartment of maize endosperm is not fully described. Whereas in other cereals, including wheat and barley, the PSV serves as the main protein storage compartment, only small, globulin-containing PSVs have been identified in maize so far. We present here a multi-scale set of data, ranging from live-cell imaging to more sophisticated 3D electron microscopy techniques (SBF-SEM), that has allowed us to investigate in detail the vacuoles in maize endosperm cells, including a novel, autophagic vacuole that is present in early developmental stages.

Crossing paths: Recent insights in the interplay between autophagy and intracellular trafficking in plants

Autophagy fulfils a crucial role in plant cellular homeostasis by recycling diverse cellular components ranging from protein complexes to whole organelles. Autophagy cargos are shuttled to the vacuole for degradation, thereby completing the recycling process. Canonical autophagy requires the lipidation and insertion of ATG8 proteins into double-membrane structures, termed autophagosomes, which engulf the cargo to be degraded. As such, the autophagy pathway actively contributes to intracellular membrane trafficking. Yet, the autophagic process is not fully considered a bona fide component of the canonical membrane trafficking pathway. However, recent findings have started to pinpoint the interconnection between classical membrane trafficking pathways and autophagy. This review details the latest advances in our comprehension of the interplay between these two pathways. Understanding the overlap between autophagy and canonical membrane trafficking pathways is important to illuminate the inner workings of both pathways in plant cells.

Specialized endoplasmic reticulum-derived vesicles in plants: Functional diversity, evolution, and biotechnological exploitation

A central role of the endoplasmic reticulum (ER) is the synthesis, folding and quality control of secretory proteins. Secretory proteins usually exit the ER to enter the Golgi apparatus in coat protein complex II (COPII)-coated vesicles before transport to different subcellular destinations. However, in plants there are specialized ER-derived vesicles (ERDVs) that carry specific proteins but, unlike COPII vesicles, can exist as independent organelles or travel to the vacuole in a Golgi-independent manner. These specialized ERDVs include protein bodies and precursor-accumulating vesicles that accumulate storage proteins in the endosperm during seed development. Specialized ERDVs also include precursor protease vesicles that accumulate amino acid sequence KDEL-tailed cysteine proteases and ER bodies in Brassicales plants that accumulate myrosinases that hydrolyzes glucosinolates. These functionally specialized ERDVs act not only as storage organelles but also as platforms for signal-triggered processing, activation and deployment of specific proteins with important roles in plant growth, development and adaptive responses. Some specialized ERDVs have also been exploited to increase production of recombinant proteins and metabolites. Here we discuss our current understanding of the functional diversity, evolutionary mechanisms and biotechnological application of specialized ERDVs, which are associated with some of the highly remarkable characteristics important to plants.

Cell Biology Methods to Study Recombinant Proteins in Seeds

Seeds are an attractive platform for the production of recombinant proteins because of their excellent storage properties and their well-developed endomembrane system, which allows accumulation of the product within specialized storage organelles. Due to the presence of these additional organelles and the resulting complexity of intracellular protein trafficking it is interesting to investigate the transport and storage of a recombinant protein within seed tissues, its interactions with endogenous reserve proteins and its impact on the ultrastructure of the endomembrane system. Possible approaches include sequential extraction procedures, subcellular fractionation and 2D as well as 3D electron microscopy techniques such as electron tomography (ET) and serial block face scanning electron microscopy (SBF-SEM), which are described and discussed in this chapter.

