The leaves of Salvia dorisiana produce methyl perillate, a compound that can easily be converted into terephthalic acid, a building block for certain plastics. © Laboratory of Plant Physiology.
The fresh scent of oranges and lemons comes from the cyclic terpene limonene, which is used as an aroma and flavouring in numerous products. At present, limonene is mainly extracted from residues from the manufacture of orange juice. But that source is under pressure due to diseases and the use of pesticides in the cultivation of citrus fruit. Monoterpenes can be synthesized from sources such as petroleum, but plants produce them too. Jongedijk recently obtained a doctorate for a study of how plants make terpenes and how you can use that method in the production of bioplastics.
For the proof of principle, Jongedijk used methyl perillate, a natural derivative of limonene. She found that you only need two chemical steps to turn methyl perillate into terephthalic acid, a building block for the polymer used for PET bottles (polyethylene terephthalate). In theory, this means she has solved the trick of making PET from monoterpenes. Jongedijk chose a sage species, Salvia dorisiana, as the source for the methyl perillate. This herb smells of fruit. ‘They call it the fruit-scented sage.’
Jongedijk was able to identify four genes in the sage plant that are involved in producing methyl perillate. She inserted the genes in a model plant of the tobacco family, which indeed went on to produce methyl perillate. Proof, says Jongedijk, that she had got the right genes. These genes may provide a commercial route for making PET from monoterpenes, but that does not mean the PET bottles are ‘growing on the tobacco plants’; extracting methyl perillate from plants is technically challenging. But the genes she found can also be inserted in microorganisms such as yeasts. Jongedijk has already developed a strain of yeast that can make limonene.
The yeast pathway has commercial potential, says Jongedijk, especially if the focus is on specialist materials, such as plastics for LCD screens, rather than cheap, bulk PET plastic as the end product.