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The implemented protoplast isolation protocols for above-ground and root tissues successfully enable efficient digestion of the cell wall and the release of intact single cells from all Arabidopsis thaliana tissues. Across the tested tissue types, which included flowers, stems, leaves, and roots, the enzymatic digestion process produced well-separated protoplasts with minimal numbers of undigested cell groups (Figure 2A-D). Evans Blue dye assays confirmed that the protoplasts that displayed spherical morphology with smooth plasma membrane boundaries are viable cells (Figure 1C). For each tissue type, spherical protoplasts were counted. Flowers contained on average 97,875 ± 7,531 (9,788 cells/mg, n = 3), stems 119,025 ± 22,508 (550 cells/mg, n = 4), leaves 210,483 ± 15,572 (225 cells/mg, n = 3), and roots 225,363 ± 52,561 (599 cells/mg, n = 4).
The protoplast suspensions showed a broad distribution of cell sizes and the presence of green or pale plastids, reflecting the heterogeneous cellular composition of each harvested tissue (Figure 3A-D). To confirm this, a protoplast isolation with a transgenic companion cell-specific YFP reporter line was performed, and the suspension contained viable small protoplasts with a YFP signal representing companion cells (Figure 3E). This underscores that the used protoplast isolation protocol is suitable for difficult-to-isolate vascular cells with rigid cell walls.
With the presented optimized method, one can isolate a high number of viable protoplasts with a minimal amount of debris from various tissues of adult plants. To improve the purification of above-ground samples, we included a Ficoll cushion cleaning step to remove released organelles and other cell debris from the digested samples while preserving the transcriptome quality (Figure 4).
In the scRNAseq cDNA libraries generated from the protoplasts, highly expressed housekeeping transcripts such as ACTIN2 and rare/low expressed transcripts such as the homeodomain transcription factor KNAT1, a marker specific to cambial and meristematic cells, as well as the companion cell markers SUC2 and PARCL1, were detected by specific cDNA PCR assays15 (Figure 5). This indicates a broad cell-type representation and heterogeneity of the obtained protoplast population, which was confirmed further by analyzing the scRNAseq data produced by this method15.

Figure 1: Workflow of protoplast isolation steps. Six main steps are shown: (1) buffer preparation, (2) tissue digestion, (3) filtration, (4) wash and Ficoll purification, (5) counting and quality control, and (6) continuing with downstream experiments. Please click here to view a larger version of this figure.

Figure 2: Images of protoplast preparation steps. (A) Representative picture of the chopping stage of the leaf sample (Step 2.1). The upper image shows the harvested tissue with a few drops of buffer in a standard-sized Petri dish and a razor blade for chopping. The image below, on the left, represents the tissue after a few vertical strokes with the razor blade (see arrow). The image below, on the right, represents the chopped sample, ready for the next step. (B) Image of a Ficoll cushion after centrifugation in a 1.5 mL tube (Step 4.3.7). The white arrow indicates the middle layer, which contains protoplasts. (C) Representative microscope image of leaf protoplasts incubated in Evans Blue dye. Scale bar: 250 µm. Please click here to view a larger version of this figure.

Figure 3: Microscope images of generated protoplasts. Protoplast images from each tissue type are displayed. (A) Flower, (B) Stem, (C) Leaf, (D) Root. Scale bar: 250 µm. (E) Confocal microscope images of a companion cell-specific YFP reporter line. The first image (left) represents the expression of the YFP companion cell reporter in the leaf vasculature (green color). The second image (right) represents protoplasts from cells expressing the YFP reporter (green signal). Purple/blue color represents chloroplast auto-fluorescence. Scale bar 10 µm. Please click here to view a larger version of this figure.

Figure 4: Microscope images of leaf protoplasts before (A) and after (B) Ficoll cushion cleaning. Scale bars: 250 µm. Please click here to view a larger version of this figure.

Figure 5: Quality control PCRs of scRNAseq libraries. Representative gel images of quality control PCRs of root and above-ground tissue single-cell RNAseq libraries. Tested genesSUC2, PARCL, KNAT1, ACT2 (A to D, respectively), and for each tissue: flower, stem, leaf, root from left to right, respectively. cDNA (+) is used as a positive control, and water is used as a negative control. Please click here to view a larger version of this figure.
| Root Protoplasting Buffer | Flower/Stem/Leaf Protoplasting Buffer |
| 600 mM Mannitol | 400 mM Mannitol |
| 2 mM MgCl2 | - |
| 0.1% BSA | 0.1% BSA |
| 2 mM CaCl2 | 10 mM CaCl2 |
| 2 mM MES pH 5.7 | 20 mM MES pH 5.7 |
| 10 mM KCl | 20 mM KCl |
| Adjust pH to 5.5 with 1M Tris Base if necessary | Adjust pH to 5.5 with 1M Tris Base if necessary |
Table 1: Protoplast isolation buffer compositions. Protoplast isolation buffer recipes for root and above-ground (flower, stem, leaf) tissues are provided.