Fluorescent detection of fluid phase endocytosis allows for in vivo estimation of endocytic vesicle sizes in plant cells with sub-diffraction accuracy

Advances in structural, spatial, and temporal mechanics of plant endocytosis

Endocytic trafficking underlies processes essential for plant growth and development, including the perception of and response to abiotic and extracellular stimuli, post-Golgi and exocytic trafficking, and cytokinesis. Protein adaptors and regulatory factors of clathrin-mediated endocytosis that contribute to the formation of endocytic clathrin-coated vesicles are evolutionarily conserved. Yet, work of the last ten years has identified differences between the endocytic mechanisms of plants and Opisthokonts involving the endocytic adaptor TPLATE complex, the requirement of actin during CME, and the function of clathrin-independent endocytosis in the uptake of plant-specific plasma membrane proteins. Here, we review clathrin-mediated and -independent pathways in plants and describe recent advances enabled by new proteomic and imaging methods, and conditional perturbation of endocytosis. In addition, we summarize the formation and trafficking of clathrin-coated vesicles based on temporal and structural data garnered from high-resolution quantitative imaging studies. Finally, new information about the cross-talk between endocytosis and other endomembrane trafficking pathways and organelles will also be discussed.

Fluid-phase and membrane markers reveal spatio-temporal dynamics of membrane traffic and repair in the green alga Chara australis

We investigated the mechanisms and the spatio-temporal dynamics of fluid-phase and membrane internalization in the green alga Chara australis using fluorescent hydrazides markers alone, or in conjunction with styryl dyes. Using live-cell imaging, immunofluorescence and inhibitor studies we revealed that both fluid-phase and membrane dyes were actively taken up into the cytoplasm by clathrin-mediated endocytosis and stained various classes of endosomes including brefeldin A- and wortmannin-sensitive organelles (trans-Golgi network and multivesicular bodies). Uptake of fluorescent hydrazides was poorly sensitive to cytochalasin D, suggesting that actin plays a minor role in constitutive endocytosis in Chara internodal cells. Sequential pulse-labelling experiments revealed novel aspects of the temporal progression of endosomes in Chara internodal cells. The internalized fluid-phase marker distributed to early compartments within 10 min from dye exposure and after about 30 min, it was found almost exclusively in late endocytic compartments. Notably, fluid cargo consecutively internalized at time intervals of more than 1h, was not targeted to the same vesicular structures, but was sorted into distinct late compartments. We further found that fluorescent hydrazide dyes distributed not only to rapidly recycling endosomes but also to long-lived compartments that participated in plasma membrane repair after local laser injury. Our approach highlights the benefits of combining different fluid-phase markers in conjunction with membrane dyes in simultaneous and sequential application modus for investigating vesicle traffic, especially in organisms, which are still refractory to genetic transformation like characean algae.

Calcium deprivation enhances non-selective fluid-phase endocytosis and modifies membrane lipid profiles in Arabidopsis roots

Ecological studies have revealed significant decreases in calcium (Ca) levels in various soils, and widely occurring physiological Ca deficits worldwide. These changes may cause decreases in plant diversity and increases in plant vulnerability to environmental stress, but the underlying cellular mechanism is not well understood. In this study, we found that in Arabidopsis thaliana roots, deprivation of Ca²⁺, but not other minerals, dramatically enhanced plasma membrane invagination, endosome formation, and trafficking to the vacuole through the trans-Golgi network and pre-vacuole compartment, a typical pathway of endocytosis. Antagonist and cellular tracing analyses using non-bioactive, membrane-impermeable fluorescent probes indicated that this type of endocytosis is not regulated by receptors, instead representing a non-selective, non-specific fluid phase-based process. We performed lipid-profiling analysis of roots in response to Ca²⁺ deprivation, finding increased phosphatidylcholine (PC), Lyso-PC, phosphatidylethanolamine (PE), Lyso-PE, phosphatidylinositol (PI) and triacylglycerols (TAG) biosynthesis but deceased phosphatidic acid (PA) and diacylglycerols (DAG) biosynthesis. The increased TAG contents and molar ratio of PC/PE in these roots might reflect a cellular response to maintain membrane stability and the balance between the membrane and storage lipids. This study demonstrates the essential role of Ca in maintaining plasma membrane stability and the selectivity of plant root cells, and it highlights the potential deleterious effect of decreased Ca levels in surface soil on plant growth.

