Despite the ubiquitous nature of symbioses across the globe, evidence of their origins is scant to say the least. Mostly we look for clues of their origin hidden within the fossil record. Excitingly, a series of fossils discovered in Scotland reveal what very well be the early days of plant-cyanobacterial interactions. Thanks to these exquisitely preserved fossils, we now have the earliest record of an association between these two groups of organisms.
The fossils themselves date back to the early Devonian, some 400 million years ago. They hail from a hot spring community which allowed wonderfully detailed preservation of everything down to the cellular level. Needless to say, this was a drastically different time for life on this planet. Plants were really starting to dominate the landscape. In the case of the fossil discoveries in question, one plant in particular is the star of this show.
Meet Aglaophyton major. This odd looking plant would have been a common site in these sorts of habitats. It largely consisted of a small, leafless stem that branched as it ambled over the ground. These stems bore the stomata, which allowed gas exchange to occur. Every once in a while, a stem would throw up a reproductive structure called a sporangium, which housed the spores. At the ground level, the stems would occasionally produce root-like rhizoids that have been found in association with fossilized mycorrhizal fungi in the soil.
In total, A. major only stood about 18 cm in height. Though abundant, it was relatively small compared to some of the other vegetation coming online at this point in time. It is likely that A. major could tolerate occasional flooding. In fact, some have speculated that flooding may have been necessary for the germination of its spores. It's this periodic inundation with water that likely led to an interesting and tantalizing relationship with cyanobacteria.
Cyanobacteria are probably best known for their contribution of oxygen to Earth's early atmosphere. What's more, many also fix nitrogen. That is why the fossil discovery of A. major with cyanobacteria in and around its cells is so exciting. These 400 million year old fossils provide the first evidence of a plant and cyanobacteria in an intimate association.
As mentioned above, the fossilization process was so thorough that it preserved subcellular structures. After thin sectioning some A. major stems, a team of researchers found filaments of cyanobacteria in the process of invading the plant and taking up residence. The cyanobacteria appears to be entering the plant through the stomatal openings along the stem. Once inside, the cyanobacteria show signs of colonazation of substomatal chambers as well as intercellular spaces within the plants tissues.
Although the authors cannot say whether this association was mutualistic or not, it nonetheless represents a model situation detailing how such a symbiotic relationship could have evolved in the first place. Because the cyanobacteria in question here is thought to be aquatic, the only way for it to move into the plant would have been during periodic flooding events. The idea that this could be simply an infection following the death of the plant was considered. However, the non-random distribution of cyanobacteria within A. major cells suggests that this relationship was no accident.
For now, the relationship between A. major and cyanobacteria was likely an "on-again–off-again incidental association" centered around flood events. The fact that A. major was already associated with mycorrhizal fungi at this point in Earth's history certainly suggests that the genetic adaptations necessary for symbiotic relationships were already in place. Though it isn't a smoking gun, these fossils provide the earliest evidence of plants' relationship with cyanobacteria.
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