What do contaminating fungi look like in a tissue culture?
In an orchid tissue culture study, fungal contaminants grew predominantly as colonies that were white and grey in color and had a rough surface shape [1]. The researchers identified the contaminants mostly as belonging to two broad groups of fungi, Deuteromycetes and Zygomycetes, based on their outward appearance [1]. The amount of contamination varied by the type of plant tissue used: flower stalk explants had the highest contamination rate at 81%, while leaf explants produced the most colonies, at 28 [1]. These observations come from a single orchid species, so the specific colors and shapes may differ in other plants.
In date palm tissue culture, a different set of fungi was found: Alternaria alternata, Chaetomium globosum, and Nigrospora osmanthi, with A. alternata being the most prevalent [5]. This shows that the fungi that show up in tissue culture are not a fixed set; they depend on the plant and the culture conditions.
Why does fungal growth vary from one culture to another?
The pH of the tissue culture medium can change how fungi grow. In date palm tissue cultures, the radial growth and dry weight of the contaminating fungi differed significantly across different pH levels, even though the shoot growth of the date palm itself was not affected [5]. This means the fungi are responding to the chemical environment, not just to the presence of plant tissue.
Other organisms in the culture can also change fungal growth. When live cells of the bacterium Klebsiella oxytoca were added to date palm tissue culture medium, they inhibited fungal growth by about 92.6% for several contaminating fungi, including Aspergillus flavus, Penicillium chrysogenum, and Meyerozyma guilliermondii [2]. A cell-free filtrate from the bacteria also inhibited fungal growth, though the effect varied with the age of the filtrate and the fungus tested [2]. Separately, plant essential oils have been reported to inhibit both mycelial growth and fungal contamination in date palm tissue culture [4].
Can fungal growth in tissue culture be stopped?
Yes, in several ways. In jojoba tissue cultures infected with Aspergillus flavus and Aspergillus niger, fungal growth appeared on the medium and on the plant by day 5 and day 10 in control experiments, but no fungal growth was observed even after day 10 when the plants were treated with a chemical solution designed to create fungal "ghost cells"—cells emptied of their contents [3]. The same study proposed that this approach could be developed into a spray for controlling fungal infections in tissue culture or in the field [3].
Adding K. oxytoca filtrate to the tissue medium also improved plant growth traits: at a 10% concentration, the average number of branches, fresh weight, and dry weight reached 20.67, 6.25, and 1.05, respectively, and the contamination rate was 0% compared to 100% in the control [2]. These results are from specific fungi and specific plant species, so the exact percentages should not be assumed to apply universally.
Sources used in this answer
Characteristics of fungi contaminant in tissue culture of Bromheadia finlaysoniana (Lind.) Miq
Fungal contaminants in orchid tissue culture grew mostly as white and grey colonies with rough surfaces, belonged mostly to Deuteromycetes and Zygomycetes, and contamination was highest (81%) in flower stalk explants while leaf explants had the most colonies (28).
Effect of Klebsiella oxytoca Bacteria on the Growth of Fungi Contaminating Date Palm Tissue Culture Media (Phoenix dactylifera L.)
Live Klebsiella oxytoca cells inhibited several date palm tissue culture fungi by about 92.6%, and a 10% sixth-day filtrate reduced contamination to 0% while improving branch number, fresh weight, and dry weight.
The Development of a Phytopathogenic Fungi Control Trial: Aspergillus flavus and Aspergillus niger Infection in Jojoba Tissue Culture as a Model
In jojoba tissue culture, fungal growth appeared by day 5 and day 10 in controls but was absent after day 10 when plants were treated with a fungal ghost cell chemical solution.
… the efficiency of plants essential oils against common fungal contamination affecting tissue culture of date palms (Phoenix dactylifera L.) by in vitro culture
Plant essential oils inhibited mycelial growth and fungal contamination in date palm tissue culture.
The study of pH effects on contaminated fungal growth dynamics of date palm tissue culture
In date palm tissue culture, radial growth and dry weight of Alternaria alternata, Chaetomium globosum, and Nigrospora osmanthi differed significantly across pH levels, while date palm shoot growth was unaffected.