A subscription to JoVE is required to view this content. Sign in or start your free trial.
This protocol describes in detail how the plant material for immunolocalization of Arabinogalactan proteins and pectins is fixed, embedded in a hydrophilic acrylic resin, sectioned and mounted on glass slides. We show cell wall related epitopes will be detected with specific antibodies.
Plant development involves constant adjustments of the cell wall composition and structure in response to both internal and external stimuli. Cell walls are composed of cellulose and non-cellulosic polysaccharides together with proteins, phenolic compounds and water. 90% of the cell wall is composed of polysaccharides (e.g., pectins) and arabinogalactan proteins (AGPs). The fluorescent immunolocalization of specific glycan epitopes in plant histological sections remains a key tool to uncover remodeling of wall polysaccharide networks, structure and components.
Here, we report an optimized fluorescent immunolocalization procedure to detect glycan epitopes from AGPs and pectins in plant tissues. Paraformaldehyde/glutaraldehyde fixation was used along with LR-White embedding of the plant samples, allowing for a better preservation of the tissue structure and composition. Thin sections of the embedded samples obtained with an ultra-microtome were used for immunolocalization with specific antibodies. This technique offers great resolution, high specificity, and the chance to detect multiple glycan epitopes in the same sample. This technique allows subcellular localization of glycans and detects their level of accumulation in the cell wall. It also permits the determination of spatio-temporal patterns of AGP and pectin distribution during developmental processes. The use of this tool may ultimately guide research directions and link glycans to specific functions in plants. Furthermore, the information obtained can complement biochemical and gene expression studies.
Plant cell walls are complex structures composed of polysaccharides and glycoproteins. Cell walls are extremely dynamic structures whose architecture, organization and composition vary according to cell type, localization, developmental stage, external and internal stimuli1. Arabinogalactan proteins (AGPs) and pectins are important components of the plant cell wall. AGPs are highly glycosylated proteins and pectins are homogalacturonan polysaccharides whose composition, amount and structure vary greatly during different plant developmental stages2,3,4. AGPs and pectin studies have revealed their involvement in several plant processes such as programmed cell death, response to abiotic stresses, sexual plant reproduction, among many others5. Most of these studies started with information obtained from immunolocalization studies.
Given its complexity, the study of cell walls requires many different tools. Detection of glycan epitopes using monoclonal antibodies (mAbs) is a valuable approach to resolve polysaccharide and glycoprotein distribution along this structure. There is a large collection of mAbs available to detect glycan epitopes and the specificity of each mAb is continuously being improved as well6. The technique here described is applicable to all plant species, and is a perfect tool to guide future research directions that might involve more expensive and complex techniques.
In this technique, specific antibodies are chemically conjugated to fluorescent dyes such as FITC (fluorescein isothiocyanate), TRITC (tetramethylrhodamine-5-(and 6)-isothiocyanate) or several Alexa Fluor dyes. Immunofluorescence offers several advantages, allowing a clear and quick subcellular localization of glycans that can be directly observed under a fluorescence microscope. It is highly specific and sensitive, since the preparation of the sample can effectively protect the natural structure of the antigen, even if present in lower amounts. It allows the detection of multiple antigens in the same sample and most important, offers high quality and visually beautiful results. Despite the great power offered by fluorescence immunolocalization studies, they are often regarded as difficult to perform and implement most probably due to the lack of detailed protocols allowing the visualization of the different steps of the procedure. Here, we provide some simple guidelines on how to perform this technique and how to obtain high quality images.
For the protocol presented here, samples must first be fixed and embedded using the most appropriate fixative. Although considered as a time consuming and relatively tedious technique, proper fixation and embedding of the plant sample is the key to ensure a successful immunolocalization assay. For this purpose, the most usual is chemical fixation using crosslinking fixatives, like aldehydes. Cross-linking fixatives establish chemical bonds between molecules of the tissue, stabilizing and hardening the sample. Formaldehyde and glutaraldehyde are cross-linking fixatives, and sometimes a mix of both fixatives is used7. Formaldehyde offers great structural preservation of tissues and for extended periods of time, producing small tissue retractions and being compatible with immunostaining. Glutaraldehyde is a stronger and stable fixative usually used in combination with formaldehyde. The use of glutaraldehyde has some disadvantages that must be taken into account as it introduces some free aldehyde groups into the fixed tissue, which may generate some unspecific labeling. Also the crosslinking between proteins and other molecules occasionally may render some target epitopes inaccessible for the antibodies. To avoid this, the quantity and duration of the fixation must be carefully defined.
After fixation, samples are embedded in the proper resin to harden before obtaining the sections. London Resin (LR-White) acrylic resin is the resin of choice for immunolocalization studies. Unlike other resins, LR-White is hydrophilic, allowing the antibodies to reach their antigens, with no need of any treatment to facilitate it. LR-White has also the advantage of offering low auto-fluorescence, allowing a reduction in background noise during immunofluorescence imaging.
There are many staining techniques available to detect different components of the cell wall, such as Alcian blue staining, toluidine blue staining or Periodic acid–Schiff (PAS) staining. None of these offers the power of immunolocalization analyses8. This approach gives greater specificity in the detection of glycans, offering vaster information regarding cell wall composition and structure.