A computational approach for inferring the cell wall properties that govern guard cell dynamics

Guard cells dynamically adjust their shape in order to regulate photosynthetic gas exchange, respiration rates and defend against pathogen entry. Cell shape changes are determined by the interplay of cell wall material properties and turgor pressure. To investigate this relationship between turgor pressure, cell wall properties and cell shape, we focused on kidney-shaped stomata and developed a biomechanical model of a guard cell pair. Treating the cell wall as a composite of the pectin-rich cell wall matrix embedded with cellulose microfibrils, we show that strong, circumferentially oriented fibres are critical for opening. We find that the opening dynamics are dictated by the mechanical stress response of the cell wall matrix, and as the turgor rises, the pectinaceous matrix stiffens. We validate these predictions with stomatal opening experiments in selected Arabidopsis cell wall mutants. Thus, using a computational framework that combines a 3D biomechanical model with parameter optimization, we demonstrate how to exploit subtle shape changes to infer cell wall material properties. Our findings reveal that proper stomatal dynamics are built on two key properties of the cell wall, namely anisotropy in the form of hoop reinforcement and strain stiffening.

Single molecule microscopy in 3D cell cultures and tissues

From the onset of the first microscopic visualization of single fluorescent molecules in living cells at the beginning of this century, to the present, almost routine application of single molecule microscopy, the method has well-proven its ability to contribute unmatched detailed insight into the heterogeneous and dynamic molecular world life is composed of. Except for investigations on bacteria and yeast, almost the entire story of success is based on studies on adherent mammalian 2D cell cultures. However, despite this continuous progress, the technique was not able to keep pace with the move of the cell biology community to adapt 3D cell culture models for basic research, regenerative medicine, or drug development and screening. In this review, we will summarize the progress, which only recently allowed for the application of single molecule microscopy to 3D cell systems and give an overview of the technical advances that led to it. While initially posing a challenge, we finally conclude that relevant 3D cell models will become an integral part of the on-going success of single molecule microscopy.

Analysis of fluid-phase endocytosis in (intact) plant cells

Endocytosis is a continuous process at the plasma membrane at least of all eukaryotic cells. Regardless of the molecular machinery, which drives the formation and uptake of endocytic vesicles, it is reasonable to assume that this process inevitably collects external fluid. Hence, at least for the majority of apoplastic solutes, the endocytosis of the fluid phase is likely to be an inevitable process. Due to its independence from the molecular machinery and low selectivity with respect to the cargo, it is thus perfectly suited to be used as a tracer to follow the activity of all endocytic events. Here we describe simple protocols based on fluorescence microscopy, which yield quantitative information about endocytic vesicle sizes-with sub-diffraction accuracy-as well as the size exclusion limits for these uptake routes.

Architectural remodeling of the tonoplast during fluid-phase endocytosis

During fluid phase endocytosis (FPE) in plant storage cells, the vacuole receives a considerable amount of membrane and fluid contents. If allowed to accumulate over a period of time, the enlarging tonoplast and increase in fluids would invariably disrupt the structural equilibrium of the mature cells. Therefore, a membrane retrieval process must exist that will guarantee membrane homeostasis in light of tonoplast expansion by membrane addition during FPE. We examined the morphological changes to the vacuolar structure during endocytosis in red beet hypocotyl tissue using scanning laser confocal microscopy and immunohistochemistry. The heavily pigmented storage vacuole allowed us to visualize all architectural transformations during treatment. When red beet tissue was incubated in 200 mM sucrose, a portion of the sucrose accumulated entered the cell by means of FPE. The accumulation process was accompanied by the development of vacuole-derived vesicles which transiently counterbalanced the addition of surplus endocytic membrane during rapid rates of endocytosis. Topographic fluorescent confocal micrographs showed an ensuing reduction in the size of the vacuole-derived vesicles and further suggest their reincorporation into the vacuole to maintain vacuolar unity and solute concentration.

