Plants change chemical defences to suit habitat

September 8th, 2008 - 10:08 am ICT by IANS  

Washington, Sep 8 (IANS) Closely related plants produce their own natural chemical cocktails, adapting each set to the individual plant’s specific habitat. Comparing anti-fungals produced by tobacco and henbane, researchers at the Salk Institute for Biological Studies discovered that only a few mutations in a key enzyme are enough to shift the whole output to an entirely new product mixture.

The findings not only gave the Salk scientists a glimpse of the plants’ evolutionary past, but may help them fine-tune the production of natural and environmentally friendly fungicides and pesticides as well as new flavours and fragrances by turning “enzymatic knobs” in the right direction.

“Most people are familiar with the word biodiversity, but ‘chemodiversity’ - the extraordinary tapestry of natural chemicals found in plants - is just as important for life, the appearance of new species and the survival of many different ecosystems on the earth,” said Howard Hughes Medical Institute investigator Joseph P. Noel, who led the study.

For the current study, postdoctoral researcher and co-author Paul O’Maille probed the metabolic pathways that members of the nightshade family, which includes tobacco, tomatoes, potatoes, peppers and henbane, use to produce terpenes-compounds that impart aromatic odours and flavours to foods.

In many cases, they are also modified in the plant to produce so-called phytoalexins, which are natural forms of anti-fungal and antimicrobial compounds found in many different plants.

Henbane (Hyoscyamus muticus) and tobacco (Nicotiana tabacum) each rely on a different phytoalexin to successfully defend themselves against fungi typical for their habitat.

Yet the more than 500 amino acids that make up the chemical factories in each - known as sesquiterpene synthases - are nearly identical to one another, with very minor differences accumulated over approximately several million years of evolutionary change.

Using structural analyses, O’Maille and his colleagues had earlier discovered that changing only nine of the 550 amino acids shifts the production from tobacco-specific phytoalexins to the henbane versions and vice versa.

This time, they were trying to understand the many possible roads that cross the evolutionary divide between tobacco and henbane sesquiterpene synthases.

O’Maille created a gene library that encoded all possible amino acid combinations, 512 in total, and produced and analysed the mutant proteins, paying specific attention to the chemical output and efficiency of each enzyme. “This was the first systematic effort to link DNA sequence variation with chemical complexity,” said O’Maille.

Now Salk researchers are planning to extend their studies to other members of the nightshade family, including tomato, potato, pepper and eggplant, to see how the simplified laboratory system is recapitulated by Mother Nature.

These findings were published in the Sunday online edition of Nature Chemical Biology.

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