Engineering Plants That Fertilize Themselves to Save the World

Engineering Plants That Fertilize Themselves to Save the World

If you’ve ever had a garden, you’ve probably given your plants some kind of food during the growing season to keep them healthy and happy. Maybe it was some compost or a store-bought fertilizer that you mixed into their water.  That’s because not all soils are the same, and some are lacking in the important nutrients that plants require for lush, consistent growth.  

Plants That Fertilize Themselves

In fact, humans have relied on fertilizers to nourish and boost the yield of their crops for thousands of years. Fertilizers help us grow a lot more plants in whatever soil is available.  And thanks to the advent of synthetic fertilizers in the early 20th century, it became easier than ever to give crops a nutrient boost… so much so that, today, around half of the world's population relies on these types of fertilizers to grow their food. 

But the convenience of synthetic fertilizers comes at a cost to the entire planet. And no, I’m not talking about runoff, though that's a problem, too.  You see, the way they’re made uses a lot of energy, which comes primarily from fossil fuels. So using synthetic fertilizers is having an impact everywhere. 

Relying on them simply isn’t a sustainable option for the future, especially as the world’s population continues to grow. So researchers are looking at other ways to keep crop yields high while moving away from fossil fuel dependence and increased greenhouse gas emissions. And one really clever way to do that might be to give nature a little boost. Some researchers are hoping to genetically engineer plant-associated microbes, or even the plants themselves, to make their own fertilizer! 

To talk about that, we have to understand why they need fertilizer. Plants require a variety of nutrients for growth, in different amounts, but there are three primary ones often lacking in soil: nitrogen, phosphorus, and potassium. Nitrogen is arguably the most important nutrient on the list and the one that I will be focusing on. Because sure, we’re all carbon-based lifeforms, but without nitrogen, life as we know it wouldn’t exist. 

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It’s a key component of chlorophyll, which plants,  need for photosynthesis. It’s also vital in the formation of amino acids, which are the building blocks of proteins. And it turns out, though, that there is tons of nitrogen all over our planet. However, most of it is in the form of nitrogen gas, which plants can’t use for food. 

They require a little help to get it into a usable form. That help can come from biological processes, but often, it comes from fertilizers: substances we add to soils to make them more fertile. Fertilizers can be made by people, or they can be natural, meaning they can come from living creatures or straight from the Earth. Like, as far back as 8000 years ago, farmers were adding manure to their crops to give them a nutrient boost. 

Compost is another common way of nourishing crops, especially in a backyard garden.  But there’s just a lot less bang for the buck in those natural nutrient sources than in the ones we cook up in labs. This is why synthetic fertilizers are the primary way crops are fertilized today, especially where lots are grown in the same soil, year after year after year.  

The biggest perk of synthetics is that they contain more of these essential compounds than natural fertilizers do, so you don’t need as much of them to feed your plants! One of the issues, though, is that even though less of them is required, people often over-apply these fertilizers, and the excess gets washed out of the soil and into the nearby waterways.  

This is what you probably hear about when problems with fertilizer are brought up, that this excess, called runoff, pollutes the water and promotes the growth of harmful things like toxic algae that use up all the water’s oxygen.  

And while this is definitely a problem in many places, there’s an even bigger, planet-wide problem with synthetic fertilizers that has nothing to do with runoff. And that’s that their production is a significant contributor to climate change. 

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Right now, fertilizer is produced with the Haber-Bosch process, which involves combining hydrogen and nitrogen gas at high temperatures and pressures to create ammonia. Additional processing turns ammonia into ammonium nitrate, a form of nitrogen that plants can use as food. But, those high pressures and temperatures require a lot of energy, most of which currently come from fossil fuels. 

Now, researchers are working on switching to more renewable forms of energy, which would help a bit with the emissions. But it’s not really feasible right now because they're too expensive.  This nitrogen-making process simply takes too much energy. 

So if the fertilizer production industry switched to existing renewable energy sources tomorrow, the cost of fertilizer would skyrocket beyond what's reasonable. And since almost half the world’s population relies on synthetic fertilizers, it’s just not an option, at least not yet. Natural fertilizer production can, and should, be bumped up to lessen our dependence on synthetics. But what if there’s a way to engineer plants to make their own food, thereby decreasing the need for fertilizers all together? 

Researchers have been investigating exactly this, and a few attempts show real promise. Essentially, there are two ways of doing this: one, you can get the microbes living amongst the crops’ roots to do it, or you can get the plants to do it themselves.  Both methods involve giving these organisms the means to convert nitrogen from the air into a usable food source. This is where the process of nitrogen-fixing comes into play.  

Some microbial species use an enzyme called nitrogenase to convert nitrogen gas from the atmosphere into ammonia, a process known as biological nitrogen fixation. The microbes use the nitrogen they fix for food; any leftover is released as ammonia, which the plant can use. Legumes are particularly interesting because they have a very unique relationship with nitrogen-fixing microbes known as rhizobia. 

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These nitrogen-fixers attach themselves to the roots of the plants, triggering the plants to form round bumps called nodules that they can move into.  

