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Plants depend on Earth’s gravitational pull to direct their growth, react to environmental stimuli, and stay anchored in the soil. However, the exact chemical process behind this ability remains unclear.
In a new study, Anastasia Teplova from the Friml group at the Institute of Science and Technology Austria (ISTA) investigates the mechanism behind this process. She and the team found that using the force of gravity, roots weave their way through the soil to provide a plant with both structural support and essential nutrients.
Taking a square Petri dish off her lab bench, Anastasia Teplova brings it to light. Inside, happily growing seedlings of the model organism in biology, the tiny mouse-ear cress (Arabidopsis thaliana, A. thaliana), are embedded in a nutrient-rich media. She picks up another Petri dish and sets the previous one down again.
She said, “Look closely. The seedlings here seem different from the others, don’t they?”
“Their fine roots, which normally grow downward, are directed in the opposite direction. They’re modified and lack three proteins from the protein family called NGR (Negative Gravitropic Response of Roots), which causes this phenomenon. The plant can still survive but has lost its ability to sense gravity.”
Vibrant green shoots of plants are reaching toward the sun to absorb as much light as possible. Beyond our line of sight, another world opens up, one in which roots are indiscernibly growing and perforating the ground. However, how do they manage to do that? “Gravitropism is a fairly intricate and well-regulated process,”
Shoots show negative gravitropism, growing upward against gravity, whereas roots show positive gravitropism, growing downward with the pull of Earth. With the aid of specialized plastids (compartments) known as “amyloplasts,” columella cells at the forefront of the root, sometimes referred to as the root tip, feel gravity in the root.
Teplova continues, “Amyloplasts are filled with starch and are way heavier than their surroundings. In a horizontal root, gravity causes them to accumulate on the lower side of the columella cells. This triggers a signaling cascade that results in an accumulation of the plant hormone auxin on the lower side of the root, causing downward bending of the root. Step by step, this facilitates the root’s gradual descent into the soil to access nutrients and water.”
Gravitropism schematic. Due to gravity, amyloplasts (small brown circles) sediment to the lower side (thin green lines) of columella cells (yellow) in the root tip, causing a redistribution of auxin (red) and downward bending of the root. © Anastasia Teplova/ISTA
Gravitropism model has largely stood the test of time. However, a number of aspects of the molecular interaction are still unclear, such as the relationship between auxin dispersion and gravity sensing. Teplova uses her PhD study to try to unravel that riddle.
Teplova goes right to work in the microscope laboratory. She examines the objective of a specially-made microscope and gently rotates the knobs to change the image. Teplova focuses on a mouse-ear cress seedling’s root while positioned on a platform.
Teplova said, “You can place a plant vertically into the microscope and then rotate it in a full 360-degree. We take live movies of the seedlings growing in the microscopy chamber to see how they respond to gravity.”
The proteins found in columella cells—invisible to the human eye—are the primary focus of the scientists’ investigation. They must first have a luminous dye label applied to them. For instance, in a recent paper by Teplova and Co., the researchers labeled NGR (loss of NGR proteins causes chaos in root growth) and then closely examined where it localizes inside cells in response to gravity detection.
“When we rotated the plant, NGR moved to the new bottom side of columella cells, along with the amyloplasts. Similarly, another protein called “D6 protein kinase (D6PK)”, which activates specific proteins that create the auxin flow, follows the same pattern. When testing mouse-ear cress lacking NGR, D6PK no longer relocates. In essence, if one mechanism fails to function, the other is also affected, thus suggesting an interplay between those two.”
The results provide insight into the processes that allow plant roots to change their development direction in reaction to gravity. They offer the missing piece that ties auxin distribution to amyloplast sedimentation. Numerous questions still need to be answered. Determining the interaction between these proteins will be one of the next steps.
Journal Reference:
I. Kulich, J. Schmid, A. Teplova, L. Qi & Jiří Friml. 2024. Rapid translocation of NGR proteins driving polarization of PIN-activating D6 protein kinase during root gravitropism. eLife. DOI: 10.7554/eLife.91523
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