Thursday, October 05, 2006

The Plant bloodhound

It’s time for some scienceblogging again!

Before you read on, pause for a moment, and answer this question.

Can plants smell things?

I mean, think seriously. It’s a plant. How could a plant detect odors? Volatile chemicals? Sure, plants themselves could smell nice or nasty, but do they sense their own smell?

Now, think of a rather common garden weed called the dodder (a.k.a. strangleweed). If you are an enthusiastic kitchen gardener, and grow potatoes or tomatoes in your backyard, it is more than likely that this weed has at some point of time entered your garden. Its actually a sorry looking plant, with the leaves hardly visible, as it vines itself around the host plant, but it does sometimes have nice looking flowers. The dodder seedling can survive only for a short time, but in this time needs to hunt out a host, and start growing on it. This vine produces roots (or haustoria) that penetrate the host plant, and sucks out the nutrients from that poor plant.

In some fascinating work, scientists now report that this plant actually sniffs out its prey.

(image from here)

Yup, you heard that right. This plant hunts down its host by smelling it out.

Well, more or less.

The researchers designed some nice experiments, where they first traced the growth of dodder seedlings, and found that they moved towards tomato plants. Then the researchers carried out a more controlled experiment, where they put the seedlings in a special chamber with only two outlets. One lead to four real tomato plants, the other to four artificial plants. The seedling, with almost a relentless focus, grew towards the real tomato plants. Next, the researchers did the same experiment, but without any plants. In one side they kept real plant extracts, in the other, solvent alone. Guess what? The dodder grew straight towards the plant extract.

Ok, you say. Perhaps the dodder can sense any plant extract, since it parasitizes many plants. But that isn’t being really perceptive.

But guess what, this plant can differentiate between plant smells. If the experiment is carried out with the dodder seedling planted between tomato plants on one side, and wheat plants on the other side, the dodder preferentially grows towards the tomato plants. The wheat plant is an unsuitable host for the dodder, and it does not survive well on wheat. And this data shows that the way the dodder differentiates between tomato and wheat plants is by the volatile compounds they emit.

This plant practically smells out a new home.

Being a (former) biotechnologist, I couldn’t but appreciate the observation that if the dodder didn’t like growing towards the wheat plants, perhaps the volatile compound in wheat that differs from the tomato plant could possibly be used some day as a dodder-icide.

A fascinating piece of work, and it certainly has made me rethink the whole concept of plant sensory systems (after all, we have them for survival. There’s no reason to think that plants could not use something this useful). This study also raises so many questions. How do the plants detect volatile chemicals? What are the receptors that do this? Do all plants still retain this system, or do parasitic plants preferentially retain them? Such exciting times.

Now, here’s my question again.

Can plants smell things?

(You can read the full paper of this story here, or listen to an NPR broadcast here).


Michael Higgins said...

Hi Sunil
Fascinating story. I would never have guessed that plants would be so sophisticated- but it shows what billions of years of evolution can do.

Btw did you create that graphic yourself? You are indeed multitalented.

Anonymous said...

Nice find Sunil. I like the way you express your excitement on the subject in your posts. Keep it up.
I was a little surprised that plants detecting/signaling volatile compounds is a controversial area. Isnt fruit ripening by ethylene strong evidence in favor of remote sensing by plants?

Sunil said...

Michael......absolutely. I chanced upon it while scanning Science (as I usually do) for papers that are closer to my own research, and I was very impressed by the elegance of the experiments as well as the impact of the story. No, I didn't create the graphic (I've credited the original source in the post, but it is fine print)....I just found it.

Are you going to start blogging again? is what I do, and what keeps me going. As far as your question goes, it is an excellent one. But the controversy does indeed exist. With ethylene, it is argued that it is due to artificially high levels of ethylene (non-physiological or concentrations that will never exist in nature). This means that the volatile (in this case ethylene) could at those concentrations diffuse through to high enough concentrations to activate internal receptors. But in this study they unequivocally showed that the dodder grows towards tomato plants in a normal garden or field. That is what was perhaps the neatest observation.

Anonymous said...

I guess thats true. If you gas a plant, it will do what you want!

Anonymous said...

I wonder ho many times the experiments were repeated to conclude the preferential growth. I shall take a look at the paper...

Wonder if the tomato plant itself is required or just a bottle of mashed tomatoes nearby (periodically freshened), to lure the dodders...

could it be that the "enemies" of wheat are also the enemies of dodder, for it to flinch away from it?

Sunil said...

Sarat....yup. I thought this experiment was rather well done.

Arunn....the experiment was done enough times to make it conclusive. Yes.....tomato plant extract was sufficient to lure the dodders....since they also further did isolate the specific active volatile compound (as I mentioned later in the post), and also found the odorants that were different in wheat.

It doesn't seem like the "enemies" of wheat are also enemies of dodder.....dodder does grow on wheat, but prefers tomato significantly. It looked (from their experiments) like wheat had a specific volatile compound that was repulsive to dodder.

What fun.

Anonymous said...

Regarding your question:

"Can plants smell things?"

If you define smell as reacting to chemical compounds carried in air, then it seems they do. But we have to be careful because the common definition of "smell" assumes a central nervous system.

Sunil said...

yes indeed, colluvial.... you are absolutely right.

I just wanted a fairly catchy title (i still dont say plants can smell things :-), and also wanted to question how we perceive plants. After all...most of us (and i'm not a plant biologist) don't think of plants as being able to detect volatile chemical cues etc....