1. Sample Preparation: fixation, dehydration, and LR-White embedding
2. Slide preparation
NOTE: Glass slides must be clean, free of any dust, grease or any other contaminants. Even new slides must be cleaned as some suppliers use oils and detergents to prevent the slides from sticking together. Any grease or detergent will interfere with the section adhesion to the slide, even if treated with poly-L-lysine. Lint and dust will affect the specimen’s observations and very possibly ruin the experiment. Teflon coated slides with reaction wells are perfect for this task. They are affordable, reusable and drastically reduce the amount of antibody solution needed. With proper cleaning, excellent quality fluorescent immunolocalization can be performed at a very affordable cost.
3. Sample trimming and sectioning
4. Immunolocalization
NOTE: The fluorescent immunolocalization procedure relies on sequential use of two antibodies. The primary antibody is raised against a specific target antigen. The secondary antibody is raised specifically against the primary antibody and for fluorescent techniques is conjugated to a fluorophore (FITC in this specific protocol). The primary antibody will be used to detect the target antigen in the sample, and the secondary antibody will be used to mark the location where the primary antibody connected to the sample after washing off the excess of primary antibody. Controls are an important part of this assay and must always be performed to insure the accuracy of the observations. One well on the slide should be reserved for use as a negative control, where the primary antibody treatment will be skipped, and therefore no signal should be observed at the end of the experiment. A positive control must be included in the experiment by treating one well with an antibody which labelling is already known and certain. The positive control is used to confirm the secondary antibody labelling effectiveness and reaction conditions while the negative control tests the secondary antibody specificity.
In a successful experiment, the secondary antibody will specifically pinpoint the location of the specific epitope in bright green, in a consistent manner, allowing for the characterization of the cell wall composition at a certain development stage of the cell, tissue or organ. For example the LM6 antibody has an high affinity for 1,5-arabinan, a compound with type-I rhamnogalacturonan that can be found abundantly labelling the cell wall of the developing Quercus suber anther (Figure 2A
The fluorescent immunolocalization method in plants here described, while seamlessly straightforward, relies on the success of several small steps. The first of which is sample preparation and fixation. During this first step, a mixture of formaldehyde and glutaraldehyde is used to crosslink the majority of the cell components. The formaldehyde in the solution provides a mild and reversible fixation while the glutaraldehyde provides a strong more permanent linkage; the balance between the two fixatives provides the appro...
The authors declare no conflicts of interest.
The authors received support from the EU project 690946 ‘SexSeed’ (Sexual Plant Reproduction – Seed Formation) funded by H2020-MSCA-RISE-2015 and SeedWheels FCT PTDC/BIA-FBT/27839/2017. AMP received a grant from the European Union’s MSCA-IF-2016 project (no. 753328). MC received a grant from FCT PhD grant SFRH/BD/111781/2015.
Name | Company | Catalog Number | Comments |
25% (w/v) Gluteraldehyde | Agar Scientific | AGR1010 | aq. Solution, methanol free |
8 wells Glass reaction slides | Marinfeld | MARI1216750 | other brands may be used |
Acetic acid | Sigma-Aldrich | A6283 | |
Anti-Rat IgG (wole molecule)-FITC antibody produce in GOAT | Sigma-Aldrich | F6258 | |
cover slips, 24 mm x 50 mm | Marinfeld | MARI0100222 | The cover slip should cover all the wells. Other brands may be used |
ddH2O | na | na | |
Ethanol absolute | na | na | |
Fluorescent brightner 28 | Sigma-Aldrich | F-6259 | |
Gelatin capsules | Agar scientific | AGG29211 | The capsule size sould feat the size of the sample. |
Glass vials | na | na | Any simple unexpensive glass vials that can be sealed, may be used. The vials may be clean with 96% etanol after use to remove LR-White residue and reused. |
LR-white medium grade, embdeding resin | Agar Scientific | AGR1281 | LR-White comes in several forms the medium grade provides na adequate cutting suport for most tissues for harder tissues a harder grade of LR-white may be recomendable. If possible use a resin already mixed with the polimeration activator (benzoyl peroxide), if not please folow the instructions of the suplier to prepare the resin. |
Non fat dry milk | Nestlé | na | any non fat dry milk is adequate |
Oven | na | na | generic laboratory oven |
Petri dish, 10 cm x 10 cm square | na | na | |
PIPES | Sigma Aldrich | P1851 | |
Rat generated Monoclonal Anti-Body | Plant probes | na | Several antibodies that recognize cell wall components are available at both the Complex Carbohydrate research center (CCRC, Georgia University USA) and Plant Probes (Paul Knox Cell Wall Lab, at Leeds University UK). A short list of some commonly used MABS and where they can be purchased is presented in Supplemental Table 1 |
Razor blades | na | na | regular razor blades |
SDS | Sigma-Aldrich | L6026 | |
Toluidine Blue-O | Agar Scientific | AGR1727 | |
Tween 20 | Sigma-Aldrich | P9416 | |
Ultramicrotome | Leica Microsystems | UC7 | |
upright epifluorescence microscope with UV and FITC fluorescence filters | Leica Mycrosistems | DMLb | |
vaccum chamber | na | na | |
vaccum pump | na | na | |
Vectashield | vecta Labs | T-1000 | Other anti-fade may be used. Please do check for compatibility with FITC and the Fluorescente brightner 28. (Note: for a non-commercial alternative, (see Jonhson et al 198218) An antifade medium can be made by mixing 25 mg/mL of 1,4-Diazobicyclo-(2,2,2)octane (DABCO) in 9:1 (v/v) glycerol to 1xPBS. Adjust pH to 8.6 with diluted HCl.) |
Request permission to reuse the text or figures of this JoVE article
Request PermissionThis article has been published
Video Coming Soon
Copyright © 2025 MyJoVE Corporation. All rights reserved