Million years ago, land plants originated from an ancestral green alga that underwent crucial transformations to overcome the challenges associated with the transition from aquatic to terrestrial habitats (Figure 1). Among key adaptive traits that pushed the conquest of land, the ancestors of land plants evolved protection mechanisms to counteract environmental stresses (such as high irradiance, drought, and pathogen attack) and developed new strategies for nutrient uptake.
Regarding mineral nutrition, iron is an essential element for plant life because it is needed to produce chlorophyll – a crucial component for photosynthesis. Indeed, iron deficiency is a serious plant disorder that starts with leaf yellowing and can lead to the death of the entire plant!
Iron concentration and bioavailability depend on the habitat, being greater in freshwater than in seawater. Iron is also abundant in terrestrial environments, but it is found in the poorly soluble form Fe3+ and becomes unavailable if soil pH is higher than 6.5. However, plant roots can easily absorb the soluble form Fe2+, after reduction of the rhizosphere by specific enzymes. So, the main questions are…
How and when did green plants evolve mechanisms to uptake metals from early soils and to deal with iron shortage in terrestrial habitats?
To better understand which changes marked the evolution of land plants, the group of Evolutionary Genomics (see definition below) led by Professor Luiz-Eduardo Del-Bem, Department of Botany of the Institute of Biological Sciences, has studied the mechanisms controlling metal uptake in green organisms living in terrestrial and aquatic ecosystems. In their latest study, published in the New Phytologist, the authors investigated the origin and diversification of a group of proteins related to Zinc-regulated, Iron-regulated transporter-like Protein (ZIP) transporter IRON-REGULATED TRANSPORTER1 (IRT1), which are involved in Iron capture in several land plants and also in the chlorophyte that lives in stagnant water and damp soil.
Is the strategy of iron uptake conserved among all green organisms?
Evolution refines solutions to problems, rather than constantly creating new solutions. So an interesting question is, do all green organisms take up iron in a similar way? If they do, then that suggests that they all share a common ancestor that was able to take up iron. If there are different ways, then there was a divergence in how green organisms evolved deep in evolutionary time.
“ZIP/IRT proteins are everywhere and serve as transporters of metals like zinc, iron, and manganese. IRT1 was originally described in the model species Arabidopsis thaliana and similar proteins were later found not only in the crop species rice but also in the green alga Chlamydomonas reinhardtii. Therefore, it was assumed that iron uptake in algae and land plants was basically the same. However, it’s easy to absorb iron in water but it’s harder to get it in soil”, says Luiz-Eduardo Del-Bem.
When the authors had a deeper look at ZIP/IRT transporters, they discovered that green organisms could have acquired different strategies during evolution. By using bioinformatic approaches, Del-Bem and co-workers identified almost 500 homolog ZIP/IRT proteins in the genomes of more than 50 extant species belonging to the Plantae* kingdom. The authors constructed a phylogenetic tree (Figure 2) based on the similarities among the primary sequences of these IRT-related proteins, and found two deeply divergent clades, called X (IRTs from Arabidopsis, rice, and Marchantia) and Y (IRTs from Chlamydomonas).
This analysis indicates that generic metal transporters evolved to fulfil a more specific function in iron uptake at least two times (or maybe more) during plant evolution. The specialized version found in land plants could be traced back to charophytes, the ancestors of land plants that likely evolved these transporters when moving from aquatic to terrestrial environments.
What are differences and commonalities among different iron transporters?
Another interesting point is that critical amino acids for iron transport that were described in Arabidopsis are not conserved among Clade X and Clade Y proteins, corroborating that these transporters have the same function but not the same sequence.
Moreover, Wenderson Felipe Costa Rodrigues and Ayrton Breno P. Lisboa – the first and second authors of the article – analysed the predicted 3D structures of these proteins by combining standard procedures with new methods such as modelling by using AlphaFold. Results of the comparison come as something of a surprise as it seems that iron transporters of Clade X and Y are separated in the evolution but convergent in the structure.
Why is it so important to study the evolution of iron transporters in plants?
Iron is crucial for a plant life as its deficiency impairs photosynthesis and seriously affects plant growth. However, the strategies that green organisms employ for iron acquisition are not very well studied, despite their physiological and ecological importance. Increasing knowledge in this field can be an advantage for both basic and applied plant research.
