## 4 Biological Functions  
As already stated, a distinctive feature of SMs is that they are dispensable for the fungus under controlled growth conditions, so that mutants unable to produce them are not affected in their viability in the laboratory. However, the persistence of these biosynthetic processes in fungi imply adaptive advantages in their natural environment. In some cases the functions are obvious and in other cases they are less clear. Some illustrative examples of known SM functions are mentioned below. Melanin protects against UV radiation, facilitating the survival of the fungus under strong sun exposure. However, it also exerts protective effects against other sources of stress, such as oxidative or thermal stress, as well as mechanical damage [83]. SMs can fulfill more specific physiological functions. Siderophores are NRPs that act as high-affinity iron chelators, which are used by some fungi to scavene environmental iron or to sequestrate internal reactive iron [84]. Fusarubin and 5-deoxybostrycoidin-based melanin provide dark pigmentation to perithecia in _F. fujikuroi_[85] and _F. granimearum_[86], respectively. Other SMs are used for more than one purpose. \(\beta\)-carotene is useful as a protective agent against oxidative stress [87], but it is also used as a source of derivatives with more specific functions. In mucormycotina fungi, such as _B. trispora_ or _P. blakesleeanus_, \(\beta\)-carotene is cleaved to produce sexual hormones called trisporic acids, which are needed to carry out the sexual cycle [88; 89]. However, in _F. fujikuroi_[30] and _Ustilago maydis_[90] the same caroten is cleaved to produce retinal, the prosthetic group of rhodopsins.  
Many secondary metabolites play roles in the interactions of fungi with other organisms, both in terms of competition and pathogenesis [91]. There are many examples where fungi produce antibiotics to avoid competition. For example, _Beauveria bussiana_ produces the polyketide oosporein to limit bacterial growth in the parasitized insect [92]. It is well known that the interaction between fungi and plants, either mutualistic or pathogenic, involves the simultaneous production of molecular signals from the interacting species [93; 94]. Moreover, some SM gene clusters are expressed in the host plant but not in others. The _F. fujikuroi FUB1_ gene, coding for a PKS for the synthesis of the toxin fusaric acid, is expressed when infecting its host but not when infecting other plants [95]. The participation of SMs in pathogenesis is not easily predictable. In _Pyricularia oryzae_, the causative agent of rice blast disease, melanins are required for pathogenesis, but no role is apparently played by tenuazonic acid, a hybrid NRP/PKS mycotoxin, by metriapyrones, polyketide compounds with antibacterial activity, or by pyriculols, phytotoxic polyketide compounds [96]. The dependence on specific needs in their ecological niches explains why many fungal SM clusters are not expressed under laboratory conditions. However, their functions may be investigated by activating them in a targeted way [97; 98].  
The relation of SMs with pathogenesis has been investigated by analyzing the virulence of mutants with alterations in their syntheses. As an example, mutants of fusaric acid production in _F. oxysporum_ revealed that this SM contributes importantly to virulence in tomato plants and in immunocompromised mice [99]. Another interesting example was provided by a study of the role of gibberellins in _F. fujikuroi_, which are assumed to participate in the infection of rice because of the characteristic over-elongation of the seedlings for the \"bakanae\" disease caused by this fungus. Infection of rice with a non producing _F. fujikuroi_ mutant showed a lack of over-elongation in the infected seedlings and a reduced cell invasion capacity of the hyphae in the plant [100].