Clathrin-independent endocytosis contributes to uptake of glucose into BY-2 protoplasts

In eukaryotic cells, several pathways exist for the internalization of plasma membrane proteins and extracellular cargo molecules. These endocytic pathways can be divided into clathrin-dependent and clathrin-independent pathways. While clathrin-dependent pathways are known to be involved in a variety of cellular processes in plants, clathrin-independent pathways have so far only been identified in animal and yeast cells. Here we show that internalization of fluorescent glucose into BY-2 cells leads to accumulation of the sugar in compartments of the endocytic pathway. This endocytic uptake of glucose was not blocked by ikarugamycin, an inhibitor of clathrin-dependent endocytosis, suggesting a role for clathrin-independent endocytosis in glucose uptake. Investigations of fusion and fission of single vesicles by membrane capacitance measurements revealed stimulation of endocytic activity by extracellular glucose. Glucose-stimulated fission of vesicles was not affected by addition of ikarugamycin or blocking of clathrin coat formation by transient over-expression of HUB1 (the C-terminal part of the clathrin heavy chain). These data demonstrate that clathrin-independent endocytosis does occur in plant cells. This pathway may represent a common mechanism for the uptake of external nutrients.

Ion transport, membrane traffic and cellular volume control

Throughout their development, plants balance cell surface area and volume with ion transport and turgor. This balance lies at the core of cellular homeostatic networks and is central to the capacity to withstand abiotic as well as biotic stress. Remarkably, very little is known of its mechanics, notably how membrane traffic is coupled with osmotic solute transport and its control. Here we outline recent developments in the understanding of so-called SNARE proteins that form part of the machinery for membrane vesicle traffic in all eukaryotes. We focus on SNAREs active at the plasma membrane and the evidence for specialisation in enhanced, homeostatic and stress-related traffic. Recent studies have placed a canonical SNARE complex associated with the plasma membrane in pathogen defense, and the discovery of the first SNARE as a binding partner with ion channels has demonstrated a fundamental link to inorganic osmotic solute uptake. Work localising the channel binding site has now identified a new and previously uncharacterised motif, yielding important clues to a plausible mechanism coupling traffic and transport. We examine the evidence that this physical interaction serves to balance enhanced osmotic solute uptake with membrane expansion through mutual control of the two processes. We calculate that even during rapid cell expansion only a minute fraction of SNAREs present at the membrane need be engaged in vesicle traffic at any one time, a number surprisingly close to the known density of ion channels at the plant plasma membrane. Finally, we suggest a framework of alternative models coupling transport and traffic, and approachable through direct, experimental testing.

Endocytosis in plant-microbe interactions

Plants encounter throughout their life all kinds of microorganisms, such as bacteria, fungi, or oomycetes, with either friendly or unfriendly intentions. During evolution, plants have developed a wide range of defense mechanisms against attackers. In return, adapted microbes have developed strategies to overcome the plant lines of defense, some of these microbes engaging in mutualistic or parasitic endosymbioses. By sensing microbe presence and activating signaling cascades, the plasma membrane through its dynamics plays a crucial role in the ongoing molecular dialogue between plants and microbes. This review describes the contribution of endocytosis to different aspects of plant-microbe interactions, microbe recognition and development of a basal immune response, and colonization of plant cells by endosymbionts. The putative endocytic routes for the entry of microbe molecules or microbes themselves are explored with a special emphasis on clathrin-mediated endocytosis. Finally, we evaluate recent findings that suggest a link between the compartmentalization of plant plasma membrane into microdomains and endocytosis.