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One of the most important events in the history of evolution occurred about 500 million years ago, with the spread of plant life to the world. 39, water to the ground. In order for plants to thrive in this new environment, root systems had to move downward, following two main objectives: anchoring in the soil and a source of water and nutrients for the growth of aerial parts of the soil. plant. This mechanism – called gravitropism – has been studied extensively in flowering plants such as Arabidopsis thaliana. However, it has never been systematically compared throughout the plant kingdom and its evolutionary origin remains a mystery.
Low, low, low – but at a different speed
Now, Yuzhou Zhang, post-doctoral fellow in Professor Ji's group? Friml and his team gained a broader view of how and when root gravitropism evolved. The researchers selected several plant species representing moss lines, lycophytes (mosses and firmosses), ferns, gymnosperms (conifers) and flowering plants and allowed their roots to grow horizontally to observe if and when they began to bend to follow gravity. As a result, root growth due to gravity was found to be very rudimentary and slow in the most primitive terrestrial plants (mosses) as well as in basal vascular plants (lycophytes and ferns). Only seed plants (gymnosperms and flowering plants), which appeared about 350 million years ago, had a faster and therefore more efficient form of gravitropism.
The power of starch
But what stage of evolution allowed this fast and efficient root gravitropism in seed plants? By badyzing the different phases of gravitropism – the perception of gravity, the transmission of the gravitropic signal and, ultimately, the growth response itself – the researchers discovered two crucial components that evolved together. The first was revealed to be an anatomical feature: plant organelles called amyloplasts – densely filled with starch granules – sediments in response to gravity and thus functioning as gravity sensors. However, this sedimentation process was observed only in gymnosperms and flowering plants, with amyloplasts eventually concentrating in the root tip. In contrast, in older plants, amyloplasts remained randomly distributed within and above the root tips, thus not functioning as gravity sensors as was the case in plants.
A special PIN code for auxin
After perception through the amyloplasts, the signal of gravity is then transmitted from cell to cell by auxin, a growth hormone. In genetic experiments, the researchers identified a specific transport molecule in the model plant Arabidopsis thaliana, PIN2, which directs auxin flow and thus root growth. While almost all green plants carry PIN proteins, only the PIN2 specific molecule of seed plants gathers on the pusher side of epidermal root cells. This specific localization – unique to seed plants – leads to the polarization of the transport cells that allows the root to carry auxin towards the shoot and thus allows the auxin – based signaling of the plants to grow. move away from the location of the perception of gravity towards the growth control zone.
Plants as teachers for humanity
With these two anatomical and functional components identified, the authors have gained valuable information on the evolution of root gravitropism, one of the crucial adaptations of ground seed plants. But even the practical implications of these results are conceivable: "Now that we have begun to understand what plants need to develop a stable anchorage in order to reach the nutrients and water in the deep layers of the soil, we can eventually find ways to improve the growth of cultivated plants and other plants in very arid areas, "said Zhang, who joined IST Austria in 2016. He adds," Nature is much smarter than we are. there are so many things that we can learn from the plants that can benefit us. "
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Material provided by Institute of Science and Technology of Austria. Note: Content can be changed for style and length.
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