Scientific information regarding secondary metabolites

[Adivino]

I'm not sure if this is the right place to write this, but since it contains more or less advanced terminology, I decided to put it here. I'll try to copy interesting information and ideas from a very very good book: Biotechnology, Secondary Metabolites (2nd edition), Ramawat and Merillon. Ed. SP. I will try to add more relevant things in the future.

How do alkaloids naturally occur in plants?

In plants, alkaloids generally exist as salts of organic acids such as acetic, oxalic, citric, malic, lactic, tartaric, tannic acid, etc. Some weak basic alkaloids (nicotine, etc) occur free in nature. A few alkaloids also occur as glycosides of sugar (e.g., glucose, rhamnose and galactose) e.g., alkaloids of solanum group (solanine), as amides (piperine) and as esters (atropine, cocaine) of organic acids. [p. 30]

Biological functions of alkaloids:

The biological functions of alkaloids within the plants are not clearly understood but it's clear that they are not produced in plants for a single function, but for many. The following functions have been observed in different plant species. Alkaloids are considered as:
1. Reserve substances to supply nitrogen, but very little evidence is available about this function.
2. End product of detoxification mechanisms; otherwise their accumulation in plants might cause damage to the plants, e.g., in tobacco, 10% of carbon metabolism is directed to synthesize nicotine biosynthesis. This, it is an energy-expensive process.
3. Poisonous substances to protect the plant itself from insects and animals. Nicotine has insecticidal properties. Sheep avoid grazing lupin plants with high alkaloid content. Some cacti repel fruit fly but Drosophilia pachea is resistant and breeds on cactus.
4. Plant stimulants or regultators, e.g., alkaloids inhibit rye and oat seedling growth. Colchine inhibits cell division.
5. Reservoirs for protein synthesis.
6. Excretory product of plant.
7. Inhibition of enzyme activity by alkaloids is also known. [p. 42]

Time-course study of growth and secondary metabolite production:

The growth curve has three phases: a) lag, when there is almost no growth; cells adjust to the new medium after subculture; b) exponential phase of growth when primary metabolism increases and tissue proliferates rapidly with the consumption of medium nutrients; and c) stationary phase when primary metabolism and cell proliferation come to a halt as nutrients in the medium are exhausted (nutrients become the limiting factors). There may be a decline in dry weight of the cells as cells utilize stored reserve material. If we look at secondary metabolites synthesis during these phases, it is evident that synthesis is almost at its lowest during the lag phase (secondary metabolites accumulation of previous passage get diluted further due to cell proliferation) and the early exponential phase. It is at the end of the exponential phase that increased synthesis of secondary metabolites is evident, as primary metabolites start converting into secondary metabolites, instead of cell-building material. It is during the stationary phase that there is no more cell growth and all primary metabolites are diverted into the synthesis of secondary metabolites. Therefore, the highest amounts of secondary metabolites are accumulated in the stationary phase cultures. It is at this stage that, if cells are not harvested, there may also be a degradation of secondary metabolites with primary metabolites. At this stage both nutrient medium and cells are exhausted of carbohydrates and other nutrients. [p. 62]