# Fungi of the _Trichoderma_ Genus: Future Perspectives of Benefits in Sustainable Agriculture
## 5 Application of _Trichoderma_ Fungi in Promoting Plant Growth
### Colonization and Modification of the Root System of Plants  
_Trichoderma_ sp. shows high resistance to phenols, phytoalexins, terpenoids, aglycones, and flavonoids produced by plants at the time of infection by foreign microorganisms. This ability allows these fungi to quickly colonize and modify the root system of plants [31; 32]. In  
Figure 3: Direct mechanisms of _Trichoderma_ fungi in promoting plant growth (author’s own scheme). turn, the expansion of the root system contributes to increasing its colonization by beneficial soil microorganisms and the easier uptake of minerals by plants from the ground [126, 127, 128]. Colonization of the root by _Trichoderma_ sp. is preceded by the production of chemical signaling substances by the plant [21, 129]. This process leads to the adsorption of these microbes on the surface of underground plant structures and the formation of mycelium. These microorganisms then penetrate the tissues of the plants and then become established in the plant tissues [87]. Fungi of the genus _Trichoderma_ are capable of ectomycorisis, in which flakes fill the intercellular spaces of the cortex of the primary roots [58, 125]. Species of these microorganisms that exhibit mycorrhizal potential include _T. album_, _T. aureusvoiride_, _T. fertile_, _T. glaucum_, _T. hamatum_, _T. harzianum_, _T. koninigii_, _T. lignorum_, _T. longipilis_, _T. nunbergii_, _T. polysporum_, _T. pseudokoningii_, _T. pubescens_, _T. strictiplilis_, _T. strigosum_, _T. verruculosa_, _T. virens_, and _T. viride_ [130]. _Trichoderma_ sp., like other mycorrhizal species, produce cysteine-rich hydrophobins which are responsible for host recognition and fungal adhesion to hydrophobic root fragments [32]. Root penetration is facilitated by cellulose-binding expansins produced by these microorganisms, which disrupt the structures of plant cell walls by creating channels in them, which additionally enable plants to absorb more minerals from the substrate [21, 55, 79, 131]. [PERSON] et al. [132] observed that TgSWO protein extracted from _T. guizhouense_, like expansins, promoted cucumber growth by modifying its root system. The protein loosened and expanded the cell walls of the roots, thus outcompeting pathogenic fungi and bacteria for nutrients, an enhancement in the number of roots, stem diameter, root fresh weight, and shoot fresh weight of cucumber. In turn, [PERSON] et al. [133] proved that the root colonization process by _T. harzianum_ was facilitated by the citric acid, malic acid, and oxalic acid produced by this strain. Increased adhesion of these fungi to the surface of cucumber roots caused a significantly enhanced shoot height, and shoot and root dry weights by 63.7%, 55.1%, and 36.1%, respectively, compared to the control. [PERSON] et al. [134] documented that _T. harzianum_ also produced a cysteine-rich protein which, by inducing root hair growth, increased the available absorption area in the root system of cucumber. This change facilitated a greater uptake and translocation of carbon, lithium, manganese, and potassium in the plant, thus leading to an enhancement in crop biomass. According to [PERSON] et al. [34], and [PERSON] and [PERSON] [135], the main phytohormes responsible for modifying the root system of plants are auxins, particularly indole-3-acetic acid (IAA). A low level of IAA in the plant induces primary growth and lengthening of existing roots, while disturbed transport leading to a higher concentration of auxins stimulates the formation of new native and lateral roots [32]. According to [PERSON] and [PERSON] [136], one of the mechanisms of _Trichoderma_ responsible for promoting plant growth is the ability of the fungi to solubilize inaccessible forms of phosphorus and the production of phytohormes and phytoregulators by these microorganisms. Therefore, the researchers analyzed the ability of 10 _Trichoderma_ strains to synthesize siderophores and auxins. The results of the studies showed that the species _T. asperellum_, _T. atrovide_, _T. harzianum_, _T. koningii_, _T. longiphrachiatum_, _T.