Plant cells have additional cell walls in addition to membranes. Are CytoTracer® dyes applicable to plant cell staining?
Yes, CytoTracer® dyes can be used in plant research, particularly to study intracellular transport, plant immune responses, and the state of vacuoles without prior disruption of cell walls. Here are some examples of this kind of research [1, 2, 3, 4].
CytoTracer® dyes are membrane-permeant molecules that enter cells by passive diffusion. The presence of a plant cell wall does not automatically prevent their uptake, because the cell wall is porous to small molecules (typically allowing diffusion of compounds up to several nanometers in size). However, successful labeling depends on several factors:
- Cell type and tissue structure. CytoTracer® dyes work best with isolated cells such as protoplasts, suspension-cultured cells, pollen tubes, root hairs, and thin tissues. Uptake can be slower or heterogeneous in highly lignified or heavily cutinized tissues.
- Efflux and compartmentalization. Plant cells possess numerous transporters and large vacuoles that can sequester or export fluorescent compounds, reducing labeling intensity or retention.
- Esterase activity. Since some CytoTracer® dyes rely on intracellular esterases for activation, staining efficiency can vary between different plant species, tissues, and developmental stages.
- Plant cell autofluorescence. Lignin, chlorophyll, phenolic compounds, and other cell components can create significant background interfering with CytoTracer® dyes fluorescence.
The table below compares the major sources of intrinsic plant autofluorescence with the spectral properties of the four CytoTracer® dyes.
| Plant component | Major fluorophores | λEx (nm) | λEm (nm) | Overlap with CMAC (345/465) | Overlap with CMFDA (492/517) | Overlap with CM-BDP (514/542) | Overlap with CMTMR (540/565) |
| Chlorophyll a (chloroplasts) | Chlorophyll a | 430, 662 | 660–700 | None | Minimal | Minimal | Low–moderate (tail of broad autofluorescence, but not chlorophyll peak itself) |
| Chlorophyll b (chloroplasts) | Chlorophyll b | 453, 642 | 650–660 | None | Minimal | Minimal | Moderate |
| Lignin (secondary cell walls, xylem) | Phenylpropanoid dehydrodimers | 320–405 | 400–550 | High | Moderate | Moderate | Low |
| Hydroxycinnamic acids (ferulic, p-coumaric acids) | Cell wall phenolics | 310–380 | 400–500 | High | Low–moderate | Low | Minimal |
| Flavins (FAD, FMN, riboflavin) | Flavoproteins | 340–470 | 515–540 | Low | High | High | Low |
| Suberin/Cutin | Lipid-associated phenolics | 330–380 | 400–500 | High | Low–moderate | Low–moderate | Minimal |
| Tannins and other phenolics | Vacuolar phenolics | 320–420 | 400–650 | High | Moderate | Moderate | Moderate |
| Carotenoids | Carotenes, xanthophylls | 400–520 | Very weak (500–600) | Low | Low | Low–moderate | Low–moderate |
| NAD(P)H | Reduced pyridine nucleotides | 340–360 | 440–470 | High | None | None | None |
Note that spectral overlap alone does not determine whether a dye is useful. In many plant systems, the autofluorescence intensity is low, and the intracellular signal from CytoTracer® dyes is sufficiently bright to provide good contrast. For example, lignin and phenolic compounds have broad excitation spectra, but their excitation efficiency at the wavelengths used for CMAC imaging is often much lower than their maximum. In many tissues, the wall signal is present but not overwhelming.
Moreover, cell wall autofluorescence forms a thin peripheral signal, whereas CytoTracer® dyes generate a bright intracellular fluorescence. The spatial separation often makes the dye easy to distinguish from the wall background.
To conclude, CytoTracer® dyes can penetrate plant cell walls and label living plant cells, but staining efficiency and signal-to-background ratio depend strongly on tissue type and intrinsic plant autofluorescence. Optimization may be required for each plant species and tissue.