Hannah Rees: giving wheat jet lag
Hannah Rees, image courtesy of Hannah Rees
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Food security is an ever increasing problem for the planet. You’ve probably already noticed that your weekly food shop at the supermarket is costing a lot more than it used to. Wheat in particular is in short supply. Yes, the war in Ukraine has been a factor, as Russia and Ukraine are the first and fifth largest exporters of wheat in the world, but it’s not the only factor.
In 2021, heatwaves and droughts reduced wheat yields in the US and Canada, the second and third largest wheat exporters, and in the same year excessive rain flooded crops in China. This year, India decided to ban the export of wheat as record breaking heatwaves caused widespread crop failures, and swathes of Pakistan are still underwater, right when farmers would normally be planting next year’s crop.
Clearly we need to make wheat a more reliable and resilient crop in the face of our ever changing climate, and that’s where geneticists like Dr Hannah Rees from the Earlham Institute in Norwich come in. Kat Arney sat down with Hannah to find out how understanding the basic biology of wheat is helping us produce a more future-proof plant:
Kat: What is wheat to the world in terms of a crop?
Hannah: Wheat, as a whole, accounts for about 20% of the calories that we eat as a human race across the world. So it's really important that we know everything there is to know about it in terms of its biology, but also that we are looking at how to increase yields and make that sustainable for the future.
Kat: So what are some of the challenges that we face with trying to grow wheat and meet this demand as the world is changing around us?
Hannah: Well, a lot of the demand for wheat comes from the switch to Western diet. So the western diet is very much based around wheat. And rather than being focused on rice, a lot of Asian communities are now switching to wheat centric diets. So the demand for wheat is really increasing.
Hannah: But also, wheat is really good at being able to be grown all over the world. So wheat is very much a global crop, but it faces challenges. So if you don't get predictable rainfall at critical parts of the wheat development, you can have catastrophic crashes in crop yields. And that is one of the sort of worrying things about climate change, as we get more unpredictable weather patterns, can we actually design resilient wheat? So switching from a focus of just increasing wheat yields at any cost to having wheat that's really robust to fluctuating weather conditions such as drought, but also flooding and unpredictable patterns basically.
Kat: So you are coming at understanding wheat from understanding the body clock of wheat. Now, I think we can all understand that plants, they're seasonal. They grow at certain times of the year, they don't grow other times of the year, you harvest them at certain times of the year, and you can think about things like flowers that come out at certain times of the day... But I'd never really thought of wheat as having like a body clock, I guess, a plant clock. So what's going on here? What is wheat's clock, what's its day like?
Hannah: So actually having a circadian clock is really important for plants because plants make their energy from sunlight. And being able to predict when dawn is going to happen in advance of it happening makes plants really efficient.
Hannah: So if you can forecast in an hour, there will be light, you can get ready and prepare for the onset of light, and it makes the plants really a lot more efficient in terms of photosynthesis.
Kat: So we're not talking about the Sun comes out and the plant wakes up. This is actually a plant getting ready to wake up. So it's running on some kind of underlying cycle.
Hannah: Yeah, exactly. You can equate it more towards you getting hungry an hour before lunchtime, like you haven't seen your food yet, but you know it's coming. And that's a similar thing for the plant.
Kat: So what do we know about the wheat clock? Like what makes up a clock inside an organism?
Hannah: So this is a question that a lot of people ask me is, you know, "how can plants have a clock without a brain?" And it's really interesting. Within each cell in a plant, it has its own molecular oscillator, so it has its own mini clock inside each cell in the plant. And these genetic loops are based on transcriptional translational feedback loops.
Hannah: So you have a gene that is transcribed into mRNA, and then that mRNA is translated into a protein. And in the case of this molecular oscillator, that protein sits in front of a target gene and represses it. So these are transcription factors.
Hannah: And what happens at the most basic level is you've got a nighttime gene and a daytime gene. And the nighttime protein sits on the daytime gene and represses, it stops it from being expressed. And the daytime protein does the same to the nighttime gene. So you have this tick-tock of proteins repressing the genes for the nighttime and vice versa. And this is at the very most basic level. In plants you have many interconnected oscillators that all interact with each other. But this is the premise of actually the clock in plants, but also in insects, in humans, in mammals, they all have these negative feedback loops.
Kat: So something just builds up and builds up and builds up... and then it effectively turns itself off by turning something else on that builds up and builds up and builds up... and then round and round and round we go.
Hannah: Yes. So the input to this, it does feed in from external stimuli. So light will have an impact on these genes being turned on and off. Temperature is another stimulus that'll allow the expression of certain genes and set up this rhythm. But the important thing is that these rhythms persist even when you remove all external stimuli. So if you put plants in a completely dark box, they still have this tick-tock rhythm that's derived from genes being expressed in the perceived day or night.
Kat: So what is this clock actually controlling? I can understand that maybe you'd want to control your responses to light - light runs on a day, night cycle. But what do we know about what kinds of processes in wheat are affected by this clock?
Hannah: So the clock is really prolific, and has an enormously powerful effect on gene expression throughout the plant. We know that around a third of genes are actually rhythmically expressed: they'll have a higher expression at a certain time in the day to a different time in the day. And these are really agriculturally important genes as well.
