Agar can be replaced in plant tissue culture media, and it usually is replaced at roughly a quarter of the weight: 6–8 g/L of agar becomes 1.5–3 g/L of low acyl gellan (the material sold as Gelrite or Phytagel). That is the easy half of the change. The hard half is that gellan is calcium-sensitive and agar is not, which means your phosphate, your tap water and your autoclave cycle all suddenly matter. Most labs that try this and fail do not fail at the gelling step — they fail because the medium gelled harder than expected and the explants sat on a surface they could not penetrate, or because nobody told them why.
Why labs actually switch
Four reasons come up in practice, and only the first is about the medium itself.
- Clarity. A gellan gel is markedly clearer than agar. For anything involving roots — rooting studies, root–pathogen interaction, hairy root culture, mycorrhizal work — being able to see through the medium without excavating the plant changes how much work one plate can do. Agar scatters light; gellan does not.
- Purity. Agar is a seaweed extract, and it carries whatever came with it: residual sulphates, trace metals, and occasional lots with enough inhibitory impurity to slow a sensitive culture for reasons nobody can pin down. Gellan is a fermentation product with a defined specification, and that reproducibility is the actual selling point in a protocol that has to run identically for five years.
- Weight going into the flask. A quarter of the powder does most of the same work. Whether that matters depends entirely on your purchasing, but at 40,000 litres of medium a year the weights are worth putting against your own prices.
- Agar supply behaviour. Agar price and availability move with a red seaweed harvest. Labs that ran short during a bad season tend to qualify a second source afterwards.
What actually changes — and it is not only the dose
The substitution is not a straight swop, because the two materials set by different mechanisms. Agar sets on cooling through a thermal transition and is largely indifferent to ions. Low acyl gellan sets by ionic crosslinking with divalent cations, primarily calcium and magnesium. Every practical difference follows from that one fact.
| Property | Agar | Low acyl gellan | What it means for your protocol |
|---|---|---|---|
| Typical working dose | 6–8 g/L | 1.5–3 g/L | Roughly a quarter of the powder weight |
| Setting mechanism | Thermal, on cooling | Ionic, needs divalent cations | The medium composition now controls gelling |
| Sensitivity to Ca²⁺ / Mg²⁺ | Low | High | Tap water and hard water give inconsistent gels |
| Gel clarity | Translucent to opaque | Clear | Root and contamination observation is easier |
| Hysteresis (set / melt gap) | Sets ~40 °C, melts ~85 °C | Sets ~35–45 °C, but thermostable once set | Autoclave-stable either way; do not reheat expecting agar behaviour |
| Effective gel strength window | Broad and forgiving | Narrow | Overdosing gives a surface explants cannot push through |
| Syneresis | Moderate | Low | Less free water on the surface |
The second row is the one that ruins trials. A standard Murashige and Skoog (MS) basal medium already contains calcium and magnesium at concentrations that are enough to crosslink gellan. If you dissolve the medium in deionised water and the protocol works, then switch to a tap water rinse on the glassware or a different RO line, you can move the gel strength by a visible amount without changing a single number in the recipe.
The dose arithmetic
Taking a 1,000-litre medium preparation, which is a normal annual volume for a mid-sized commercial micropropagation lab:
| Line | Agar medium | Gellan medium | Difference |
|---|---|---|---|
| Gelling agent dose | 7 g/L (typical) | 2.2 g/L (midpoint) | −69% |
| Powder per 1,000 L | 7.0 kg | 2.2 kg | −4.8 kg |
| Powder per 1,000 L, range | 6–8 kg | 1.5–3 kg | about a quarter |
| Hydration requirement | Boil to dissolve | Boil or heat above 80 °C | No extra step |
| Chelator needed | No | Sometimes, when the water is hard | Two or three extra grams of EDTA per 1,000 L |
At about a quarter of the weight, gellan has to cost several times agar per kilogram before the gelling line stops coming down — which is where the argument usually starts and where it usually ends, because the deciding factors tend to be clarity and reproducibility rather than powder. Put your own two prices against the weights above and you will have the answer in one multiplication.
Four cases where you should not switch
These are the honest limits, and they are genotype-specific enough that you cannot borrow someone else's answer.
- Hyperhydricity-sensitive species. A well-documented minority of species — certain woody perennials, several orchids, some Eucalyptus and Prunus material — vitrify more readily on gellan than on agar. Shoots come out glassy and translucent and fail to acclimatise. If the literature for your species flags it, do not switch without a side-by-side.
