You probably don’t think much about plant roots; after all, they are hidden underground. Yet they continually change the shape of the world. This process takes place in your garden, where plants use invisible mechanisms for their endless growth.
Scientists discovered about fifteen years ago that genes at the root tip (or more precisely, the level of proteins produced by some genes) appear to pulse. It’s still a bit of 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, it would help farmers and scientists design or choose the best plants to grow in different types of soil and climate. As weather becomes more and more extreme, such as droughts and floods, damaging crops around the world, understanding how plants grow is more important than ever before.
To really understand how plants grow, you have to look at processes that take place in cells. Numerous chemical reactions and changes in gene activity are constantly taking place in 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 each plant’s developmental program, encoded in its genes.
Many people think of plants as nice-looking vegetables. Essential for clean air, yes, but for simple organisms. A step change in research is changing the way scientists think about plants: they are much more complex and more similar to us than you might think. This flourishing field of science is too delightful to do justice to in one or two stories.
This article is part of the Plant Curious series, which explores scientific studies that challenge the way you look at plants.
Some of these cell 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 in plants and animals (including humans).
Read more: How understanding plants’ body clocks can help transform the way food is grown
Natural cycles
There are many other examples of spontaneous oscillations in nature. Some are fast, such as the heartbeat and the mitotic cell cycle, the cycle of cell divisions. Others, such as the menstrual cycle and hibernation, are slow.
Usually they can be explained by an underlying negative feedback loop. This is where a process sets off a chain of events, which then suppresses the activity that triggered it. This appears to be the case for the root growth pulsation.
Shortly after the root tip gene oscillation was discovered, scientists noticed that this pulsation leaves an invisible trail. They discovered this by using fluorescent markers that are visible under a microscope. These spores remain in places where the root can grow sideways. This means that they regularly give signals that lead to the root system taking shape.
Its cause is unknown today, although scientists have ruled out theories that it could be caused by circadian oscillations.
We know there are a lot of feedback loops involved. A plant hormone called auxin appears to be crucial to the process. It wakes up a number of genes that code for proteins, such as those needed for growth. Charles Darwin postulated 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 causes the removal of ‘repressors’, proteins that can block activity in genes. Animals also have repressors in their cells.
But these repressors are activated by the genes they block. It may be that this feedback loop is causing the oscillations we see, but we don’t know for sure.
We know that auxin moves from cell to cell via a complex network of transport proteins. The way proteins travel to parts of cells depends on the surrounding levels of auxin itself. This is another feedback loop. The pulsation occurs in growing roots, where the cells at the tip are continually dividing as a result of the cell cycle (which involves separate feedback loops).
What a mystery
Scientists often turn to mathematics to help explain things. Researchers have used geometry to study the visible part of plants since ancient times. 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 used DST tools to try to show how auxin patterns are affected by rounds of cell divisions.
If these rounds of cell division were properly 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 all divide at the same time, so any pulsation of auxin would be quite irregular.
When my team looked under the microscope for fluorescent auxin markers, we discovered a lack of regularity in auxin, in the parts of the root where the target genes regularly oscillate.
This suggests that the root tip gene oscillation may be related to root growth, but does not occur at the same time as root stem cells divide.
Although still mysterious, we are now better equipped to decipher this enigma. The answer probably does not lie with a single process, but with an interplay between different processes. We know the protagonists, but the rules they play have yet to be discovered.
Read more: Why do cauliflowers look so strange? We’ve cracked the math behind their ‘fractal’ shape
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 their academic appointment.