Thursday, 23 November 2017

The chemistry of aromas: a Plants Are Chemists interview with V.S. Pragdheesh

What do a grape candy, an industrial bird repellant, and perfume all have in common? They are all derived from plants of course! Many of the molecules that go into these products are chemicals that plants make and release into the air around them (think lavender, Figure 1A). These airborne molecules are called volatiles [vall-a-tyles]. To discuss them in this month’s post we interview an expert: V.S. Pragadheesh (Figure 1B), a chemical ecologist (a researcher who studies how chemicals made by living things play roles in ecosystems) at the National Centre for Biological Sciences in Bangalore, India.

Figure 1: A source of plant volatiles you may be familiar with, and this weeks interviewee. A) That lavender smell? Chemicals made by the plant that are released into the air and enter your nose - these are plant volatiles! B) This week’s interviewee - V.S. Pragadheesh from the National Centre for Biological Sciences in Bangalore, India.
Plants are Chemists: So, you study “plant volatiles” - what are those exactly?

V.S. Pragadheesh: There is an old saying that “a flower shop does not need an advertisement”. This is because we recognize flowers simply by their smell! Aroma, or fragrance, is created by chemical compounds which very easily escape from the liquid state (even from the solid state sometimes) into the vapor state - and that’s how they reach us. Chemicals with this ability are called volatiles. Plants produce a huge diversity of such volatile compounds which are called “plant volatiles”.

Plants are Chemists: As humans, how do we experience these airborne volatiles?

V.S. Pragadheesh: A great example to know how we experience these airborne volatiles is food. Whether it’s buttered popcorn, or a masala dosa, or a pizza, or whatever, every dish has some smell. Some volatile compounds in the dish escape from the food and reach our nose through the air. These specialized compounds activate our smell receptors (present in the nose) and these receptors send the signal to the brain through neurons.  The brain combines the signals from different neurons and makes a signature for the dish in our brain as a particular smell. The brain even combines these smell signals with taste signals to create a characteristic flavour.

Plants are Chemists: Very cool! I know that many of the volatiles used in industry are synthesized in large chemical reactors - but where did scientists discover these compounds in the first place?

V.S. Pragadheesh: These compounds are identified mostly from plants! Plants biosynthesize these volatile chemicals to adapt themselves to various biotic (living) and abiotic (non-living) stresses of different environments.  The pleasant smell of rose, the earthy smell of vetiver, the fruity smell of an apple or mango are olfactorily pleasant to humans, but they have several roles in plant biology such as pollinator attractants (Figure 2, left), defensive compounds (Figure 2, middle), mutualist attractants, indicators for seed dispersal (Figure 2, right), and many other unknown functions.
Figure 2: Some functions of plant volatiles. Left: volatiles can attract pollinators; Center: plant volatiles can kill insects that damage plants; Right: plants can signal to one another to synchronize seed maturation and the attraction of seed dispersal agents (birds, mammals, etc.)
Plants are Chemists: Can you explain a little more about how plants utilize these volatiles as a signal for their benefit?

V.S. Pragadheesh: Of course. For instance, it is to a plant’s advantage to only allow its seeds to be dispersed once the seeds have reached maturity. That is why unripe fruits (which contain immature seeds) have a strong, acidic odor that repels most seed dispersing agents (birds, mammals, etc.). During ripening, the acids present in the unripe fruits are converted to more volatile chemicals called esters, which impart the fruit with a fresh, ripe scent. This is why a ripe fruit smells better than an unripe one. The smell of ripe fruits attracts birds and mammals to eat the fruit and disperse the seeds at the complete maturity of its seeds.

Plants are Chemists: Can you give us an example of a plant volatile that we might be familiar with?

V.S. Pragadheesh: One common volatile is an ester present in flavours like grape, apple, and citrus. It is called methyl anthranilate (Figure 3). Methyl anthranilate is also found in many flower fragrances such as jasmine, tuberose, lemon, champak, etc. It is also the characteristic flavor of grape candy.

Fig. 3. Methyl anthranilate. This molecule is made up of a carbon ring (black) with a nitrogen atom (blue) attached, as well as two oxygens (red). It is found in many fruits and flowers, has a distinct “grape” flavor, and repels birds.
Plants are Chemists: Interesting! So, what plants make methyl anthranilate and why do they bother to make it anyway?

V.S. Pragadheesh: Corn and grapes are two common species that make methyl anthranilate. Due to its unpalatable nature to birds, methyl anthranilate acts as a defensive compound for plants to protect themselves from birds.

Plants are Chemists: Corn and grapes are two very different plants - one is a vine and the other is in the grass family. If such different plants can both make methyl anthranilate, does that mean that all plants can?

V.S. Pragadheesh: Well, the easy answer is no, not all plants have this ability. There is also a slightly more complicated answer as well - which has to do with the fact that corn and grapes each make methyl anthranilate in a different way. Maize and grapes use different biosynthetic pathways to produce methyl anthranilate. These two plants use different substrates and different enzymes to produce the same compound, methyl anthranilate. This phenomenon is called convergent evolution. It is easy to explain convergent evolution with a common example like flight. Flight has evolved in insects, mammals (bats), and birds: the same trait (flight) evolved in several different classes of organisms.  Convergent evolution onto methyl anthranilate in maize and grapes also a similar process.

Plants are Chemists: Wow, that is incredible! So, is methyl anthranilate the only technique to deter birds?

V.S. Pragadheesh: Pin wheels, streamers, reflective art, owl, and cat figures are some classical techniques to deter birds, and while these might work sometimes, a pretty good alternative is synthetic methyl anthranilate. Unlike other chemicals, methyl anthranilate specifically deters birds and can be used to protect diverse crops. Further, it is a food grade chemical - safe for human consumption as it is made by many other plants we already eat.

Plants are Chemists: Very nice. Do you have any last comments before we end the interview?

V.S. Pragadheesh: Next time when you smell a flower or plant, appreciate the interesting chemistry in it. The plant is communicating with the environment around it by transferring the chemical information. Chemicals are the language of nature and plants are the best chemists! It is very interesting to talk to you, Luke. Thank you very much.

For more information, see:
[1] Pichersky, E., Noel, J.P. and Dudareva, N. (2006) Biosynthesis of plant volatiles: Nature’s diversity and ingenuity. Science, 311, 808-811.
[2] Pichersky, E. and Lewinsohn, E. (2011) Convergent evolution in plant specialized metabolism. Annu. Rev. Plant Biol., 62, 549–66.
[3] Cummings, J.L., Mason, J.R., Otis, D.L., Heisterberg, J.F. (1991) Evaluation of dimethyl and methyl anthranilate as a canada goose repellent on grass. Wildi. Soc. Bull. 19, 184-190.