In the evolutionary history of Planet Earth, one of the most important events occurred about 500 million years ago with the diversification of Plantlife from water to land. Firstly, for Plants to survive on land, they had to adapt to the airy and water-scarce environment. In this process, another mysterious life form, Fungi, directly helped the Plantlife to adapt to the completely new environment. Before Plants came on land, Fungi created Plant-friendly environments by dissolving rocks of barren land, producing carbon-based gases and nutrients-based soil to help the Plantlife thrive on land.
One of the most constant environmental stimuli that a Plant encounters on land is gravity. Even with the help of Fungi, Plants also had to evolve to survive and thrive on land, for this, complex root system was mandatory to grow downwards, following gravity with two main purposes— 1) anchoring in the soil strongly and, 2) providing a source of water and nutrients for the growth of the parts of the Plant above the ground. This evolved mechanism of Plants sensing gravity is called gravitropism.
From previous studies, it is now evident that highly developed seed Plants manifested deep root systems with highly advanced gravitropism. From the beginning, since when and how the Plants evolved to sense gravity, remained unknown until now. A group of researchers from the Institute of Science and Technology Austria (IST Austria) has identified essential elements and processes which started to develop in seed plants around 350 million years ago.
Gravitropism – how plants sense gravity
Gravitropism, also known as geotropism, is a coordinated process of differential growth in Plants’ roots in response to gravity pulling on it. It can also be observed in Fungi. Gravity can be either “artificial gravity” or natural gravity. It is a general feature of all higher and many lower Plants as well as other organisms. Charles Darwin was one of the first to scientifically document that Plants’ roots show positive gravitropism and stems show negative gravitropism.
Gravitropism has been widely studied in flowering plants such as Thale Cress (Arabidopsis Thaliana). However, it has never been systematically compared throughout the Plant kingdom, and its evolutionary origin remains a mystery.
Research methods and findings
The research team lead by Professor Jiri Friml had demonstrated a broader insight into how and when root gravitropism evolved over time. To do this, the researchers selected multiple Plant species expressing the lineages of Lycophytes, Mosses, Ferns, Gymnosperms (conifers), and Flowering Plants. Then they put their roots to grow horizontally to observe if and when they started to bend downwards to follow the gravity.
The researchers found out that primitive Plants like Mosses as well as in the basal vascular Plants like Lycophytes and Ferns have a very rudimentary and slow gravity-driven root growth. On the other hand, the seed Plants like Flowering Plants and Gymnosperms showed a faster and thus more efficient form of gravitropism. According to the authors of the study, in the evolution of Plants, the Flowering Plants first appeared around 350 million years ago, and thus since then, the Plants have perfected gravitropism.
But which evolutionary step enabled this efficient and fast root gravitropism in seed plants?
How Plant roots sense gravity?
Through investigating and analyzing the distinct aspects of gravitropism such as gravity perception, the transmission of the gravitropic signal, and ultimately the growth response itself—the researchers found two crucial elements of this mechanism, which gradually evolved hand in hand.
According to the authors, the first element turned out to be an anatomical feature—Plant organelles called amyloplasts which are densely filled with starch granules that sediment at the bottom of the root tip in response to gravity, and this way function as gravity sensors. However, as mentioned above, this sedimentation process was only witnessed in the Flowering Plants and Gymnosperms, with the amyloplasts ending up highly concentrated at the very bottom of the root tip. By contrast, in primitive Plants, the amyloplasts remained scattered within and above the root tip, thus not functioning as well established gravity sensors as was the case in the Plants.
The second element is a functional one- the process of how PIN proteins activate the growth hormone auxin through amyloplasts, and thereby Plants perceive gravity and transmit the gravity signal from cell to cell by the auxin. In previous genetic experiments, the researchers identified a specific transporter molecule PIN2 in the model Plant Thale Cress, which directs auxin flow and thus regulates root growth. While PIN proteins can be observed in almost all green plants, only the specific PIN2 molecule in seed plants gathers at the shoot-ward side of the root epidermal cells. This specific localization is unique to seed Plants that lead to the polarization of the transporter cells, which in turn enables the root to transport auxin towards the shoot and, as a result, auxin-based signaling to shift from the place of gravity perception to the region of growth regulation.
This is the mechanism of gravitropism while a Plant is growing on land, but what if a Plant could grow in an environment where the gravity is tensed to zero?
Plants growing in space provide insights into root growth
In 2017, scientists untangled the competing influences of gravity and water on Plant roots by growing a cucumber during spaceflight.
On the land, Plant roots typically grow to find water, and this process is known as hydrotropism. As described above, roots are also influenced by gravity and tend to grow downwards. To find out whether water or gravity has the greater influence on root growth, scientists grew cucumber Plants in the microgravity environment onboard the International Space Station. In this experiment, scientists observed that hydrotropism or water had more influence in controlling root growth than gravitropism.
According to Dr. Hideyuki Takahashi, senior author of this study, the result of this study is paving the way for new research to find ways to utilize roots’ ability to sense moisture gradients for controlling root growth orientation and efficiently growing Plants in future space farms.
With these two crucial components identified, the research team has been providing valuable insights into the evolution of root gravitropism, which is one of the essential adaptation abilities of the seed Plants on land. Now that scientists have started to understand what Plants need to grow stable and anchored in order to reach water and nutrients in deep layers of the soil, it is possible that eventually, scientists may be able to figure out ways to improve the growth of crops and other Plants in very arid areas of the world.
Fungi and Plants evolved on land for millions of years, gathering wisdom of the ages to survive and thrive through immeasurable number of trial and errors, which the scientific world is gradually finding out. Every aspect of the Plants and Fungi world contain wisdom for humans to learn from through observation. According to Yuzhou Zhang, the lead author of the study, millions of years of evolution has made Nature much smarter than humans, and there is so much humanity can learn which can eventually benefit all of humankind.
- Morohashi, K., et al. (2017). Gravitropism interferes with hydrotropism via counteracting auxin dynamics in cucumber roots: clinorotation and spaceflight experiments. New Phytologist Foundation, [online] Volume, 215(4), p. 1476-1489. Available at: https://doi.org/10.1111/nph.14689 [Accessed 8th September 2021].
- Zhang, Y., Xiao, G., Wang, X. et al. (2019). Evolution of fast root gravitropism in seed plants. Nature Communication, [online] Volume, 10, p. 3480. https://doi.org/10.1038/s41467-019-11471-8 [Accessed 8th September 2021].