And one of the things that these people love to remember and reminisce about is weather. So we're going to talk about the long term memory of weather. It's a common theme for us, we like to talk about how hot it was last summer. We remember great droughts, extremes in weather, blizzards, hurricanes or just plain cold, very cold winters, or very hot and very dry summers. You may be surprised to learn that plants also remember weather. To talk about this, I'm going to bring in a guest lecturer who's actually an expert in long term memory of plants, my friend and colleague, Professor Nir Ohad. Hi, Nir. >> Hi. >> Thanks for joining us here in Coursera and helping us with the class. >> Sure, it's a pleasant to be here. >> Bringing Nir in actually, for two reasons. One, he's an international expert who does recently wrote a book about plant memory and plant long term memories. And second, he's a very close friend and colleague, and we've been working together almost 30 years. And why am I talking about this is because that some of you may have the idea of science as being a solitary endeavor, when actually it's very social. We did our PhDs in the same lab together. We made contact when we were postdoctoral fellows, and we're lucky enough that our labs are right next to each other. So that we interact and collaborate and share ideas all the time. So, Nir, quick question. >> Yes. >> Do plants have a long term memory of weather? >> Yes indeed, and that is exactly what we're going to talk about in this class today. >> So, I'm going to leave him now here to teach you about actually the complex mechanisms that are involved in plant long term memory. >> So the question that we're going to place in this class today is, do plants remember experiencing a cold period? Why is it important to monitor passing through a cold period? Plants growing in regions where summer is short need to exploit as much time available to complete seed production, and therefore starting the process as early as possible is crucial. Moreover, it could be risky for a plant to grow and flower too early as winter could still last, jeopardizing their offsprings. As we can see in our next slide, where plants have just flowered into a blizzard of snow. As an example, let us examine the case of most important crop plant, wheat. To flower most strains of wheat need first to undergo a cold winter in the variety of winter wheats. Planted in fall in sprouts they get covered by snow and only after that they could flower, making grain in the spring. If no winter, no flowers, no grain. A severe problem occurred in the early 1920s of the previous century. In the former Soviet Union, warm winters destroyed wheat yields, which led to famine. Soviet scientist Trofim Lysenko discovered 1928 that if seeds were put into a freezer before planting, they would flower and make grain regardless of being exposed to winter. Lysenko called this term grout jarovization. To describe a chilling process he used to make the seed of winter cereals behave like spring cereals. This term was translated as Vernalization, from the latin word, [FOREIGN] for western text. So, the term was going to use from now on is vernalization to express the transition through winter allowing plants to flower. >> So, Nir what your saying is that the wheat seeds are first put in a refrigerator before planting then that has the same effect on the wheat as when the seedling is exposed to a cold winter? In both cases, whether the seed or the seedling is exposed to the cold, the wheat flowers. But what does the cold actually do? >> It turns out the plants, like us, use signs or marks to assign for the cold they experience. Just to demonstrate we use different ways to remember ourself things not to forget for example, we turn our watches or we write a note such as do not forget. Or we just scribe on our hand do not forget to remind us of the things which was important. While using all of the above memos we did not change of ourselves anything. How do plants write their memos to record cold? They do it by placing a chemical sign. But where? This mechanism of writing signs was solved only in recent years using genetic and molecular tools. The marks are placed on the DNA or protein which anchor the DNA. There are proteins of which their function is to place such marks called writers. Before going on, I would like to review some concept, and integrate these new information. As we should all know, our DNA is composed of four types of building blocks called nucleotides. Which are abbreviated G,A,T and C which are found in long strands millions of nucleotides long. In a human cell each of these strands would reach about two meters long. Obviously, a two meter long string could not fit into a microscopic cell, let alone in the even smaller nucleus of the cell. The way that the DNA fits in the nucleus is that it is highly compact, the double helix of the DNA wraps around proteins called histone forming what is known as chromatin. This wrapping literally caused the DNA to be arranged in little beads on a string. These beads can twist even more, just like an overlay twist rubber band compacting the DNA and proteins into highly condensed and packed structures. These structures are dynamic. Different parts of the chromatin can unravel or pack up again. The importance of the packing is that when genes are found in highly compacted regions, they are usually turned off. The genes are inactive. On the other hand when a gene is in a relatively open, less compact region, then it can be turned on. Genes that are turned on, those that are transcribed, are found in areas of the chromatin that are unraveled, while genes that are turned off are in regions that are more condensed. The writer proteins, as I've called them, influence the packing of the chromatin by changing the chemical structure of histones. The change is called methylation. Histones that are methylated pack tightly, turning off genes around them. While histones that are unmethylated, enable the DNA to be less tightly packed and the genes can be turned on. Back to our main theme. Plants which experience cold activate writers. These are proteins that place marks at particular genes without changing the DNA sequence. This type of mechanism is designated an epigenetic mechanism, meaning above the level of genetics, as it affects the read out of the DNA without changing the DNA itself. This marks caused particular genes to become gradually silent and eventually, to stop exerting their function. So which genes are silenced and what are their function? The pioneering work of the British team lead by Professor Caroline Dean and Professor Rick Amasino from the USA, discovered that flowering is activated due to the suppression of particular genes designated FLC which blocks flowering. >> Let me just re-emphasize what you said, Nir. The function of the FLC gene is to repress flowering. In other words until a plant starts to flower the FLC gene is active. That is, it's inhibiting the flowering. >> As winter progress, a writer protein place marks on the repressor gene until it is silenced thus allowing flowering to take place. As in math minus times minus give rise to plus. The same in this case, where the cold leads to suppress the suppressor, allowing flowering. Let us review this complex process again. The specific gene involved is FLC, which stands for flowering local seed. FLC function by inhibiting flowering. Once the plant goes through a period of cold weather, the FLC gene is no longer transcribed. In other words, the gene is turned off. But that doesn't mean that plants will immediately start to flower. It only means that plants can flower if other conditions such as light and temperatures cooperate. Professor Dean and Amasino studies showed us that in fact mutant arabidopsis plants lacking the writer protein which add the methyl group on the histone express FLC continuously, even when exposed to cold and dust, they cannot flower.
