Dear Colleagues,

Secondary metabolites are organic compounds that are not directly involved in the growth, development, or reproduction of an organism. Unlike primary metabolites, which are essential for basic life processes, secondary metabolites often confer adaptive roles that can help an organism to survive in its environment.

Secondary metabolites are incredibly diverse in structure and function, representing a wide variety of compounds that can be grouped into different classes based on their biosynthetic origins and chemical structures.

Many secondary metabolites play roles in defense against herbivores, pathogens, or competitors. Some act as attractants for pollinators or seed-dispersing animals. Others provide protection from UV radiation or assist with allelopathy (where a plant releases chemicals to inhibit the growth of nearby competing plants).

Classes of secondary metabolites, articles that are interesting for publication in this issue:

Alkaloids: Nitrogen-containing compounds that often have potent effects on animals. Examples include caffeine, nicotine, morphine, and quinine. Many are used as drugs or have pharmacological effects.

Terpenoids (or isoprenoids lipids): They comprise the largest class of secondary metabolites. Examples include steroids, carotenoids, and many essential oils.

Phenolic compounds: These include flavonoids, tannins, and lignins. They can have antioxidant properties and play a role in plant defense.

Polyketides: This diverse group includes many antibiotics, anticancer agents, and other bioactive compounds.

Pharmaceutical relevance: Due to their bioactivity, many secondary metabolites have been utilized in human medicine. For instance, the alkaloid compound salicylic acid, derived from willow bark, led to the development of aspirin. Many antibiotics are secondary metabolites produced by bacteria and fungi.

Ecological importance: Secondary metabolites play crucial roles in plant–plant, plant–animal, and plant–microbe interactions. They shape the dynamics of ecosystems, influencing feeding behaviors, mutualistic relationships, and competitive interactions.

Biotechnological applications: With advances in synthetic biology and metabolic engineering, there's growing interest in harnessing microbes like bacteria and yeast to produce valuable secondary metabolites, either by transferring the metabolic pathways from native producers or designing novel pathways.

In summary, while secondary metabolites are not required for the basic metabolic processes of an organism, they play significant roles in its interaction with the environment, defense, and survival and have vast importance for humans in terms of medicine, agriculture, and biotechnology.

Which symbionts produce secondary metabolites?

Many symbiotic organisms produce secondary metabolites that play roles in the mutualistic relationships that they establish. These compounds can have a wide range of functions, ranging from defense to communication. Some symbionts that produce secondary metabolites include:

Endophytic fungi: These are fungi that live inside plant tissues without causing any apparent harm. Many endophytes produce secondary metabolites that can protect the host plant from herbivores, pathogens, or environmental stresses. For example, some grass species harbor endophytic fungi that produce alkaloids toxic to grazing animals, providing the grass with a form of chemical defense.

Mycorrhizal fungi: These fungi form symbiotic associations with the roots of most plants, helping them to take up nutrients from the soil. In return, the plant provides the fungus with carbohydrates. Some of these fungi also produce secondary metabolites that can deter herbivores or protect against pathogens.

Rhizobia: These are nitrogen-fixing bacteria that form nodules on the roots of leguminous plants. While the primary role of rhizobia is to convert atmospheric nitrogen into a form usable by plants, they also produce secondary metabolites that might be involved in communication and establishing the symbiotic relationship.

Marine invertebrates and their microbial symbionts: Many marine organisms, such as sponges, corals, and ascidians, harbor microbial symbionts that produce a plethora of secondary metabolites. Some of these compounds deter predators, inhibit the growth of competing organisms, or protect against pathogens. Often, the exact producer (host or symbiont) of a particular secondary metabolite in these associations can be challenging to determine, but there's evidence that many of these compounds are synthesized by the microbial partners.

Leaf-cutter ants and their fungal cultivars: Leaf-cutter ants cultivate a particular type of fungus as their primary food source. In return, the fungus receives a steady supply of leaf material. Some secondary metabolites produced by the fungus protect the cultivar from parasitic fungi and other potential threats.

Lichens: Lichens are mutualistic associations between fungi and photosynthetic partners (algae or cyanobacteria). Many lichens produce secondary metabolites, visible as unique pigments or detected chemically, that might play roles in protection from UV radiation, herbivory, or microbial invasion.

These are just a few examples. The production of secondary metabolites in symbiotic relationships is widespread and represents a rich field of study, especially given the clear ecological and potential pharmacological importance of these compounds.

Prof. Dr. Valery M. Dembitsky
Guest Editor

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• microbial symbionts
• plants
• lichens, fungi
• seaweeds
• microalgae
• marine invertebrates
• secondary metabolites
• human health
• fungal endophytes
• bacteria