The Nitrogen-Fixing Abilities of Cycads


Long before the first legumes came onto the scene, the early ancestors of Cycads were hard at work fixing atmospheric nitrogen. However, they don't do this on their own. Despite being plentiful in Earth's atmosphere, gaseous nitrogen is not readily available to most forms of life. Only a special subset of organisms are capable of turning gaseous nitrogen into forms usable for life. Some of the first organisms to do this were the cyanobacteria, which has led them down the path towards symbioses with various plants on many occasions. 

Cycads are but one branch of the gymnosperm tree. Their lineage arose at some point between the Carboniferous and Permian eras. Throughout their history it would seem that Cycads have done quite well in poor soils. They owe this success to a partnership they struck up with cyanobacteria. Although it is impossible to say when exactly this happened, all extant cycads we know of today maintain this symbiotic relationship with these tiny prokaryotic organisms. 

Cross section of a coralloid cycad root showing the green cyanobacteria inside.

Cross section of a coralloid cycad root showing the green cyanobacteria inside.

The relationship takes place in Cycad roots. Cycads don't germinate with cyanobacteria in tow. They must acquire them from their immediate environment. To do so, they begin forming specialized structures called precoralloid roots. Unlike other roots that generally grow downwards, these roots grow upwards. They must situate themselves in the upper layer of soil where enough light penetrates for cyanobacteria to photosynthesize.

The cyanobacteria enter into the precoralloid roots through tiny cracks and take up residence. This causes a change in root development. The Cycad then initiates their development into true coralloid roots, which will house the cyanobacteria from that point on. Cycads appear to be in full control of the relationship, dolling out carbohydrates in return for nitrogen depending on the demands of their environment. Coralloid roots can shed and reform throughout the lifetime of the plant. It is quite remarkable to think about how nitrogen-fixing symbiotic relationships between plants and microbes have evolved independently throughout the history of life on this planet.

Photo Credits: [1] [2]

Further Reading: [1] [2]


Of Gunnera and Cyanobacteria

Nitrogen is a limiting resource for plants. It is essential for life functions and yet they do not produce it on their own. Instead, plants need to get it from their environment. They cannot uptake gaseous nitrogen, which is a shame because it makes up 78.09% of our atmosphere. As such, some plants have developed very interesting ways of obtaining nitrogen from their environment. Some, like the legumes, produce special nodules on their roots, which house bacteria that fix atmospheric nitrogen. Other plants utilize certain species of mycorrhizal fungi. One family of plants, however, has evolved a symbiotic relationship that is unlike any other in the angiosperm world.

A  Gunnera  inflorescence

A Gunnera inflorescence

Meet the Gunneras. This genus has a family all to itself - Gunneraceae. They can be found in many tropical regions from South America to Africa and New Zealand. Some species of Gunnera are small while others, like Gunnera manicata, have leaves that can be upwards of 6 feet in diameter. Their leaves are well armed with spikes and spines. All in all they are rather prehistoric looking. The real interesting thing about the Gunneras though, is in the symbiotic relationship they have formed with cyanobacteria in the genus Nostoc.

Traverse section of a  Gunnera  stem showing cyanobacteria colonies (C) and the cup-like structures (S) where they enter the stem.

Traverse section of a Gunnera stem showing cyanobacteria colonies (C) and the cup-like structures (S) where they enter the stem.

Gunnera produce cuo-like glands that house these cyanobacteria. The glands are filled with a special mucilage that not only attracts the cyanobacteria, but also stimulates it to grow. Once inside the glands, the cyanobacteria begins to grow into the plant, eventually fusing with the Gunnera cells. From there the cyanobacteria earn their keep by producing copious amounts of usable nitrogen and in return, the Gunnera supplies carbohydrates. This relationship is amazing and quite complex. It also offers researchers an insight into how such symbiotic relationships evolve.

Photo Credit: [1] [2] [3]

Further Reading: [1] [2] [3] [4] [5]