Will the leguminous Prince find her? And what root will he take?
This story can also be read on Medium.
Once upon a time, there was a Sleeping Beauty. She had waited for years, floating in a thin layer of water and surrounded by giant rocky aggregates.
This Sleeping Beauty is not a princess imprisoned in a castle and surrounded by a moat full of water. She is a soil bacterium, a bacterium of the soil. She swims in the soil water, and moves between soil clusters.
Sleeping Beauty comes from a noble family, the Rhizobia. A family of bacteria whose special skill is fixing nitrogen.
Nitrogen is the most important element for plants. Nitrogen atoms are used an ingredients for many parts of the plant body — including DNA, RNA and the many hard-working proteins. But nitrogen is not easy to find in the ground. It’s water-soluble, which means it gets “leached” — that is, dissolved and carried away by the rain.
That’s why rhizobia don’t search for nitrogen in the soil. Instead, they take it from the air.
Nitrogen atoms are very common in the sky. That’s where they stay, in the form of ‘dinitrogen’. Dinitrogen — what we know as N₂ or “nitrogen gas” — is one of the easiest things to find in the sky. In fact, it makes up 78% of the Earth’s atmosphere. So it must be very frustrating for plants, who have this great source of nourishment floating everywhere but can’t get to any of it.
Plants can’t eat nitrogen through their leaves. They’re not built for it. The only way for plants to get nitrogen is through their roots, which can suck up ammonium (NH₄) or nitrate (NO₃) through their leaves. Which means they have to wait for it to get into the soil, either through natural processes or when someone pours in nitrogen-filled fertilisers.
That’s also why lots of plants grow near a lightning-strike. As the lightning slices through the air, it breaks apart the dinitrogen into separate nitrogen atoms. Some of these atoms then fall down with the rain, mixing with oxygen to form nitrates in the ground that the plant can use. So a bolt of lightning may injure a tree, but it also fertilises the soil for the next generation of plants to grow.
What about the rhizobia? They’re not plants. They’re bacteria, and they have just the right enzymes to digest bacteria from the sky. Enzymes are little workers inside the cell that do stuff: in this case, they take in dinitrogen, transform it into ammonium, and release it into the soil for plants to eat. This is what is known — with a term plants will certainly relate to — as “fixing” nitrogen.
But our own rhizobium, the Sleeping Beauty, is not fixing anything. She is still fast asleep, waiting and waiting for her Prince Charming to come to her.
Prince Charming is not actually a prince. He is a root hair, a hair of root. A long, thin, woody string, winding its way out from the root of a plant. He feels his way through the soil, probing the ground and deciding which direction to grow in. One day, he arrives under a dark forest, and, being adventurous, decides to explore it. And then, a strange signal attracts his attention.
The sweet scent — or is it feel? — of fat and sugar molecules.
Fat and sugar. These are the molecules that Sleeping Beauty sends out in her sleep, tiny strings of atoms which her Prince can recognise and grow towards. Not everyone can understand this message and create a relationship with Sleeping Rhizobia Beauty. Only a certain select family can do that: the Legumes. They’re diverse family of plants, but you probably already know some legumes, such as alfalfa, clover, pea, and bean.
Legume Prince Charming is charmed by the signal. He grows rapidly towards it, until he and the princess are practically touching. Does he kiss her? Well, not quite. But what happens next is, in some way, quite similar.
Not all humans kiss the same way or for the same reasons. It differs from culture to culture. But when people do kiss, in a romantic way, scientists think there’s also a biological reason behind it.
When two humans kiss, it’s also a chance for their bodies to exchange pheromones. These chemical signals contain information about the people behind them, such as their health or genetic make-up. For example, women have been found to prefer partners with immune-system genes that are different from their own. Probably because that’d help her children get a wider range of immune-system protection.
For the legume and the Sleeping Beauty, it’s not so much about finding the right person, but about finding the right species.
As soon as the root hair is in contact with her, a chemical conversation is started with her future plant host. The plant, on the other hand, checks the identity of the bacterium by emitting flavonoids.
Flavonoids are a kind of carbon-based molecules made by plants. Their name comes from the Latin flavus, meaning “yellow”, because that’s their natural colour. But they’re not just about yellow: flavonoids are responsible for colour in plants, and they can make flowers in any shade from red to purple. Other kinds of flavonoid have other jobs, including blocking harmful radiation, controlling the cell cycle, or acting as chemical messengers.
In this case, chemical messengers is exactly what these flavonoids are. And they’re designed specially to attract Rhizobia bacteria. Because Sleeping Beauty is from the Rhizobia family, the plant will consider her as a potential mutualistic partner.
On receiving the princely plant’s flavonoids, Sleeping Beauty replies with signals of her own. Known as “Nod factors”, these molecules help the root to be sure of her identity. As for why it’s called a “Nod factor”, I’ll tell you about that in a bit.
When he receives her signals, the root hair curls around Sleeping Beauty, holding her in an embrace. The bacterium is then taken up, through the root, and into a special room where she can multiply. This room is called the “cellular infection compartment”. Yes, the bacteria is technically “infecting” the plant’s root, although in this case it’s an infection that’s loved and welcomed.
As the princess and her progeny live and multiply in this room, the root grows an extra protective structure around them. This is the nodule. It’s a sort of ball you can see, attached to the outside of the root.
Inside the nodule is the Rhizobia bacteria’s own little world. The plant works to create the optimum environment, with the perfect temperature, nutrients, and chemical make-up, for the bacteria to live comfortably and fix nitrogen. Apart from protection them, the nodule also regulates the bacteria’s oxygen supply.
And now, can you guess the reason behind the name “Nod factor”?
Yes — “Nod” is short for “Nodulation”. The nodulation factor, in other words, tells the plant went to nodulate — that is, create a nodule for the bacteria to live in.
You know haemoglobin? The oxygen-carrying protein that gives blood its colour? The nodules of leguminous plants have something similar.
Leghaemoglobin, as it is called, carries plants just like haemoglobin in blood. By controlling leghaemoglobin levels, the plant can give just the right amount of oxygen to the bacteria. And, it can keep oxygen away from the nitrogen-fixing enzymes, which find oxygen toxic.
Thanks to this mutualism, the Rhizobia bacteria gets protection, oxygen and sugars, while the legume plant gets its precious nitrogen. It’s an exchange where both parties are winning.
Sometimes, a group of rhizobia try to cheat and steal all the services from the plant, without giving nitrogen back. In this case, the plant has a mechanism to detect and punish bad partners. It can stop the nutrient supply to those bacteria, and let the nodule die.
Today, humankind needs these nitrogen-fixing plants and bacteria. They’re needed to reduce pollution of the environment. Most of the nitrogen fertilizer we use are made in industrial, energy-consuming, ways. Getting help from legumes and rhizobia would make us less dependent on these polluting industries.
Thousand of legume plants are able to fix nitrogen thanks to this cooperation with bacteria. Most of the bacteria who can do this are still to be discovered. Many Sleeping Beauties are waiting, waiting for scientists to find them underground.
Hold on. Are they really waiting for scientists? Probably not.
All they want are safe, comfortable nodules to live in. Whatever happens in the human world above ground, it is not their concern. They simply wait for their Prince Charming root hairs to take them home.
And then, they live happily ever after.
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Sources and references for this article can be found here.