As genetic sequencing technologies have become cheaper and more plentiful, scientists researching the history of life on Earth have found it that much easier to learn about the answers to the many mysteries that still lay unresolved. A major piece of the puzzle involves the emergence of plant life onto land and out of the oceans, just as important of a transition as the eventual evolution of both insects and other land animals due to having plants to consume.
The Path To Land
That event occurred during the middle of the Paleozoic era some 400 to 500 million years ago. The first land plants were algae and mosses similar to those found in the ocean, having evolved from that complex green algal life, much of which still lives on in their direct descendents today. The closest living examples are the charophytic algae, whereby the land plant clade, Embryophyta, emerged from their original division, the Charophyta.
Similar body plans and the phragmoplast body structure, which are a series of microtubules in the plant cell that form as a scaffold during cell division to keep the cell wall hardened, were the coinciding factors between the land plants and the charophytes that first suggested this ancestral link. Which classes within the Charophyta were the most linked has been a subject for debate, however, with research over the past few years overturning prior claims and instead suggesting that the Zygnematophyceae class is the sister group to the land plants.
This is an interesting revelation for the tree of life, but is less helpful for biologists studying how terrestrial plants evolved. That’s because this specific class appears to have lost morphological complexity and many of their secondary structures over time, making them no longer reflect the rich biological systems they must have had when the land plants split off back in ancient times. So, while they are the most correct answer to the question of the origins of terrestrial plants, they are also the least useful.
A Global Investigation
Because of that, a collaborative team of researchers from Japan, Germany, the UK, Canada, Belgium, France, the United States, the Czech Republic, Denmark, South Africa, Austria, the Netherlands, and Australia decided to forego looking into any species from that class and instead went with what was originally thought to be the close relative class, the aptly named Charophyceae. These species still contain all the complexity that would be expected for terrestrialization to be possible within a population.
Thus, as a representative organism, they chose the genome of Chara braunii to be the one that provided answers to how land plants became possible and what biological alterations led to that form of selection being even feasible. Using RNA sequencing and complementary DNA, they were able to process and annotate the genome and its parts of interest.
They also used a meta-genomic analysis to identify bacterial DNA that had potentially contaminated the results and removed those additional sequences. The transcriptome was sequenced to aid in genome annotation and to check for duplication events in the existing genes.
Genomes Across Eras
The focus of their study was on the shared genes between C. braunii and land plants, which they decided to term land plant heritage genes (LPHGs). The model organism Arabidopsis thaliana was taken as a comparative land plant, due to its genome being one of the more well studied among land plant species. A major gene difference they found among cell wall construction was one named TANGLED1, which guides the aforementioned phragmoplasts to the right locations within the cell. Since all algae, including this charophytic one, lack this gene but still have phragmoplasts themselves, it is likely to be a gene that was highly relevant to terrestrial development.
Another important aspect was the inclusion of phytohormones. Since these allow plants to respond to environmental stimuli and control their own development cycles, they became especially relevant for land plant species and many of these hormones have presumed ancestral versions in related algal species. This was found to be true for the regulator hormone auxin, though it appears to have a different pathway and usage in C. braunii than in land plants at large. So, while the genes were likely present in the ancestor algae, the evolution of land plants included adaptation of it for new particular functions and ways of metabolizing it.
Homologs for ethylene pathways also exist in the algae and the capability to bind it, but without the entire setup of the signaling pathway that must have been constructed during land plant terrestrialization. When it comes to abscisic acid, only the very early precursors to making it seem to exist in algae, but it is likely to be produced directly from these precursors, rather than through the complex pathway that land plants formed later on. All of these differences showcase that the sequences and beginnings of the pathways were set up before the ocean and land plant split, but that true complexity of the pathways were made by land plants as selection pressures arose during their movement onto land.
Yet another crucial feature of land-based plants is their ability to use photorespiration, the metabolic process helping photosynthesis to function in an oxygen-rich atmosphere. Similar genes involved in the system, including close to full respiration activation, appear to have existed in the ancestral species and likely changed to fit what was needed during the trip to land and direct interaction with air over water.
A Peek Into Paleozoic Time
Significant amounts of other features, similarities, and presumed loss of functions were seen in the studied C. braunii genome. A relevant note is that the algae species appears to have more proteins than its related algae, but less overall than land plants. These secondary gains over others in their family likely came about from gene duplication and loss of other components. In many ways, while being one of the closest relatives to land plants, the inherent complexity in the algae also helped it to develop its own differences from not just land plants, but its own water brethren as well.
All in all, certain aspects, such as phytohormones, photorespiration, and not mentioned storage proteins, clearly were around back before the terrestrial split, only becoming more convoluted after that point. Other aspects of land plant genomes, such as their non-mobile vegetative stage and filamentous growth such as with their roots were systems that had no analog in algal species and had to be formed separately after terrestrialization.
The latter may have become critical characteristics later on, but it was nonetheless the former, the land plant heritage genes, that were the true helps in allowing plants to develop on land and they have those traits thanks to the oceanic algae that they came from. There is still much we can learn about this era of evolutionary history, with this being but the first step using genomics into that region. Modern technology will continue to allow us to probe into the history of humans, animals, plants, and life itself as we seek knowledge about how our world came to be.
Photo CCs: Chara sp reproductive structure from Wikimedia Commons