Hannah: So they are genes that are regulating flowering time. They are genes that are regulating responses to cold and to heat. And actually there are genes which effectively make sunscreen for the plant in advance of daylight happening, so it's already protected by the time day hits. There are genes which make the plant really unpalatable towards caterpillars. So the plant knows when the caterpillars are gonna wake up. And the plant thinks “right, I need to increase my production of these really nasty tasting chemicals so that I'm less likely to be eaten”.
Hannah: And photosynthesis, as I've already mentioned, is regulated by the clock. It can get ready with all of its photosynthetic machinery, so it's ready to hit the ground running when daylight happens.
Hannah: And starch as well. So throughout the night, the plant doesn't just switch off. It actually uses the nighttime period for growing. It can know the exact length of the nighttime, so it knows exactly how long it's got to do its growing. And it can predict if you've got a really short night, it can predict that it's gotta grow really quickly and utilise the available starch because it's only got a short amount of time to grow. Whereas if you have the same plant under a longer night, it will actually take its time and use the starch more slowly because it knows it's got a longer night in which to grow.
Kat: This is absolutely fascinating. It's a whole sort of secret in a world of wheat that I never really knew existed. But the one thing I do know about wheat is that it's got a lot of genes. I've done an episode recently where we talk about how humans have just over 20,000 genes and wheat has over 100,000 genes because it's kind of evolved in this really complex way. It's like - am I right? - it's like three different ancestral plants that have all smushed themselves together.
Kat: So this seems like it's gonna be quite complicated in terms of controlling all these genes. How does that all work?
Hannah: Yeah, you're exactly right. I think the genetics of the circadian clock in plants is already more complicated than it is in humans because you have to orchestrate all of these independent clocks in every single cell. And in wheat, you have the added complication that hexaploid wheat - which was formed from three genes merging into one basically, hybridising into one plant - it means that for every one gene in a diploid plant, you have three sister genes. And in one way, this means the plant has more genes to play with. So from an evolutionary point of view, you can devote one gene to its original function and still develop new functions for these extra genes.
Hannah: And this is something that we are interested in seeing. How often does this happen and how often are the three sets actually conserved? And is it useful to have three copies just in case you lose one? Does it give the plant, um, extra security?
Hannah: And we were interested in this from a circadian point of view, and what we were wondering is where you have these three sister copies of the same gene, are they always expressed in the morning? Are they always expressed in the evening? Or are they allowed to start having different functions?
Kat: And what did you find? Because this seems like a hell of a challenge if not just you've got one clock in wheat, you've got like three clocks and three different sets of genes. How's it running?
Hannah: So what we found was a complicated picture. As always, there's never a simple answer. But what we found were some sets of three genes where you have exactly equal balance between the three copies in each of the sub genomes, in those cases, it seems to be really important that that gene is always expressed very precisely at the same time in the day.
Hannah: In other cases, it seems to be that one copy said, "I've got this covered" you know "you two can take some time off. Like I've got, I'm gonna take on responsibility for the role of this gene".
Kat: I'll do the morning shift.
Hannah: Exactly. And what you see then is a slow decrease in the amplitude of expression of those genes.
Hannah: And then, in still other sets of three genes, you've got genes that are originally the same gene in these three ancestral species but now, one gene is being expressed in the middle of the day, and the other one is being expressed in the middle of the night. So we don't know what they're doing yet, but clearly they're being allowed to have very different functions.
Kat: Wow. So that literally is like, I'm doing the morning shift, you're doing the late shift guys.
Hannah: Yeah. And the fancy name for this is "neofunctionalisation". But it's exactly like you're saying partitioning roles between the three.
Kat: This is just absolutely wonderful. But, you know, it's very interesting, it's very cool. I'm sure it's very cool science. You can get some nice papers out of this! But we know that obviously the climate is changing and wheat is an important crop, as we said. So, I mean, this is good knowledge to have, but how can we use this knowledge to actually do more, to protect wheat as a crop and make it into a more sustainable, resilient crop? Can we tinker with this? What's going on?
Hannah: So one idea that we have is, I don't know if you're familiar with humans being larks or owls, like different body clocks to match different jobs perhaps, and we have the same idea in wheat. So if we can find a particular variety of wheat that really has a body clock suited to a particular latitude or growing region, then it's very much choosing the right plant for the right place to make sure the plants are happy and increase yields.
Hannah: It's been shown that if you have a plant and you put that plant in a environment in which it's well matched to the external day/night cycle, you have higher yields and higher photosynthetic rate, and it's able to outcompete other plants that have mismatched body clocks.
Hannah: So, flowering time, and like I've mentioned before, resilience to drought, salt stress, nitrogen availability. All of these things we might be able to find candid crops for particular locations.
Hannah: And actually, inadvertently throughout human history, we've selected for varieties that have higher yields or varieties that we like because they flower at a particular time. And unknowingly, farmers have actually been selecting for clock genes.
Hannah: Actually one well known clock gene allowed for the development of what you might know as "spring wheat". So there are two different growing seasons for wheat. So there are wheats that farmers put in in the autumn, and it sits in the fields over winter and flowers very early in the spring. And then there's spring wheat, which you can put in at the spring, and it will flower in the summer. And it's actually a knockout for a circadian clock gene, which has allowed spring wheat to be developed because it doesn't have to wait for the daylight increases before it will flower.
Hannah: So they've selected for genes which give the plant a circadian rhythm that really excels in a particular environment. So these things are all happening and it's whether we can optimise them further by having an actual biological understanding of what the clock is doing in wheat.