“On one side, we are curious to know more about mineral nutrition in plants and to discover the function of the metal transporters we have identified. On the other side, novel findings can have important applications in agriculture as they can be used to achieve better functionality of metal transporters, thus optimizing iron uptake”, declares Del-Bem. For instance, new knowledge can be employed in biofortification programmes (i.e., crop improvement to achieve nutritional value of food) to ameliorate the distribution and localization of iron inside the plant body, for example in seeds.
Despite great advances achieved by using comparative genomics, plant evolution is still bewildering scientists…
To date, the “standard explanation” for land plant evolution has been that someday multicellular plant-like charophytes just crawled out of water and land plants emerged as the first terrestrial organisms of their lineage. However, Luiz-Eduardo proposes that probably the very first terrestrial green plant was a unicellular charophyte and not a big plant emerging from water (Figure 3). Indeed, these ancestral organisms (that still exist today) share several molecular machineries with land plants (e.g., they can synthesise Xyloglucan, use the same iron transporters, have similar systems to protect from pathogens), which appeared at similar time-point in evolutionary time.
My idea is that, if we go back in evolution, there was a time on Earth when land settings were full of micro-forests, tiny unicellular organisms where microalgae were doing photosynthesis and being the primary producers taking carbon from the air… similar to woodland of present time, but in microscale.
The very first terrestrial photosynthetic eukaryotes could be older, and likely simpler, than what people think. Indeed, scientists have estimated that the first land plant appeared 500 million years ago based on the fossils of the first vegetative body, but evolutionary genomics studies advance the critical time point to 700-750 million years ago based on the divergence of a particular group of charophytes that likely lived on land (Klebsormidiaceae). The main problem is that, theoretically, DNA can be stable for 1-2 million years, thus genome sequencing could be performed on a 38,000-year-old Neanderthal bone but not on “prehistoric” plants that are so much older.
What’s next in your research?
“The goal of this project was to understand how plant nutrition, and specifically iron uptake, evolved from water to soil. Yet, we would like to know much about these transporters and their mode of action with other metals (e.g., zinc). We are currently planning benchwork to figure this out. An easy and cheap way will be to test proteins in yeast (complementation of mutants in transporters), but we would also like to characterize proteins with unknown function in different species, red algae for example, although we are aware that it’s not so straightforward to carry out functional studies with non-model organisms”.
We cannot time travel to see the first plant plant, but we can see its after-effects. By examining plants from diverse clades, with distant common ancestors, genes almost act as a time machine to get us towards that early plant. It’s through the genes that Del-Bem and colleagues discovered that the Iron uptake mechanism based on ZIP/IRT1 transporters is ancestral to land plants, which have vertically inherited genes encoding these proteins from their last common ancestor. This study is part of a bigger research programme aimed at understanding which are the genomic changes that allowed terrestrialization of green plants. We still don’t know of it will ever be possible to solve the complete puzzle of plant evolution, but we will keep you posted/informed with latest discoveries.
READ THE ARTICLE
Rodrigues, W.F.C., Lisboa, A.B.P., Lima, J.E., Ricachenevsky, F.K. and Del-Bem, L.-E. (2023) “Ferrous iron uptake via IRT1/ZIP evolved at least twice in green plants,” New Phytologist. Available at: https://doi.org/10.1111/nph.18661.
Definition of evolutionary genomics
Luiz-Eduardo Del Bem: “We can think of a process similar to that used in comparative anatomy. For example, several textbooks report a classical figure that compare anatomical structures such as the arm of a human being, the paw of a dog and a wing of a bird… they are different, but we can recognise the same bones with different shapes.
In evolutionary genomics, we compare the genomes of different species and analyse what they have in common (or not) to understand how life changed across time.
It’s a way to go backwards and track the origin of life, infer how the ancestor was. Based on the tree of life, we can try to estimate how old a molecular system is by using complex techniques that aim to compare sequences of nucleotides in DNA or amino acids in proteins.”
*Plantae kingdom (or ARCHAEPLASTIDA, photosynthetic eukaryotes) is composed of:
Viridiplantae (green plants): aquatic green algae and land plants (embryophytes), which emerged from within the green algae.
Rhodophyta (Red algae): photoautotrophic organisms (oldest groups of eukaryotic algae) that are abundant in marine habitats but relatively rare in freshwaters, no terrestrial species.
Glaucophyta: unicellular algae living in freshwater and moist terrestrial environments.