Inside, the bacteria turn atmospheric nitrogen into ammonia, providing the legumes with a constant source of nitrogen, and in return, the plant provides the bacteria with sugars. When the bacteria and plant die off at the end of the growing season, ammonia is released into the soil for other plants to take up as well, which may be why you’ve heard of farmers planting legumes as cover crops between seasons. 

This partnership adds nitrogen back to the soil. You might be thinking: well, that sounds great! Why don’t we just give these bacteria to all the plants? Turns out that that is not a simple thing to do. 

These bacteria are so reliant on the legume that they are physically unable to fix nitrogen on their own. And this relationship is oftentimes so specific that we simply can’t transfer these bacteria to other plant species. 

Other crops like sorghum, rice, and maize also have a beneficial relationship with nitrogen-fixing bacteria, though. Maize even releases a sugary mucus when it rains, which provides a safe house for its nitrogen-fixing microbes.  

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But those relationships aren’t as co-dependent, or as beneficial, as what legumes have with their microbes. These particular microbes are able to convert atmospheric nitrogen into ammonia, regardless of whether the plant is there or not. 

But, they are not as good at fixing nitrogen as legume-associated bacteria, so they can’t provide the plants with all the nitrogen that they need. And even when it comes to legumes, the nitrogen-fixing ability of their bacterial associates varies widely. Some are just better at it than others.  

So most of the time, fertilizers are still necessary to give plants a boost during the growing season, particularly in a commercial setting. To get away from this dependence on fertilizers, researchers are looking into giving these soil microbes a boost, either through modifying existing nitrogen-fixers or engineering other microbes to give them nitrogen-fixing abilities. 

Modifying microbes to have slightly different metabolic needs removes some of the limitations that nitrogen-fixers currently suffer from. One idea being investigated is blocking the uptake of nitrogen by the bacteria, so they would release more of the ammonia that they produce to the plant. Another option is more specific to the enzyme itself. 

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You see, these microbes need things like nodules or special mucus because nitrogenase is immediately deactivated by oxygen.  If researchers could engineer a microbe with a nitrogenase that is not sensitive to oxygen, that microbe could fix nitrogen anywhere. And to that end, researchers have successfully given E. coli bacteria the ability to turn nitrogen gas into ammonia by modifying them to contain nitrogenase genes known as nif genes. 

Unfortunately, the modified E. coli were unable to convert enough nitrogen gas for their own growth, let alone a plant’s. So it currently isn’t a complete solution, but the potential is there for future research!  

Now, another problem with this is that many plant species don’t naturally have a strong relationship with these kinds of microbes. So researchers are also investigating ways to help plants make their own food, without the need for microbes at all. 

One very promising idea is engineering a part of the plant cell to produce nitrogenase, so the plant could turn nitrogen gas into ammonia all by itself.  But there are some hurdles to cross here, too. One of the biggest ones is that nitrogenaserequires a lot of energy to convert nitrogen into ammonia.  

It seems that synthesizing ammonia is energetically costly no matter who’s making it!  So you’d need to put it somewhere that has enough energy to use it.  

Researchers have proposed engineering plant mitochondria to produce the enzyme since they are the main sites of ATP synthesis and could easily fuel the process. But it’s not just as simple as activating one gene to produce nitrogenase. 

Bacteria activate at least 9 nif genes when they make it, and researchers have identified at least 16genes are required for plants to produce the same enzyme. The ratio at which all of these genes are expressed is important as well. 

Also, as I mentioned before, nitrogenase is easily and irreversibly de-activated by oxygen. And oxygen is what mitochondria need to make ATP! To get around this, researchers have proposed engineering the plant’s chloroplasts to produce nitrogenase instead. 

They are also a source of ATP and could be modified to separate nitrogen fixation from photosynthesis, so the oxygen made from that doesn’t interfere. In fact, some cyanobacteria species are already able to do this! Their chloroplasts photosynthesize during the day and fix nitrogen at night.  

Plants That Fertilize Themselves.

Plant chloroplasts are evolutionarily similar to cyanobacteria, so it may be possible to engineer a similar strategy in plants. But as of right now, that's all hypothetical. No one has actually taken wheat or whatever and successfully given it the ability to fertilize itself. Some are trying it, though. 

Humans are facing the global challenge of not only feeding Earth’s ever-growing population but also combating the ever-increasing threat of climate change. And depending on synthetic fertilizers to get the job done isn’t a sustainable option for the future, seeing as the manufacturing process is significantly contributing to CO2 emissions. 

Natural fertilizers can, and will, be a part of a sustainable solution, but they also contribute to issues like runoff. So something bigger and bolder is needed, too.  

And that something may be genetic engineering to improve the nitrogen-fixing abilities of crops or the microbes that live among them. Or it may be something else. There are a lot of smart people working on these problems and coming up with innovative ways to minimize our future fertilizer use. Either way, we need real, long-term solutions, so that we can ensure that what we eat doesn’t destroy our planet.  


Article Source: SciShow

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