Fundamental Science: When evolution finds similar solutions by different paths

We live on a planet dominated by plants: about 80% of all living matter on the globe consists of a terrestrial plant. And to think that the biomass of all other forms of life amounts to around a quarter of that! But ecological relationships on Earth would be radically different were it not for the ability of these organisms to capture carbon dioxide from the air and transform it into sugars. To do so, they use sunlight and water through photosynthesis, a mechanism capable of transforming and accumulating the energy contained in the light that travels from the Sun to the Earth’s surface, in the form of chemical energy. It is easy to understand the importance of this phenomenon when even the movements of the fingers that type this text depend on this energy.

The ubiquity of photosynthesis, however, hides the molecular complexity behind the phenomenon, because not only water, light and carbon dioxide does a plant live. The chemical element iron, for example, is not only essential for the biochemical reactions that produce chlorophyll — the molecule responsible for absorbing solar energy in the form of photons — but it is also indispensable in the transport chain of electrons removed from water molecules during the process. The flow of these electrons in chloroplasts — the organelle where photosynthesis takes place — is the source of energy that fuels the capture of carbon from the air, and therefore the mechanisms that plants and algae use to obtain iron from the environment are essential for photosynthesis.

If, on the one hand, iron is the fourth most abundant element in the earth’s crust and the first in terms of mass, accounting for almost a third of the entire planet, its bioavailability for plant roots is quite limited. This happens because the pH of the soil and its oxygen content dictate the chemical form in which the iron is found.

In alkaline (high pH) and oxygenated soils, iron is rapidly oxidized and converted into insoluble ferric oxides. In acidic soils (low pH), it can break free from these oxides and become more soluble and can be captured by the roots. Land plants have proteins that, by pumping protons out of the cells, lower the pH around the roots, allowing them to absorb the soluble form of iron through the transporter protein IRT1, discovered in the 1990s. It was found that green algae also used such a protein to transport iron from the environment into cells.

It made sense to imagine that the gateway for iron in plant cells, the IRT1 protein, had evolved before plants and algae parted ways more than a billion years ago. However, a study that we developed at UFMG, in partnership with UFRGS, has just shown that the capture of this element by plants could not be more different. Our work, published in the journal New Phytologist, compared the complete genome of more than 50 species of plants and algae and found that, contrary to what was supposed, the IRT1 genes present in land plants and green algae do not have the same origin, the kinship between them is far more distant than presumed.

IRT1 proteins are part of a large family of metal transporters called ZIP. The metal that each ZIP protein transports varies between zinc, manganese and iron, with some transporters capable of transporting more than one of these. Our study showed that ZIP proteins specialized in capturing iron from the environment evolved at least twice independently: it is as if evolution had notched the entry opening for this element more than once, from different ports.

It is known that all land plants, from the largest trees to the smallest mosses, share a common ancestor. Our work shows that they use the same type of IRT1 to take up iron, while some more distant unicellular green algae use a different type of IRT1. Our data indicate that the type that acts on land plants was inherited from the first unicellular algae to leave the aquatic environment and survive on land, immersed in the atmosphere. These algae, known as charophytes, are the closest relatives of land plants.

This discovery suggests that the emergence of the mechanism that maintains the flow of iron necessary for photosynthesis and guarantees the dominance of plants in the Earth’s biomass occurred when their unicellular ancestors adapted in order to capture iron in primitive soils. This was before any organism that we would recognize as a plant existed, when the “forests” that covered the continents were still microscopic, composed of microalgae.

The story of how these microalgae gave rise to the plants that dominate terrestrial ecosystems today is the story of forests and how life definitively conquered the terrestrial environment. It helps to explain what a primate just descended from the trees does by writing this text.

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Luiz Eduardo Del Bem is a geneticist and professor at the Institute of Biological Sciences (ICB) at the Federal University of Minas Gerais (UFMG) and at the MSU-DOE Plant Research Laboratory (PRL) at Michigan State University (MSU).

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