Four apple cultivars cleared on 0.2% PPM, two did not

The haze that turns up long after initiation

Surface sterilization does not reach an endophyte, and the jar does not tell you so for months.

The paper behind this piece sets the problem out plainly: bacteria living inside plant tissue can survive the establishment protocol, stay invisible through several rounds of subculture, and only announce themselves later — as a haze around the base of shoots that had looked clean — after an extended culture period or some other stress, such as a long gap without transfer.

For a multiplication line that is lost jars and a lost month. For a germplasm collection it is worse.

The study

What the source actually did

The paper is "Effect of Plant Preservative Mixture™ on Endophytic Bacteria Eradication from In Vitro-Grown Apple Shoots", by Natalya V. Romadanova, Arman B. Tolegen and Svetlana V. Kushnarenko of the Institute of Plant Biology and Biotechnology in Almaty, Kazakhstan, with Elena V. Zholdybayeva of the National Center for Biotechnology and Jean Carlos Bettoni of the New Zealand Institute for Plant and Food Research.

First they identified what they were fighting. The contaminant that appeared around the base of the in vitro-grown apple shoots and on the surface of the medium was identified as Bacillus megaterium de Bary, by MALDI-TOF mass spectrometry, with scores from 1.7 to 2.2, which the authors describe as reliable identification.

Then the treatment. In vitro-contaminated shoots were grown on tissue culture medium supplemented with 0.2% v/v PPM™ for 12 weeks, then transferred to medium without PPM™ and cultured for a further 24 weeks. That second stage is the methodologically interesting half: an agent that only suppresses bacteria will let them back once it is withdrawn, and six months off treatment is a long enough window to catch a fair amount of that regrowth.

Six cultivars went through it, and the outcome divided. PPM™ was effective in controlling the endogenous contamination in 'Golden Delicious', 'Landsberger Renette', 'Suislepper' and 'Aport krovavo-krasnyi'; in 'KG 7' and 'Gold Rush', all the plants grown in the absence of PPM™ were still bacterially contaminated, even though they had been pre-treated for 12 weeks in PPM™-supplemented medium. The authors suggest those two cultivars warrant further testing of extended incubation in PPM™.

On the shoots themselves, the abstract frames the finding as a species-specific response of apple to PPM™, and the introduction carries the standard caution that any antimicrobial compound may have deleterious effects on the establishment and growth of tissue culture plants of certain species, so its impact on plant growth should be tested before use at a commercial level. The per-cultivar growth data sits in the paper's results. The authors also state that at the time of publication there were no previously published studies on PPM™ use to eradicate endophytes from in vitro-grown apple cultures.

Our results highlight the species-specific response of apple shoots to PPM™. — Romadanova and colleagues, writing in Plants, 2022

Mode of action

Why it works, as far as anyone knows

PPM™ is described in the paper as a broad-spectrum biocide/fungicide for plant tissue culture, comprising two isothiazolones — 5-chloro 2-methyl 3 (2H) isothiazolone and 2-methyl-3 (2H) isothiazolone, under Patent No. 5,750,402 — together with magnesium chloride, magnesium nitrate, potassium sorbate and sodium benzoate. The stated mode of action is inhibition of specific fundamental processes in the Krebs cycle and the electron transport chain. Plant Cell Technology, of Washington, D.C., introduced it to the market in the 1990s.

Two working consequences follow from the paper's account. It is heat-stable, so it can be autoclaved with the culture medium. And because it targets multiple bacterial enzymes rather than a single one, the authors argue that mutations against it are "not impossible but unlikely", set against antibiotic resistance, which they call a well-known single-gene event.

That is the contrast they draw with antibiotics. The paper's reading of that literature is that ampicillin, gentamicin, amoxiclav and similar compounds have been used against endophytic contaminants, but phytotoxicity on explant growth, cultivars that simply do not respond, and the selection of resistant strains are common problems — and that antibiotics often suppress bacteria rather than eliminate them, with contamination reappearing months after the antibiotic is taken out of the medium.

Where the evidence stops

One bacterium, one crop, six genotypes, one concentration, one exposure length, one lab's plant material. Nothing in this study tells you what 0.2% v/v will do to a different crop, or to apple carrying a different organism. And four cultivars showing no bacteria 24 weeks after withdrawal is an observation with a date on it, not a certificate.

Verify this

Identify first

The authors typed the organism to species before they treated anything. Whether the haze at the base of your shoots is a soil-associated Gram-positive Bacillus or something else decides which published results are even relevant to you — and whether a 24-week clean observation window means what you want it to mean.

Before you try it

This is a paper about apple, not a protocol for your bench. A few things worth settling before adapting it:

The rate and the exposure belong to this study. 0.2% v/v for 12 weeks, on apple, at what the paper calls the manufacturer's recommended concentration. PPM™ is a biocide; handle and dose it according to the manufacturer's own label and your lab's chemical handling rules, and do not carry a number across from a paper on a different crop.

Genotype, not just species. Two of six apple cultivars here were still contaminated after the same treatment that worked on the other four. If you trial it, trial it per cultivar.

Leave a long enough window off the biocide. The whole point of the 24 weeks on PPM™-free medium was to separate eradication from suppression. A check a fortnight after withdrawal tells you much less.

Test growth before scale-up. That is the paper's own position: any antimicrobial can affect establishment and growth, and the effect should be tested before commercial use.

Keep the record per cultivar and per batch. What you dosed, when you withdrew it, and what came back. Whether that lives in xPlant, a spreadsheet or a bound notebook, a log tells you what happened in your jars; it does not change what the bacteria do.