Flavonoid mediated selective cross-talk between plants and beneficial soil microbiome

Plants generate a wide variety of organic components during their different growth phases. The majority of those compounds have been classified as primary and secondary metabolites. Secondary metabolites are essential in plants' adaptation to new changing environments and in managing several biotic and abiotic stress. It also invests some of its photosynthesized carbon as secondary metabolites to establish a mutual relationship with soil microorganisms in that specific niche. As soil harbors both pathogenic and beneficial microorganisms, it is essential to identify some specific metabolites that can discriminate beneficial and pathogenic ones. Thus, a detailed understanding of metabolite's architectures that interact with beneficial microorganisms could open a new horizon of ecology and agricultural research. Flavonoids are used as classic examples of secondary metabolites in this study to demonstrate recent developments in understanding and realizing how these valuable metabolites can be controlled at different levels. Most of the research was focused on plant flavonoids, which shield the host plant against competitors or predators, as well as having other ecological implications. Thus, in the present review, our goal is to cover a wide range of functional and signalling activities of secondary metabolites especially, flavonoids mediated selective cross-talk between plant and its beneficial soil microbiome. Here, we have summarized recent advances in understanding the interactions between plant species and their rhizosphere microbiomes through root exudates (flavonoids), with a focus on how these exudates facilitate rhizospheric associations.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11101-022-09806-3.

Understanding In Vitro Tissue Culture-Induced Variation Phenomenon in Microspore System

In vitro tissue culture plant regeneration is a complicated process that requires stressful conditions affecting the cell functioning at multiple levels, including signaling pathways, transcriptome functioning, the interaction between cellular organelles (retro-, anterograde), compounds methylation, biochemical cycles, and DNA mutations. Unfortunately, the network linking all these aspects is not well understood, and the available knowledge is not systemized. Moreover, some aspects of the phenomenon are poorly studied. The present review attempts to present a broad range of aspects involved in the tissue culture-induced variation and hopefully would stimulate further investigations allowing a better understanding of the phenomenon and the cell functioning.

Spotlight on the overlapping routes and partners for anthocyanin transport in plants

Secondary metabolites are produced by plants and are classified based on their chemical structure or the biosynthetic routes through which they are synthesized. Among them, flavonoids, including anthocyanins and pro-anthocyanidins (PAs), are abundant in leaves, flowers, fruits, and seed coats in plants. The anthocyanin biosynthetic pathway has been intensively studied, but the molecular mechanism of anthocyanin transport from the synthesis site to the storage site needs attention. Although the major transporters are well defined yet, the redundancy of these transporters for structurally similar or dis-similar anthocyanins motivates additional research. Herein, we reviewed the role of membrane transporters involved in anthocyanin transport, including ATP-binding cassette, multidrug and toxic compound extrusion (MATE), Bilitranslocase-homolog (BTL), and vesicle-mediated transport. We also highlight the ability of transporters to cater distinct anthocyanins or their chemically-modified forms with overlapping transport mechanisms and sequestration into the vacuoles. Our understanding of the anthocyanin transporters could provide anthocyanin-rich crops and fruits with a benefit on human health at a large scale.

Plant multiscale networks: charting plant connectivity by multi-level analysis and imaging techniques

In multicellular and even single-celled organisms, individual components are interconnected at multiscale levels to produce enormously complex biological networks that help these systems maintain homeostasis for development and environmental adaptation. Systems biology studies initially adopted network analysis to explore how relationships between individual components give rise to complex biological processes. Network analysis has been applied to dissect the complex connectivity of mammalian brains across different scales in time and space in The Human Brain Project. In plant science, network analysis has similarly been applied to study the connectivity of plant components at the molecular, subcellular, cellular, organic, and organism levels. Analysis of these multiscale networks contributes to our understanding of how genotype determines phenotype. In this review, we summarized the theoretical framework of plant multiscale networks and introduced studies investigating plant networks by various experimental and computational modalities. We next discussed the currently available analytic methodologies and multi-level imaging techniques used to map multiscale networks in plants. Finally, we highlighted some of the technical challenges and key questions remaining to be addressed in this emerging field.

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.

