So yet again, and for the time being we're left without conclusive evidence that plants hear. But I should say I am open to any experiment that will change this paradigm. Perhaps into the future, I'll have to change this class to report such studies. What did I just hear myself say? Perhaps in the future I'll have to change this class to report such studies. Well here we are in the future, and I indeed have to change the class. Let's go to the studio so we can hear what we now know about what a plant hears. As I mentioned earlier in this week's lecture, which I actually recorded several years ago, the beauty of science as opposed to pseudo science is that science is self-correcting. Scientists are always open to the possibility that our conclusions are wrong and open to new data which contradicts our older claims. What's happened in the past years? Let's see. Are plants deaf? Until 2012, all the research seem to lead to this conclusion. But I just want to raise the idea that maybe we haven't actually been doing the correct experiments. I mean, plants and music are obviously the wrong experiment. If we think in terms of evolution, what relevance is music for plant evolution? Flowering plants have been evolving for over 200 million years. Music has been around for how long? Rock and roll has been around for maybe 50 years, so obviously music was not an evolutionary pressure on plant development. But could there be ecologically relevant sounds that do affect how plants develop? How plants adapt to their environment. If a plant is to hear, if a plant is to respond to a signal of a pressure wave propagating through the air, it's going to have to be an ecologically relevant sound that will affect how plants develop and adapt to their environment. So maybe we need to consider what these sounds are. Such sounds would have to give either clues such as to the presence of pollinators or to the location of resources such as water. For example, back in 2012, a report coming out of the laboratory, a professor Stefano Mancuso in Italy, hinted that roots can change their shape in response to sound waves propagating through water. As we see in this picture, his group showed that corn roots bent towards low-frequency vibrations. In this experiment for the first few days, the roots were grown in silence, then once exposed to the low-frequency vibrations, the roots bent to the direction of the source of the sound waves. This was a very intriguing result but comes with several caveats. First, this result is a single figure in one article. Second, the sound waves were propagating through water and not air. Mancuso's team recently published a follow-up study. In this paper, Mancuso's group observed that low frequency sound changed the direction of root growth of young Arabidopsis seedlings. So I hope you can see here in a figure from the article, when these plants are placed vertically, the roots normally grow down, as we'll learn more in depth in the next class. Mancuso grew the second plate with the speaker emitting low-frequency about 200 Hertz sound waves placed to the side of the plate. As I think you can clearly see, these roots grew towards the sound source Mancuso termed this phonotropism, or growth toward sound. This observation leads to two important questions: First, what is the physiological mechanism that connects the sound waves to the change in growth? Well, let's remember that for us hearing is a specialized form of Mecano-perception. What we learned so far about Mecano-perception in plants? Touching a plant causes changes in concentrations of ions such as calcium and potassium. Mancuso's group further discovered that several minutes following the exposure to the sound waves, the plants also showed significant changes in these ions. For example, this picture shows part of an Arabidopsis root as seen under a microscope and after being exposed to sound. The green color shows cells with increased calcium. This really suggests that sound waves induce changes in the concentrations of calcium and also potassium. And these changes then signal for other changes that lead to the bending of the roots. We already saw earlier that touching a plant causes changes in calcium and potassium. Now we see that sound waves also influence intracellular calcium and potassium levels. So let's take this scenario a bit further. How do sound waves influence plants at the molecular level of gene expression? As I mentioned when describing Janet Braam's research, different cohorts of genes are expressed in different tissues or in response to different stimuli. For example, the touch genes were expressed after touching a plant. To understand this, I want to digress for a second and add another level of complexity to what I taught earlier. There I said that different genes are transcribed in different tissues or following different stimuli yielding unique sets of proteins. But what is transcribed? The answer to this involves a third intermediary molecule which connects between genes and proteins. This molecule is called RNA, and the general dogma biology, is such the genes, the DNA, is transcribed to RNA molecules and then the RNA is translated into a protein. So we can use the RNA as a proxy for understanding which proteins are likely present. Because of advances in technology, scientists now have the ability to study all of the organism's genes in one experiment and following any condition such as exposure to sound waves. Using a technology called "Next-Generation Sequencing", researchers today, within a few days, can know which genes are