- Genotypes that need a soft surface. Gellan at the top of its range forms a firm surface that some explants cannot penetrate, giving poor establishment rather than poor growth. If your protocol runs agar at the low end for exactly this reason, gellan will remove an option you are currently using.
- Protocols validated against hard water you cannot change. If your facility's water supply has high and seasonally variable hardness and you have no RO capacity on the medium line, agar's indifference to ions is a feature worth keeping. Adding a chelator fixes it, but changes the medium.
- A validated commercial protocol with no budget for re-validation. For a contract micropropagation lab shipping certified material, re-establishing performance data across dozens of genotypes is a serious project. The gel change saves powder; it costs validation.
Migration in four steps
- Fix the water first. Move the medium line to deionised or reverse-osmosis water before you change gelling agent, and record hardness. This single step removes most of the variability people blame on the gum.
- Run a gel strength ladder, not a single dose. Prepare the basal medium at three gellan levels across your working range — 1.5, 2.2 and 3.0 g/L — and measure firmness subjectively plus anything quantitative you already use. Do this with the real salts, not with water alone, because the salts are part of the gelling.
- Transfer three genotypes, not one. Include your most responsive line, your most recalcitrant line, and whatever represents the largest share of your throughput. Compare at subculture two, not just at establishment; some effects only appear after passage.
- Hold everything else constant and document the change. Same salts, same pH window, same autoclave cycle, same vessel. If two things change at once you will not know which caused the difference, and for any accredited work you need that record anyway.
Failure modes
| Symptom | What it usually is | First thing to check |
|---|---|---|
| Medium did not set, or set very soft | Insufficient divalent cations, or pH too low | Water source, then pH before autoclaving |
| Set far harder than expected | Medium has more Ca²⁺ / Mg²⁺ than the dose assumed | Salt lot analysis; reduce dose rather than adding chelator first |
| Gel strength varies between batches | Water hardness variation or inconsistent dissolution | Conductivity of the make-up water, every batch |
| Explants sit on the surface, poor establishment | Dose too high for the genotype | Drop to the bottom of the range before anything else |
| Shoots glassy, translucent | Hyperhydricity response to the gellan matrix | Ventilation and gel strength together; consider reverting |
| Precipitate visible after autoclaving | Local cation concentration during heating | Dispersion order — see below |
| Medium sets unevenly down the vessel | Gelled before pouring, or inadequate mixing | Pour temperature and mixing time |
Dispersion order and what to specify
For gellan media the order matters more than for agar. Dissolve the gellan fully in hot water before adding the concentrated salt stock, not after — introducing calcium to a partially hydrated gum gives local crosslinking and visible precipitate that no amount of stirring will remove. If you must add the salts first, add a small amount of chelator and expect to adjust.
When you specify, ask for the low acyl (unsubstituted) form, a stated gel strength with the test method named, and the transmittance figure if the clarity argument is why you are doing this. Gelrite and Phytagel are trade names for low acyl gellan sold into the laboratory channel; the chemistry is the same material described here, and the difference is specification, documentation and price. High acyl gellan will give you a soft elastic gel that is wrong for this application.
Tools
- Dosage calculator — includes tissue culture at 0.15–0.30% and microbiological media at 0.8–1.0%.
- Formula pre-check — flags the hydration, pH and ion combinations that fail first.
Frequently asked
How much gellan replaces agar in tissue culture?
Work by weight into the final volume: about 1.5–3 g/L of low acyl gellan replaces 6–8 g/L of agar, so roughly a quarter. Start at the bottom of the range and move up, because firmness is driven as much by your medium's cation content as by what you weighed out.
Can I keep everything else in my existing protocol?
Usually yes on salts, pH and autoclave cycle, and no on water and vessel handling. The two things that change are the water purity requirement and how carefully the dispersion order is followed. Have a look at the failure modes above before your first pour.
Are Gelrite and Phytagel the same as gellan gum?
Yes — both are low acyl gellan gum sold under laboratory trade names. Food-grade E418 is the same polymer; what differs between grades is the specification, the documentation package and the price, not the chemistry.
Is it heat-stable enough to autoclave?
Yes. Gellan is stable to standard autoclaving and the set gel does not melt back at incubation temperature. Unlike agar it sets largely through ionic crosslinking rather than a purely thermal transition, so once it has set it stays set.
Does the same argument hold for microbiological media?
Partly, and for a different reason. There gellan is used at roughly 8–10 g/L against 12–15 g/L of agar, so the weight saving is much smaller, and it is chosen because some organisms metabolise agar rather than because it is cheaper. See agar substitutes for industrial and laboratory use for that comparison, and check the dose for your organism against the dosage calculator.