Autophagy and Nutrients Management in Plants

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

A Review of Plant Vacuoles: Formation, Located Proteins, and Functions

Vacuoles, cellular membrane-bound organelles, are the largest compartments of cells, occupying up to 90% of the volume of plant cells. Vacuoles are formed by the biosynthetic and endocytotic pathways. In plants, the vacuole is crucial for growth and development and has a variety of functions, including storage and transport, intracellular environmental stability, and response to injury. Depending on the cell type and growth conditions, the size of vacuoles is highly dynamic. Different types of cell vacuoles store different substances, such as alkaloids, protein enzymes, inorganic salts, sugars, etc., and play important roles in multiple signaling pathways. Here, we summarize vacuole formation, types, vacuole-located proteins, and functions.

Autophagy and vacuolar biogenesis during the nectary development

Different autophagy pathways are a driver of vacuolar biogenesis and are development stage specific during the extrafloral nectary development in Citharexylum myrianthum. Plant autophagy plays an important role in various developmental processes such as seed germination, pollen maturation and leaf senescence. However, studies that address the evidence of autophagy and its role in the development of plant glands are scarce and largely restricted to laticifers. Regarding nectary, studies have repeatedly pointed to signs of degradation associated with the end of the secretory cycle, without exploring autophagy. Likewise, the relationship between autophagy and biogenesis of vacuoles remains an unexplored issue. In this study, using conventional and microwave fixation in association with ultracytochemical methods for transmission electron microscopy, we investigated the occurrence of autophagy and its implication in the differentiation of extrafloral nectary in Citharexylum myrianthum (Verbenaceae) under natural conditions, focusing on the vacuole biogenesis. We described a variety of vacuole types associated with the stage of nectary epidermis development, which differs with respect to origin, function and nature of the products to be stored. Three distinct autophagy pathways were detected: macroautophagy, microautophagy (both restricted to the undifferentiated epidermal cells, at the presecretory stage) and megaautophagy (circumscribed to the differentiated epidermal cells, at the postsecretory stage). Our study clearly demonstrated that the vacuole variety and autophagy processes in the nectary epidermal cells are development specific. This study highlights the role of autophagy in vacuole biogenesis and its implications for the development of nectary and opens new venues for future studies on regulation mechanisms for autophagy in plant secretory structures under normal conditions.

Communications Between the Endoplasmic Reticulum and Other Organelles During Abiotic Stress Response in Plants

To adapt to constantly changing environmental conditions, plants have evolved sophisticated tolerance mechanisms to integrate various stress signals and to coordinate plant growth and development. It is well known that inter-organellar communications play important roles in maintaining cellular homeostasis in response to environmental stresses. The endoplasmic reticulum (ER), extending throughout the cytoplasm of eukaryotic cells, is a central organelle involved in lipid metabolism, Ca²⁺ homeostasis, and synthesis and folding of secretory and transmembrane proteins crucial to perceive and transduce environmental signals. The ER communicates with the nucleus via the highly conserved unfolded protein response pathway to mitigate ER stress. Importantly, recent studies have revealed that the dynamic ER network physically interacts with other intracellular organelles and endomembrane compartments, such as the Golgi complex, mitochondria, chloroplast, peroxisome, vacuole, and the plasma membrane, through multiple membrane contact sites between closely apposed organelles. In this review, we will discuss the signaling and metabolite exchanges between the ER and other organelles during abiotic stress responses in plants as well as the ER-organelle membrane contact sites and their associated tethering complexes.

Plant Vacuoles

Plant vacuoles are multifunctional organelles. On the one hand, most vegetative tissues develop lytic vacuoles that have a role in degradation. On the other hand, seed cells have two types of storage vacuoles: protein storage vacuoles (PSVs) in endosperm and embryonic cells and metabolite storage vacuoles in seed coats. Vacuolar proteins and metabolites are synthesized on the endoplasmic reticulum and then transported to the vacuoles via Golgi-dependent and Golgi-independent pathways. Proprotein precursors delivered to the vacuoles are converted into their respective mature forms by vacuolar processing enzyme, which also regulates various kinds of programmed cell death in plants. We summarize two types of vacuolar membrane dynamics that occur during defense responses: vacuolar membrane collapse to attack viral pathogens and fusion of vacuolar and plasma membranes to attack bacterial pathogens. We also describe the chemical defense against herbivores brought about by the presence of PSVs in the idioblast myrosin cell.

