A scientific approach to recreate metabolic evolution in plants
Plants have evolved to produce a variety of compounds that vary from one species to another. These compounds play a key role in plant survival strategies. Bitter and toxic compounds protect plants against insects and animals. Meanwhile, compounds with a good odour or coloured pigments attract pollen-carrying insects.
Among the compounds produced by plants, some substances have medicinal uses because of their physiological effects on the human body. Researchers have reported that matrine can be used as a central nervous system depressant, such as morphine, and that huperizine A can also be used as a supplement in the treatment of Alzheimer's disease. These compounds are called alkaloids, which are also metabolites produced from lysine, an amino acid, in some plant species.
How has the lysine-derived alkaloid system evolved in plants? In 2016, the Chiba University research team discovered that plants producing alkaloids derived from lysine have particular enzymes that possess bifunctional decarboxylating activity towards ornithine and lysine (lysine/ornithine decarboxylase). These enzymes were developed by a micro-mutation of the old enzyme (ornithine decarboxylase), essential for the production of polyamine used in fundamental and biological activities, such as cell division, in various organisms. These enzymes can decarboxylate and activate the amino acid lysine and then send it to the production of alkaloids in plant cells. Micromutation is crucial for the alkaloid-producing metabolism and is common for plants using lysine as a material for alkaloid production.
Since this discovery in 2016, the research team expects the micro-mutated enzyme to be a key factor in the evolution of plants to produce a certain type of alkaloid, since this mutation can only be observed in plant species producing a lysine alkaloid.
Finally, research teams from Chiba University, RIKEN and Kazusa DNA Research Institute confirmed in an article published in the Plant Journal on July 31, 2019 that alkaloids were generated by the insertion of the lysine/ornithine decarboxylating enzyme gene of Lupinus angustifolius producing lysine alkaloid derivative into Arabidopsis thaliana (A. thaliana). The team also identified the type of metabolites that were newly generated in A. thaliana, which is a model plant initially without the function of producing an alkaloid.
It is interesting to note that the research team recreated the metabolic evolution of the alkaloid-producing plant in a model plant. A. thaliana has been transformed to produce alkaloids by inserting the enzyme gene to modify metabolic flow. Expression of the gene introduced into A. thaliana resulted in the production of cadaverine (1,5-diaminopentane) from lysine, which is then metabolized by endogenous enzymes. These processes have resulted in the production of new alkaloid-like metabolites: 5-aminopentanal, 5-aminopentanoic acid and δ-valerolactam by A. thaliana.
The fact that the research team was able to identify newly generated substances by detecting changes in metabolites before and after gene introduction is a noteworthy methodological development, which is a difficult task given that plants usually contain several thousand metabolites.
"We were able to modify the metabolic flow by manipulating one of the plant's genes, and also identify the types of alkaloids generated in the cells, which is a big step for the next discovery," said Mami Yamazaki, associate professor at Chiba University, who led the experiment. "Our research has paved the way for the production of new compounds, which do not yet exist in nature, by carrying out similar genetic engineering on plants with different metabolic potentials. Such an expansion of chemical diversity is an important theme for seed development in the pharmaceutical industry."
The research team hopes that the elucidation of the metabolic mechanism in plants using the new method will stabilize the supply of medical ingredients in the coming years.