transcribed at any particular condition. Last year, a group from Yeungnam University in Korea decided to study how sound influences gene expression in Arabidopsis. Here's how it works: The scientists isolate RNA from the plant grown under normal conditions and from plants exposed to various wavelengths of sound including 500 Hertz. Using this machine, the next generation sequencing machine, the scientists can count nearly the exact number of RNA molecules arising from each gene. The more copies of a particular RNA, the more the gene is expressed. Highly expressed genes have a high number of RNA molecules, genes that aren't expressed at all have no RNA molecules, and low expressed genes have few molecules. This way the researchers can find differences in the RNA expression patterns between two or more samples. In this example, under normal conditions, gene A is expressed at low levels, gene B isn't expressed at all, and gene C is expressed at high levels. Following exposure to the sound waves, the expression of gene A doesn't change at all. Gene B is now turned on and expressed at high levels, while gene C, though it's still being expressed, is now expressed at lower levels than before. In other words, the 500 Hertz sound treatment has no effect on Gene A, induces gene B and slightly represses gene C. Using computational methods, these changes can be visualized on a color scale. The more the expression of a gene increases, we give it a red color. The more it is repressed, a green color. And if no change at all, then it's visualized as black. This way we can quickly observe the expression patterns of hundreds and even thousands of genes at one time, and under different sound treatments, and this is what the Korean group did. What they found is that 59 genes were induced following the sound treatment. But here's where it gets interesting. Out of these 59 genes, 23 were genes that had been previously identified as touched genes by Janet Braam's group. In other words, the sound waves induced the expression of 23 touch genes. Not all of the over 500 genes that Braam identified were induced by sound. But the fact that a small subgroup of these genes are sound responsive indicates that a plant's response, to sound waves is similar to ours. For us, hearing is a physical response of vibrations sensed in our ears. These vibrations or pressure waves which propagate through the air with a recurrent pattern, that's the frequency. In the first part of today's lecture we saw the plants respond to physical stimulation. Now we see that this response extends also to sound vibrations which lead to a physical stimulation. And now for the second question. What is the ecological significance to responding to these sounds? Why should a plant want to listen for example to 200 Hertz? indeed if scientists want to study plant responses to sound waves, they need to consider what the physiologically relevant sounds are that would provide an evolutionary advantage for a plan to hear. Such sounds would have to either give cues, such as to the presence of pollinators, or as to the location of resources such as water. Mancuso's study coupled with the molecular genetic one, might help explain phenomena that city engineers have known for decades. Tree roots often surround and invade water and sewer pipes, causing huge physical and financial damage, While we largely assumed that the roots were attracted to the water leaking out of the pipes, I'm open to the possibility that the roots are attracted to the sound of the running water in the pipes. But even if not, roots clearly need to know how to find sources of water. Clearly, there's more going on here than we ever imagined. If in the first edition of What A Plant Knows I wrote: "For hundreds of years, plants have thrived on earth. "and then nearly 400,000 species of plants have conquered every habitat "without ever hearing a sound," I now the to reevaluate my position. Plants may indeed respond to acoustic signals. This is the strength of the scientific method and what separates science from pseudo science. Pseudo science seeks confirmations while science seeks falsifications. As a scientist, I clearly realized that my best hypotheses and conclusions are tentative waiting to be shot down by some future study. The pseudo scientist, on the other hand, is convinced that his conclusions have been proven true. A pseudo scientist won't let contradictory results get in the way of his opinion. While there are many things we don't yet understand, it doesn't mean that there's not a scientific explanation waiting to be exposed by the proper experiment. The new studies I highlighted over the past few minutes, hint that we are on the verge of a deeper understanding of plant responses to sound waves. To sum up, Today's lecture has dealt with tactile sensitivity. In both humans and plants, touch leads to an electric signal. In humans we've seen various modalities. There's mechano-reception there's temperature reception, there's pain and there's hearing. In plants, there's definitely mechano reception. While I didn't have time to delve into it, plants also respond to temperature. Plants can differentiate between cold and heat, but there's no pain in a plant's tactile responses. Regarding hearing, well, it seems that responses to sound waves in plants, as in humans, is a specialized mechano sensitivity that has the potential for allowing long distance communication. But obviously, we're still at the beginning of understanding what a plant hears. That sums up today's lecture. We'll meet again next week.
