Metabolomic wind in the willows

The many and various uses of willow – from cricket bats and wicker baskets to aspirin and biofuel - only hint at the full potential of this long-established crop, say chemists at Rothamsted Research in Hertfordshire, UK.

Rothamsted holds the UK’s National Willow Collection, amounting to 1500 different willows (from 450 different species). Many were grown for uses that no longer exist: basket weaving and hurdle making are less in demand these days. But while their external structure holds less appeal, their internal chemistry contains untold riches.

‘What’s fantastic for a chemist is that no two species are the same,’ said Jane Ward, who leads the metabolomics team at Rothamsted and has more than 25 years’ experience of plant metabolite analysis. An organism’s metabolome comprises all its metabolites – the small molecules involved in metabolism (such as vitamins, amino acids and ethanol). The untapped potential of the willow metabolome is proving a happy hunting ground for chemists. ‘There’s a massive diversity of different phenolic compounds,’ said Ward.

Ward’s team is working with geneticists at the agricultural research centre to understand the biosynthetic pathways that produce willow’s array of phenolic compounds, including salicin, the analgesic ingredient in aspirin.

Hippocrates first noted the medicinal benefits of willow in 400BCE. But nearly 2500 years later, salicin biosynthesis remains a mystery. ‘It’s amazing that nobody knows how it’s made in the plant!’ said Ward.

Working out the biosynthetic pathway is exciting, she says, but there are many more compounds and pathways to uncover. Ward estimates that her team has discovered 100 novel molecules produced by willow. These previously unknown molecules fall into two categories: medicines or materials. ‘We are looking at molecules with anticancer activity, and neuroprotective agents that might be useful in Parkinson’s disease,’ said Ward. On the materials side, they are investigating precursors for industrial processes, and phenolic mixes, such as those used for phenolic formaldehyde resin.

The researchers are singling out the highest value molecules that could compete with established petrochemicals and their derivatives. ‘There is a play-off between volume and value,’ said Mike Beale, director of the National Centre for Plant and Microbial Metabolomics, a joint venture between the Biotechnology and Biological Research Council (BBSRC) and Rothamsted Research. Willows produce phenolic resins that can be extracted relatively inexpensively using green solvents, but these cannot compete, volume-wise, with their petrochemical equivalents.

There is greater value in molecules with medical potential, says Beale. Rothamsted researchers have discovered a molecule with possible anticancer activity ‘that has a very unusual structure,’ he said. The structure was in press at the time of writing.

Alongside the impressive array of potentially useful phenolic glycosides that willow produces, the plant is also an important biofuel crop across Europe and the US. When the most useful compounds have been extracted, the material can still be burnt as fuel – making willow a lucrative dual-purpose crop. Even better, it grows fast and is happy in waterlogged ground where food crops cannot grow.