by You probably don’t think much about plant roots; after all, they are hidden underground. However, they are continually changing the shape of the world. This process occurs in your garden, where plants use invisible mechanisms for their endless growth.
Scientists discovered about 15 years ago that genes at the tip of the root (or more precisely, the level of proteins produced from some genes) appear to pulse. It’s still a mystery, but recent research is giving us new insights.
What we do know is that this oscillation is a basic mechanism underlying root growth. If we understood this process better, we would help farmers and scientists design or choose the best plants to grow in different types of soil and climate. With increasingly extreme weather conditions, such as droughts and floods, damaging crops around the world, it is more important than ever to understand how plants grow.
To really understand how plants grow, it is necessary to look at the processes that occur inside cells. There are numerous chemical reactions and changes in gene activity that occur all the time inside cells.
Some of these reactions occur in response to external cues, such as changes in light, temperature, or nutrient availability. But many are part of the development program of each plant, encoded in its genes.
Many people think that plants are green and pretty. Essential for clean air, yes, but for simple organisms. A sea change in research is changing the way scientists think about plants: they are much more complex and more like us than you imagine. This burgeoning field of science is too charming to do justice in one or two stories.
This article is part of a series, Plant Curious, that explores scientific studies that challenge the way you view plant life.
Some of these cellular processes have regular oscillations: some families of molecules appear and disappear rhythmically every few hours. The best-known example is circadian rhythms, the internal clock of plants and animals (including humans).
Read more: How understanding plants’ biological clocks could help transform the way food is grown
Natural cycles
There are many other examples of spontaneous oscillations in nature. Some are fast like the heartbeat and the mitotic cell cycle, which is the cycle of cell divisions. Others, such as the menstrual cycle and hibernation, are slow.
In most cases, they can be explained by an underlying negative feedback loop. This is where a process triggers a series of events that then suppress the same activity it triggered. This appears to be the case for pulsing root growth.
Shortly after the oscillation of the root tip gene was discovered, scientists noticed that this pulsation leaves an invisible mark. They discovered it using fluorescent markers visible under a microscope. These marks are left in places where the root can grow sideways. This means that they provide regular signals that cause the root system to take shape.
Its cause is unknown today, although scientists have ruled out theories that it may be due to circadian oscillations.
We know there are many feedback loops involved. A plant hormone called auxin appears to be crucial to the process. It wakes up some genes that code for proteins, such as those necessary for growth. Charles Darwin hypothesized the existence of auxin and its chemical structure was confirmed about 100 years ago.
The genes that oscillate are the “targets” of auxin. When auxin enters a cell, these target genes tend to become more active. Some of these genes are related to growth, but not all. Auxin triggers the removal of “repressors,” proteins that can block gene activity. Animals also have repressors in their cells.
But these repressors are activated by the genes they block. It could be that this feedback loop triggers the oscillations we see, but we don’t know for sure.
We know that auxin passes from one cell to another through an intricate network of transport proteins. The way proteins direct travel to parts of cells depends on the surrounding levels of auxin. This is another feedback loop. Pulsation occurs in growing roots, where cells at the tip continually divide as a result of the cell cycle (which involves separate feedback loops).
What an enigma
Scientists often turn to mathematics to help explain things. Researchers have used geometry since ancient times to study the visible part of plants. A branch of mathematics developed in the 19th century called Dynamical Systems Theory (DST), has given scientists some clarity about why plant roots oscillate. Scientists have been using DST tools to try to show how auxin patterns are affected by rounds of cell divisions.
If these rounds of cell division were well synchronized, we could show that, in theory, this would produce a regular pulse of auxin.
But this doesn’t solve the mystery because cells don’t usually divide all at the same time, so any auxin pulse would be quite irregular.
When my team looked under the microscope for fluorescent markers of auxin, we found a lack of regularity in auxin, in the parts of the root where its target genes oscillate regularly.
This suggests that root tip gene oscillation may be related to root growth, but does not occur at the same time as root stem cells divide.
Although it remains a mystery, we are now better equipped to decipher this enigma. The answer is probably not a single process, but the result of an interaction between several processes. We know the key players, but the rules of the game they play are yet to be discovered.
Read more: Why do cauliflowers look so strange? We’ve cracked the math behind its ‘fractal’ form
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Etienne Farcot does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond his academic appointment.
