Playing off the last topic I wrote about, the screening hypothesis, I thought it would be interesting to focus on one of the methods through which this principle could manifest. To refresh everyone’s memories the screening hypothesis is a theory attempting to explain the chemical diversity observed in plants. Specifically, how do plants maintain elaborate biochemical pathways, at significant costs, that yield very few molecules of any benefit.
In the last blog entry I mentioned a number of strategies that plants can use on a biochemical, and pathway organization level to reduce costs or limit the possibility of losing valuable chemical diversity. One phenomena that wasn’t discussed by Firn and Jones in their initial 1991 manuscript is silent biochemical potential in plants. These are latent biochemical pathways. The enzymatic machinery for the pathway is expressed and functional, however, substrates are not supplied therefore there’s no carbon flow through the pathway, and no end products (Lewisohn, Gijzen, 2009).
Silent biochemical pathways can arise any number of ways; 1) inactivation, or re-purposing a committing enzyme by mutation, and/or 2) altered gene expression profile such that the the timing or location of expression of a key biochemical event no longer coincides with the rest of the pathway.
There are some great examples of silent pathways in plants. The earliest example that I’m aware of involves terpene biosynthesis in mint. Mutagenesis of scotch mint modified its chemistry to resemble the sweet smelling spearmint varieties, revealing that the biochemical potential to synthesize ‘spearmint’ chemicals in scotch mint exists, it’s just the commitment of carbon through those pathways that’s limiting. This work was performed by Rod Croteau in the early 1990s (Croteau, et al. 1991).
As a second example of silent pathways in plants I would like to talk about tomatoes. For hundreds of years farmers have continually selected more ‘shiny’ tomatoes because of their appeal. In performing this selection the farmer was inadvertently modifying the chemical composition of tomato. The result of this selection has manifest in our common grocery store variety which lacks the expression of a key enzyme in flavonol biosynthesis, chalcone isomerase (CHI). There have been high impact publications demonstrating this principle, and reporting genetically engineered CHI overexpressing tomatoes for the purpose of accumulating flavonols (Muir, et al. 2001). There’s interest in this trait because flavonols have significant health promoting properties (Haliwell, et al. 2005). Not suprisingly there is a patent protecting the genetic modification of tomato to overexpress CHI (Syngenta, 2003), and my PhD supervisor is the inventor. More recently, a really nice proof of silent plant metabolism was recently characterized in Arabidopsis (Weng, et al. 2012). This particular work is a stellar example of a concise and to the point scientific argument. It shouldn’t be a surprise that it’s published in Science.
This phenomena of silent plant metabolism is far more common then we realize and appears to play a part in almost every plant. We just need to be interested enough to look for it.