Research

       Plant natural products, also known as secondary metabolites, are small molecules plants have evolved to fight against environmental stresses or attract pollinators. Many of the plant natural products are also potent medicines for human diseases. Medicinal plants were the dominant form of pharmaceuticals throughout human history before the era of synthetic drugs in early 20^th century. Today, about 10% of clinically used drugs are plant derivatives in the developed countries, and 80% of the global population in the rest of the world still rely primarily on ethnobotanical remedies. While the efficiency of new drug development based on combinatorial chemistry in pharmaceutical industry has been decreasing in the last two decades, we reasoned that the complex and diverse chemical structures of plant natural products will again become an essential source of new drug entities in the modern society.

          A major obstacle for more plant natural products to enter the realm of modern medicine is the lack of efficient production methods. Many of plant-derived compounds are present at extremely low concentrations in the host plants. Total synthesis by organic chemistry is extremely difficult as many of these compounds have multiple stereocenters. To solve the production problem, our lab utilizes microbes to produce plant-based compounds through metabolic engineering and synthetic biology. Microbes provides unmatchable advantages for the production of plant-based medicines because of the short life cycle, the ease of genetic manipulation, and the convenience of industrial scale-up. As a pre-request for this strategy, genes responsible for the biosynthesis of plant-derived medicines must be discovered. However, most of the biosynthetic pathways leading to plant natural product production have not been discovered. With the technological breakthroughs in next- and third generation sequencing and mass spectrometry based metabolomic profiling, the speed of gene discovery in medicinal plants has been greatly accelerated in recent years. Research in our lab focuses on the following broad areas:   

 

1. Gene discovery in medicinal plants

          A current focus is to elucidate the complete biosynthetic pathway of cardenolides. Cardenolides extracted from Digitalis plants have been used for the treatment of heart failure and atrial fibrillation since the 18^th century. Digoxin, a compound in the cardenolide family, is on the World Health Organization’s List of Essential Medicines. The current method for digoxin production relies exclusively on plant extraction, leading to an unavoidable mixture of derivatives that complicates application. However, the genetic components of cardenolide biosynthetic pathway is largely unknown, hindering the efforts to microbially producing this class of compounds. As such, we are applying a comprehensive approach that combines differential transcriptomic analysis with comparative high-resolution metabolomics to survey transcriptomic patterns for the identification of the unknown genes involved in cardenolide biosynthesis.

2. Microbial engineering for producing plant-derived drugs

          Genetically manipulatable microbes such as Escherichia coli and Saccharomyces cerevisiae are excellent hosts for efficient production of plant natural products because of their fast growth rate and potential for high productivity of chemicals. This production strategy has no requirement for climate, soil, pesticides, fertilizers or geographic locations, compared with the farming method to produce plant-based medicines. We are currently focusing on producing terpene-derived drugs in both E. coli and S. cerevisiae by first establishing universal terpenoid-producing microbial platforms using synthetic biology and metabolic engineering. Genes responsible for complex terpenoid production, such as cardenolides in Aim 1, will be introduced into the platform strains to achieve high productivity of plant natural products.

3. Cellular trafficking of plant natural product biosynthetic pathways

          Plants synthesize natural products through pathways spanning multiple organelles such as chloroplast, mitochondrion, vacuole, and endoplasmic reticulum. Such complex organization implies extensive intracellular transport of pathway intermediates, but little is known regarding the enzymes and the mechanisms involved in the subcellular trafficking of plant natural products. Membrane contact sites between chloroplast and endoplasmic reticulum (PLAMs: PLastid Associated Membranes) have been proposed to be the route for transferring hydrophobic intermediates for terpenoids and membrane lipid biosynthesis. We are studying the function of PLAMs through genetic screening and biochemical assays using the model plant Arabidopsis thaliana to identify proteins essential for maintaining the ER-chloroplast contact sites. The findings from this research will reveal the universal mechanism of metabolite trafficking at a subcellular level.