**3. Functions of secondary metabolites**

### **3.1 Secondary metabolites as competitive weapons**

The mechanism of natural defense has been evolved in microorganisms, and they achieve this by secretion of secondary metabolites. The best example could be the antibiotics, which can kill or inhibit the growth of competing microorganisms. Studies confirm that antibiotics are also involved in germination by stimulating spore formation, which can inhibit or stimulate spore formation. Formation of secondary metabolites and spores is regulated by similar factors. Thus the secondary metabolite slows down germination of spores until a less competitive environment and more favorable conditions for growth exist. It protects the dormant or initiated spore from consumption by amoebae and cleans the immediate environment of competing microorganisms during germination [1].

### **3.2 Secondary metabolites as metal transporting agents**

Secondary metabolites act as metal precipitating or chelating agents in plants as the high bioaccumulation of the toxic trace metals can lead to abiotic stress that can cause oxidative damage to plant cells. Metal precipitation is achieved by low-molecular-weight compounds such as phenolics, amino acids, organic acids, and sugars as well as high-molecular-weight compounds such as mucilage and proteins in plants. In the rhizosphere or apoplastic space, the metals are excluded through chelation so as to avoid their entry into symplast. An example could be of the siderophores, which have high affinity for iron (Fe) and could solubilize ferric iron [14].

### **3.3 Secondary metabolites as agents for symbiotic relation with other organisms**

In symbiotic relationship both the organisms are benefited from each other. The symbiotic association between soil fungi and roots is known as mycorrhizae. Mycorrhizal roots can absorb much more phosphate than roots that have no symbiotic relationship with fungi. The fungi in turn protect the plant from damage by pathogens such as nematodes, *Fusarium*, *Pythium*, and *Phytophthera* by often using secondary metabolites such as antibiotics. Another example where the secondary metabolites mediate the symbiotic relationship is bacteria *Pseudomonas*, which act as plant growth-promoting bacteria, by colonizing the roots and producing antibiotics that limit the growth of other pathogenic bacteria as well as fungi [3].

#### **3.4 Secondary metabolites as reproductive agent**

Well-known sex hormones produced by fungi are trisporic acids, which are secondary metabolites of *Mucorales*. The trisporic acid was found in 1964 that caused enhanced carotene production in *Blakeslea trispora*. This was later shown to be the hormone that brought about zygophore production in *Mucor mucedo*. Zygophores (sexual hypae) are produced when vegetative hyphae of the two mating types of these heterothallic organisms approach one another. Trisporic acids are produced from mevalonic acid in a secondary metabolic pathway of which the early steps are present in both (+) and (−) sexes. Since distinct late steps are absent in these sexes, both strains must meet and come in contact in order to complete the pathway that forms trisporic acid [15]. Similarly, a secondary metabolite, sirenin, is also involved in sexual reproduction in *Allomyces*, a phycomycete by working as a chemotaxic hormone that brings together the female and male gametes [16].

#### **3.5 Secondary metabolites as differentiation effectors**

Differentiation occurs during the development of an organism, which can be a morphological change or chemical change. Secondary metabolites released during this time bring about differentiation. Sporulation, which is the process of formation of spores from vegetative phase, is connected with production of antibiotics. This is supported by several evidences such as antibiotic production by all sporulating microorganisms, sporulation and antibiotic synthesis are induced by depletion of some essential nutrient, genetic links between the synthesis of antibiotics and the formation of spores and antibiotics are frequently inhibitory to vegetative growth of their producers at concentrations produced during sporulation [3].

#### **3.6 Secondary metabolites as agents of communication between organisms**

Cell-to-cell communication has been hypothesized to evolve first in the unicellular organisms long before the appearance of specialized (glands, neurons, immune cells, blood cells) cells. In microorganisms small secondary metabolites act as informational cues to regulate gene expression. Homoserine lactones (HLs) are synthesized from S-adenosylmethionine by many Gram-negative bacteria that diffuse and regulate their population density. HLs function in *Psuedomonas aeruginosa*, an opportunistic pathogen responsible for many hospital acquired infection. It uses two HL signaling systems, which combined to regulate over 300 genes. HL signaling could bring about drastic changes in gene expression affecting secondary metabolism, the elaboration of virulence factors, sporulation, and biofilm formation. Similar to this is *Vibrio cholerae* that uses autoinducers such as CAI-1 to terminate host colonization, halting biofilm formation and virulence factor expression. The signaling is also seen as a mechanism of pathogenesis in Gram-positive bacteria such as *Staphylococcus aureus*. During

infection when it enters the human body, it shows complex adaptive behavior that leads to changes in population density, time, and environment-specific. Part of this behavior is controlled by at least four two-component systems, one of which is the *agr* system, which uses a modified octapeptide in signaling [17].
