Pre-immunisation by activation of messenger substances
A plant can not initially distinguish between a symbiont and a pathogen. At first it responds equally to the infection by strengthening the cell walls and producing so-called phytoalexins (antibodies). Trichoderma harzianum establishes a connection with the plant‘s roots and communicate with the plant through signalling that enables the host to accept it as a beneficial organism.
During root establishment, the beneficial fungus (also known as an endophyte) triggers metabolic and genomic changes in this plant. These changes increase the resistance of the host plant to a wide range of plant damaging microorganisms and insects.
Two observations were made regarding this:
(1) Trichoderma harzianum reduced the infestation where Trichoderma harzianum and the pathogen were simultaneously infected,
(2) There was a reduction in infestation when Trichoderma harzianum was infected at different times as well as in different locations like the pathogen. The second response of the plant is also called induced systemic resistance or pre-immunisation.
The pre-immunisation resulting from Trichoderma harzianum is achieved by intensifying the production of messengers (such as salicylic acid and jasmonic acid), which in turn can trigger the production of phytoalexins. However, the phytoalexins are only produced when when a potentially harmful fungus becomes pathogenic.