Increases in activity of proteasome and papain-like cysteine protease in Arabidopsis autophagy mutants: back-up compensatory effect or cell-death promoting effect?

Autophagy is essential for protein degradation, nutrient recycling, and nitrogen remobilization. Autophagy is induced during leaf ageing and in response to nitrogen starvation, and is known to play a fundamental role in nutrient recycling for remobilization and seed filling. Accordingly, ageing leaves of Arabidopsis autophagy mutants (atg) have been shown to over-accumulate proteins and peptides, possibly because of a reduced protein degradation capacity. Surprisingly, atg leaves also displayed higher protease activities. The work reported here aimed at identifying the nature of the proteases and protease activities that accumulated differentially (higher or lower) in the atg mutants. Protease identification was performed using shotgun LC-MS/MS proteome analyses and activity-based protein profiling (ABPP). The results showed that the chloroplast FTSH (FILAMENTATION TEMPERATURE SENSITIVE H) and DEG (DEGRADATION OF PERIPLASMIC PROTEINS) proteases and several extracellular serine proteases [subtilases (SBTs) and serine carboxypeptidase-like (SCPL) proteases] were less abundant in atg5 mutants. By contrast, proteasome-related proteins and cytosolic or vacuole cysteine proteases were more abundant in atg5 mutants. Rubisco degradation assays and ABPP showed that the activities of proteasome and papain-like cysteine protease were increased in atg5 mutants. Whether these proteases play a back-up role in nutrient recycling and remobilization in atg mutants or act to promote cell death is discussed in relation to their accumulation patterns in the atg5 mutant compared with the salicylic acid-depleted atg5/sid2 double-mutant, and in low nitrate compared with high nitrate conditions. Several of the proteins identified are indeed known as senescence- and stress-related proteases or as spontaneous cell-death triggering factors.

Annexins as Overlooked Regulators of Membrane Trafficking in Plant Cells

Annexins are an evolutionary conserved superfamily of proteins able to bind membrane phospholipids in a calcium-dependent manner. Their physiological roles are still being intensively examined and it seems that, despite their general structural similarity, individual proteins are specialized toward specific functions. However, due to their general ability to coordinate membranes in a calcium-sensitive fashion they are thought to participate in membrane flow. In this review, we present a summary of the current understanding of cellular transport in plant cells and consider the possible roles of annexins in different stages of vesicular transport.

Autophagy in Plants--What's New on the Menu?

Autophagy is a major cellular degradation pathway in eukaryotes. Recent studies have revealed the importance of autophagy in many aspects of plant life, including seedling establishment, plant development, stress resistance, metabolism, and reproduction. This is manifested by the dual ability of autophagy to execute bulk degradation under severe environmental conditions, while simultaneously to be highly selective in targeting specific compartments and protein complexes to regulate key cellular processes, even during favorable growth conditions. Delivery of cellular components to the vacuole enables their recycling, affecting the plant metabolome, especially under stress. Recent research in Arabidopsis has further unveiled fundamental mechanistic aspects in autophagy which may have relevance in non-plant systems. We review the most recent discoveries concerning autophagy in plants, touching upon all these aspects.

The identification of new cytosolic glutamine synthetase and asparagine synthetase genes in barley (Hordeum vulgare L.), and their expression during leaf senescence

