Flowers emit a sweet perfume that attracts not only pollinating insects, but human beings, who use them to create perfumes widely used in cosmetics. But how do these flowers emit them into the air? François Lefèvre and Baptiste Pierman, researchers at the UCL Life Sciences Institute, are part of an international team that has discovered the mechanism for this. They have published a study in the prestigious journal Science, where it made the cover!
What is more normal than a flower that smells good? From our point of view, this seems very natural, but underlying this emission of odorous compounds there exists an entire mechanism that was unknown up to now. “Flowers and plants produce odorous compounds from the petals. It was believed up to now that these molecules were released passively and simply diffused into the air. In this study, in which research teams from not only UCL, but also Purdue University (USA) and the University of Amsterdam (the Netherlands) participated, we demonstrated that an active transporter, namely a membrane pump, was needed to convey them into the air,” confirms François Lefèvre, who specifies that an ABC transporter called PhABCG1 is involved.
Essential odorous compounds
The volatile molecules expelled into the air by plants don’t just serve to attract pollinators: “When they are attacked, or if they are suffering from drought, for example, plants and flowers can emit these volatile molecules into the air in order to communicate this threat to the other plants surrounding them, to ‘warn’ them to develop mechanisms to resist it. All these roles are essential for the survival of the plant. So it was useful to understand how all this takes place,” continues the researcher. In short, behind the beauty of the plant or the flower are concealed mechanisms that are much more complex than we think!
These odorous molecules, which are volatile organic compounds (VOCs), are of several types, with quite different characteristics, and are well known to the cosmetics industry: terpenes, fatty acid derivatives, indoles and benzenoids. “We have studied the latter in particular, as they are abundant in the petunias that served as a model in our study,” continues Baptiste Pierman, who also participated in this study. “Thus, we see that these molecules are produced continuously in some plants, and in other plants at certain times of the day. For example, the rose emits its perfumes during the day, while the petunia emits them in the evening and at night, because the pollinating insects that visit it are nocturnal.”
The role of transporters
This periodicity in the production of odorous molecules is regulated by a transcription factor called ODORANT1. But it is also involved in the regulation of PhABCG1… “This is where the link between the transporter and the production of odorous compounds is found,” continues François Lefèvre. “We noted that this transporter was present in the petals at the time the compounds were produced. This concurrence intrigued us. So we sought to find out whether the presence of this transporter was a condition for emission of VOCs into the air or whether it was a matter of two more indirectly related phenomena.”
Membrane pumps are especially familiar to the team from the UCL Life Sciences Institute, which specialises in this field. They are found in all living things, and are very often involved in resistance to stresses. For example, plants produce them in large quantities to build up protection from external pathogens or insects, or to eliminate heavy metals.
To come back to our petunias, the researchers wanted to establish the relation between the production of PhABCG1 transporters and the emission of odorous molecules. “By mathematical modelling, our co-workers showed that if no transporter carries them outside, the plant must produce these VOCs in very large quantities so that as much is found in the air. But it seems that there exists a threshold beyond which their presence can prove to be toxic to the cell. This argues in favour of a more complex mechanism,” explains Baptiste Pierman.
Three major proofs
The PhABCG1 transporter was therefore suspected of being involved in the transport of odorous molecules outside the plant. “We established its role using three tests. First of all, we showed that when plants incapable of producing this transporter are obtained by genetic engineering, more of these molecules are found in the petals and less in the air that surrounds the flower. Then, in these flowers free from these transporters, we demonstrated that the cells suffered more from toxicity; they did poorly because they contained too much of these odorous molecules in their cells. Finally, and our team was especially involved in this area, we proved that PhABCG1 is directly involved in this transport mechanism of odorous molecules, and is not just a constituent in a cascade. We have in fact produced this transporter in cultured plant cells (BY2). We then conducted transport tests with several molecules. Two types of benzenoids (methyl benzoate and benzyl alcohol) are in fact conveyed by this transporter. This was not the case with the terpenes, for example, which demonstrates a certain specificity of the transporter,” says François Lefèvre.
Prospects
This purely fundamental research can nevertheless serve as a basis for study of the mechanisms of emission of volatile compounds by other living things such as microbes, insects or even our own body. “Our research can be useful in climatology too; these molecules, when they are in the air, serve as condensation nuclei necessary for the formation of clouds. This discovery will thus allow more precise meteorological models to be developed. Finally, this research opens up a number of prospects in the area of metabolic engineering by revealing new targets for improving the production and secretion of these compounds heavily used in perfumery and cosmetics, but also as natural aromas in the food industry. As for us, we are now focusing on other ABC transporters, as this family includes at least 120 variants in plants, the roles of which must still be determined for the most part…,” the researchers conclude.
Carine Maillard
A glance at François Lefèvre's bio
2011 : Bachelor in Bioengineering at UCL
2012 : Erasmus Programme at Wageningen UR, the Netherlands
2013 : Master in Bioengineering: Chemistry and Bio-industries, Université catholique de Louvain
Since 2013 : F.R.S. - FNRS Research Fellow, Life Sciences Institute, Université catholique de Louvain
A glance at Baptiste Pierman's bio
2011 : Bachelor in Bioengineering at UCL
2012 : Erasmus Programme at Wageningen UR, the Netherlands
2013 : Master in Bioengineering: Chemistry and Bio-industries, Université catholique de Louvain
Since 2013 : F.R.S. - FNRS Research Fellow, Life Sciences Institute, Université catholique de Louvain