Structures and Biological Activities of Secondary Metabolites from Trichoderma harzianum

The biocontrol fungus Trichoderma harzianum, from both marine and terrestrial environments, has attracted considerable attention. T. harzianum has a tremendous potential to produce a variety of bioactive secondary metabolites (SMs), which are an important source of new herbicides and antibiotics. This review prioritizes the SMs of T. harzianum from 1988 to June 2022, and their relevant biological activities. Marine-derived SMs, especially terpenoids, polyketides, and macrolides compounds, occupy a significant proportion of natural products from T. harzianum, deserving more of our attention.


Introduction
The unique marine environment with high pressure, high salinity, and low temperature, breeds unique marine microorganisms [1,2]. Secondary metabolites obtained from marine-derived fungi have attracted considerable attention in recent years for potential use in the discovery of unique structures and diverse biological properties [3,4].
The biocontrol fungi Trichoderma spp. (sordariomycetes) are widely spread in the environment [5], such as in the ocean. With the deepening of marine science and technology exploration, more and more Trichoderma sp. strains have been discovered from marine sources. From marine and terrestrial environments, there are no fewer than 250 Trichoderma species discovered so far [6]. Trichoderma species are famous for producing plentiful secondary metabolites [7]. Among them, Trichoderma harzianum probably contributed the most secondary metabolites (SMs) originating from Trichoderma species [8,9]. The SMs from T. harzianum showed antifungal activity [10]. Additionally, cytotoxicity [11] and antimicrobial activity [12], and so on, have also been found in its SMs.
The SMs of T. harzianum have not been summarized in detail or systematically. Up to now, nearly 200 compounds of T. harzianum have been reported. The secondary metabolites of T. harzianum include terpenoids, polyketides, peptides, alkaloids, and lactones. Herein, this review reports the isolated compounds of T. harzianum and their bioactivities. Furthermore, details of the source organisms were analyzed for marine and terrestrial sources. A total number of 180 compounds are presented in this review with 58 cited references. These references cover the time period from 1988 to June 2022.

Alkaloids
Fleephilone (152), a new HIV REV/RRE binding inhibitor, was produced by T. harzianum WC 47695 [27] isolated from sandy soil with plant debris collected in Fort Lauderdale, USA. Compound 152 showed inhibitory activity against REV-protein binding to RRE RNA with an IC50 value of 7.6 μM, and exhibited no protection against HIV infection at concentrations up to 200 μg/mL. Harzianic acid (153) was isolated from T. harzianum SY-307, which exhibited antimicrobial activity against Pasteurella piscicida sp. 6395 [51]. Isoharzianic acid (154), a new stereoisomer of compound 153, was isolated from the T. harzianum strain M10, together with Harzianic acid (HA) [52]. HA was able to promote plant growth and strongly bind iron [52]. An OSMAC approach using multiple culture conditions or co-cultures has been applied to access the chemical diversity of T. harzianum XS-20090075 [20].
All compounds from T. harzianum with their biological activities and habitats were summaried in Table 2. As an analysis, the percentage of marine sources and terrestrial

Lactones
Two lactones, nafuredins C (169) and A (170), were isolated from the mangrovederived fungus T. harzianum D13, and the new compound 169 exhibited antifungal activity against Magnaporthe oryzae, with an MIC value of 8.63 µM [50]. From T. harzianum XS-20090075, four known compounds, xylogibloactones A and B (167, and 168), nafuredin A (170), and dichlorodiaportin (171) [20,56,57] were isolated. Compound 170 exhibited antifouling activity with the EC 50 value of 21.4 µg/mL [20]. 6-Pentyl-2H-pyran-2-one (172) and 2(5H)-furanone (173) were isolated from T. harzianum T-4 [21], while δ-decanolactone (174) was isolated from T. harzianum T-5 [21]. Compound 172, a volatile organic compound from T. harzianum [58], had the ability to inhibit primary root growth and induce lateral root formation. Peniisocoumarin H (175) was isolated from the mangrove-derived fungus T. harzianum D13 [50]. Two new lactones, harzialactones A (176) and B (177), together with a known compound R-mevalonolactone (178), were isolated from T. harzianum OUPS-N115 [32]. T. harzianum OUPS-N115 was separated from the sponge Halichondria okadai, and the cytotoxicity of compounds 176-178 against the P388 cell line was tested. The results showed no significant cytotoxicity [32]. Two lactones harzianolide (179) and T39butenolide (180) were isolated from T. harzianum T39 [30] (Figure 6). sources from the SMs distribution were exhibited, including the specific source ratio (Figure 7). The structure type proportion and the bioactivity distribution of the SMs isolated from T. harzianum were also shown (Figures 8-10).   All compounds from T. harzianum with their biological activities and habitats were summaried in Table 2. As an analysis, the percentage of marine sources and terrestrial sources from the SMs distribution were exhibited, including the specific source ratio (Figure 7). The structure type proportion and the bioactivity distribution of the SMs isolated from T. harzianum were also shown (Figures 8-10).    All compounds from T. harzianum with their biological activities and habitats were summaried in Table 2. As an analysis, the percentage of marine sources and terrestrial sources from the SMs distribution were exhibited, including the specific source ratio (Figure 7). The structure type proportion and the bioactivity distribution of the SMs isolated from T. harzianum were also shown ( Figure 8, Figure 9 and Figure 10).   Mar. Drugs 2022, 20, x 10 of 18 sources from the SMs distribution were exhibited, including the specific source ratio (Figure 7). The structure type proportion and the bioactivity distribution of the SMs isolated from T. harzianum were also shown (Figures 8-10).

Conclusions
This review covers papers on metabolites isolated from T. harzianum. From the SMs' distribution point of view, marine sources account for 45%, while terrestrial sources were 38%. From marine sources, 31 compounds were from sponges-derived T. harzianum strains, 30 compounds were isolated from soft corals-derived T. harzianum strains, 10 compounds were from brown alga-derived T. harzianum strains, 6 compounds were from mangrove samples-derived T. harzianum strains, and 3 compounds were from marine sediment samples. T. harzianum strains and their secondary metabolites were mainly derived from sponges (39%) and soft corals (38%). From the terrestrial sources, 46 compounds were purified from soil samples-derived T. harzianum strains, 13 compounds were from endogenous and 5 compounds were purified from mushroom-derived fungal strains. Compounds derived from terrestrial soil samples account for 67%. For the structure type proportion of the SMs isolated from T. harzianum, the peptides, polyketides, and terpenoids account for 31%, 27%, and 26%, respectively, followed by alkaloids (8%) and lactones (8%). Marine-derived terpenoids and polyketides have 39 and 28 natural products among the 47 and 48 total compounds, respectively. Notably, 91 of the 180 SMs exhibited bioactivities. Antifungal activity was exhibited by 27 natural products, and 17 compounds possessed phytotoxicity activity, while antibacterial and cytotoxicity activity SMs number were all 14. In the research on phytotoxicity and cytotoxic active products, almost all the active natural products were from marine-derived T. harzianum strains. Moreover, 120 of the 180 compounds were new.
In summary, organic compounds are abundant in the SMs of T. harzianum, they may be used as a fungicide, antibacterial, antineoplastic, and weedicide, both in clinical and agricultural applications. The marine sources molecules (marked * in this paper) with their unique molecular and diverse activities, could be the basis for the development of new drug-forming lead compounds.
Author Contributions: Conceptualization, X.P. and R.G.; writing-original draft preparation, R.G.; review and editing, X.P., R.G., G.L. and Z.Z. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.