Glutamine synthetase and asparagine synthetase are two master enzymes involved in ammonium assimilation in plants. Their roles in nitrogen remobilization and nitrogen use efficiency have been proposed. In this report, the genes coding for the cytosolic glutamine synthetases (HvGS1) and asparagine synthetases (HvASN) in barley were identified. In addition to the three HvGS1 and two HvASN sequences previously reported, two prokaryotic-like HvGS1 and three HvASN cDNA sequences were identified. Gene structures were then characterized, obtaining full genomic sequences. The response of the five HvGS1 and five HvASN genes to leaf senescence was then studied. Developmental senescence was studied using primary and flag leaves. Dark-exposure or low-nitrate conditions were also used to trigger stress-induced senescence. Well-known senescence markers such as the chlorophyll and Rubisco contents were monitored in order to characterize senescence levels in the different leaves. The three eukaryotic-like HvGS1_1, HvGS1_2, and HvGS1_3 sequences showed the typical senescence-induced reduction in gene expression described in many plant species. By contrast, the two prokaryotic-like HvGS1_4 and HvGS1_5 sequences were repressed by leaf senescence, similar to the HvGS2 gene, which encodes the chloroplast glutamine synthetase isoenzyme. There was a greater contrast in the responses of the five HvASN and this suggested that these genes are needed for N remobilization in senescing leaves only when plants are well fertilized with nitrate. Responses of the HvASN sequences to dark-induced senescence showed that there are two categories of asparagine synthetases, one induced in the dark and the other repressed by the same conditions.

Global analysis of the role of autophagy in cellular metabolism and energy homeostasis in Arabidopsis seedlings under carbon starvation

Germination and early seedling establishment are developmental stages in which plants face limited nutrient supply as their photosynthesis mechanism is not yet active. For this reason, the plant must mobilize the nutrient reserves provided by the mother plant in order to facilitate growth. Autophagy is a catabolic process enabling the bulk degradation of cellular constituents in the vacuole. The autophagy mechanism is conserved among eukaryotes, and homologs of many autophagy-related (ATG) genes have been found in Arabidopsis thaliana. T-DNA insertion mutants (atg mutants) of these genes display higher sensitivity to various stresses, particularly nutrient starvation. However, the direct impact of autophagy on cellular metabolism has not been well studied. In this work, we used etiolated Arabidopsis seedlings as a model system for carbon starvation. atg mutant seedlings display delayed growth in response to carbon starvation compared with wild-type seedlings. High-throughput metabolomic, lipidomic, and proteomic analyses were performed, as well as extensive flux analyses, in order to decipher the underlying causes of the phenotype. Significant differences between atg mutants and wild-type plants have been demonstrated, suggesting global effects of autophagy on central metabolism during carbon starvation as well as severe energy deprivation, resulting in a morphological phenotype.

Plant vacuole morphology and vacuolar trafficking

Plant vacuoles are essential organelles for plant growth and development, and have multiple functions. Vacuoles are highly dynamic and pleiomorphic, and their size varies depending on the cell type and growth conditions. Vacuoles compartmentalize different cellular components such as proteins, sugars, ions and other secondary metabolites and play critical roles in plants response to different biotic/abiotic signaling pathways. In this review, we will summarize the patterns of changes in vacuole morphology in certain cell types, our understanding of the mechanisms of plant vacuole biogenesis, and the role of SNAREs and Rab GTPases in vacuolar trafficking.

Degradation of organelles or specific organelle components via selective autophagy in plant cells

Macroautophagy (hereafter referred to as autophagy) is a cellular mechanism dedicated to the degradation and recycling of unnecessary cytosolic components by their removal to the lytic compartment of the cell (the vacuole in plants). Autophagy is generally induced by stresses causing energy deprivation and its operation occurs by special vesicles, termed autophagosomes. Autophagy also operates in a selective manner, recycling specific components, such as organelles, protein aggregates or even specific proteins, and selective autophagy is implicated in both cellular housekeeping and response to stresses. In plants, selective autophagy has recently been shown to degrade mitochondria, plastids and peroxisomes, or organelle components such as the endoplasmic-reticulum (ER) membrane and chloroplast-derived proteins such as Rubisco. This ability places selective-autophagy as a major factor in cellular steady-state maintenance, both under stress and favorable environmental conditions. Here we review the recent advances documented in plants for this cellular process and further discuss its impact on plant physiology.