When the roots of plants have learned to follow gravity

An essential adaptation to soil colonization by plants is the gravitropic growth of the roots, which has allowed the springs to reach water and nutrients and firmly anchor the plants in the soil.

In a new study by IST Austria, scientists provided detailed information on the evolution of an effective root rot gravitropic mechanism in seed plants.

In order for plants to thrive in this new environment, root systems must grow downward, respecting two fundamental goals: anchoring in the soil and a source of water and nutrients for the growth of parts of the plant on floor.

This mechanism – called gravitropism – has been widely considered in flowering plants, for example, Arabidopsis thaliana. Be that as it may, it has never been effectively examined in the plant kingdom and its early development remains a mystery.

The study offered a broader view of how and when root gravitropism evolved. For the study, scientists selected many plant species representing moss lines, lycophytes (mosses and firmosses), ferns, gymnosperms (conifers) and flowering plants and let their roots grow horizontally to monitor whether and when they began to bend down. gravity.

As a result, root growth due to gravity was exceptionally simple and slow in the most primitive terrestrial plants (mosses), as in basal vascular plants (lycophytes and ferns). Only seed plants (gymnosperms and flowering plants), which occurred about 350 million years ago, showed a faster and more productive type of gravitropism.

But, what allowed the rapid and efficient gravitropism of the roots in seed plants?

To determine the answer, scientists badyzed different phases of gravitropism: the perception of gravity, the transmission of the gravitropic signal and, finally, the response of growth itself. They found that two crucial components play a vital role.

The first was revealed to be an anatomical feature: plant organelles called amyloplasts – densely filled with starch granules – sedimented 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.

After recognition by amyloplasts, the signal of gravity is also transmitted from cell to cell by auxin, a growth hormone. In the genetic examinations, badysts distinguished a particular transporter molecule in the model plant Arabidopsis thaliana, PIN2, which regulates auxin flow and, as a result, root growth.

Although virtually all green plants carry PIN proteins, only the particular PIN2 atom of seed plants badembles on the pusher side of epidermal root cells. This specific localization – unique to seed plants – causes the polarization of the transporter cells, thus allowing the root to move auxin towards the shoot and thus to the displacement of auxin-based signaling. from the point of recognition of the gravity to the zone. growth regulation.

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.

Yuzhou Zhang, a postdoc of Professor Ji's group? Friml said: "Now that we are beginning to understand what plants need to develop a stable anchorage in order to reach the nutrients and water in the deep soil layers, we could eventually find ways to improve the growth of cultivated plants and other plants in very arid areas. Nature is much smarter than us; we have a lot to learn from plants that could be beneficial to us. "

The results of the study are published in the journal Nature Communications.