Trichoderma harzianum – a naturally occurring antagonist in the soil

Fungal species of the genus Trichoderma are omnipresent in the environment and especially in the soil. As natural antagonists, they play an important role as efficient users of growth-promoting factors such as food and space. An antagonist is understood to be a species that competes with another species. Fungal species of the genus Trichoderma and in particular Trichoderma harzianum thus represent a natural antagonist to many soil organisms and thus also to other fungal species.

Trichoderma harzianum

Trichoderma harzianum is characterized by a very short life cycle and high growth rates of up to 2 cm/day under optimal conditions. A special feature of Trichoderma harzianum are the green-pigmented spores. In the soil Trichoderma harzianum feeds on dead organic matter or other fungi.

Important structures of Trichoderma harzianum

Conidia

The green-pigmented spores of Trichoderma harzianum are formed asexually by the transformation of special hyphae. They serve as spreading organs for asexual reproduction. Source: Empa.

Chlamydospore and conidia

Chlamydospores are thick-walled (blue stained in photo) resting spores that help Trichoderma harzianum to survive adverse environmental conditions. In comparison to the Chlamydospores green-pigmented conidia are much smaller. Source: Empa.

Hyphae

Filamentous cells of Trichoderma harzianum, which are divided into sections by partitions (septa). The mycelium describes the totality of all hyphae of a fungus. Source: Empa.

Phialids and Hyphae

Phialids (singular Phialidus) are bottle-shaped cells on which the conidia of Trichoderma harzianum are formed. Source: Empa.

Trichoderma helps plants to protect themselves against pathogens by inducing a pre-immunisation in the host plant

Trichoderma strains have been recognized long back as biological agents, for the control of plant disease and for their ability to increase root growth and develop­ment, crop productivity, resistance to abiotic stresses, and uptake and use of nutrients. A current field of research investigates the induced resistance in plants triggered by Trichoderma. Various plants (both mono and dicotyledonous spe­cies) when pretreated with Trichoderma showed increased resistance to pathogen attack (Harman et al., 2004).

pre-immunisation by trichoderma

Figure 1: The pre-immunisation triggered in the plant by Trichoderma harzianum leads to the production of messenger substances (such as salicylic acid, abscisic acid and jasmonic acid) in the plant, which triggers the production of antibodies as soon as a pathogen occurs.

Trich­oderma strains capable of establishing such root interaction (as Trichoderma harzianum) also induce metabolic and genomic changes in plants (Mastouri, 2010; Singh et al., 2011). These changes lead to increased resistance of the host plant against a wide range of plant-pathogenic microorganisms and viruses. Two observations were made: (1) Trichoderma reduced infestation at the site where Trichoderma and the pathogen were inoculated, (2) a reduction in infestation was also achieved when Trichoderma was inoculated at different times and sites than that of the pathogen. This second response of the plant is also called induced systemic resistance or pre-immunisation. The pre-immunisation of the plant by Trichoderma produces signaling molecules which lead to the production of compounds associated with stress resistance in the plant as soon as the pathogen appears (Figure 2).

Figure 2: Trichoderma is able to grow within the first two cell layers of the host’s root. Through the production of proteins and enzymes, the production of signaling molecules is stimulated, which are responsible for the transmission of signals in a pathogen attack and trigger defense reactions in the plant. In addition, cell wall fragments in the root of the host plant can be released by Trichoderma, which in turn leads to a defense reaction of the plant (excerpt from Harman et al. 2004).

Trichoderma harzianum – an efficient user of space and food

Trichoderma harzianum is highly competitive for food and space, as it can grow rapidly and uses food sources efficiently. Other terrestrial organisms are thus deprived of the space and growth factors they need for their development. In addition, the high competitive power of Trichoderma harzianum is reflected in the high production of conidia and chlamydospores (survival stage of the fungus).

Growth comparison of Trichoderma harzianum and a wood decaying fungus (time-lapse over 4 weeks)

Trichoderma harzianum – supports nutrient uptake and promotes plant growth

Trichoderma harzianum not only strengthens the resistance of plants to unfavourable environmental conditions, but also increases the availability of nutrients present in the soil and thus supports the plant in its nutrient uptake and growth. At the same time, various studies have shown that Trichoderma harzianum stimulates the formation of new roots (adventitious roots) and thus additionally supports the water and nutrient uptake of the plant. Thus Trichoderma harzianum, if applied in time, can contribute to reduced drought stress.

Formation of Adventitious Roots (I)

The soil of the Chinese fig (Ficus microcarpa) was treated with Trichoderma harzianum. After two weeks already numerous adventitious roots have formed. Source: Ribera Regal, J., Tang, A., Schubert, M., Schwarze, F.W.M.R. (2015). Journal of Tropical Forest Science.

Formation of Adventitious Roots (II)

These adventitious roots of the Banyan fig in Brisbane were treated with Trichoderma harzianum after 12 weeks. Source: Black, F.W.M.R., Jauss, F., Spencer, C., Hallam, C., Schubert, M. (2012). Biological Control 61, 160-168.

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