Getting to the root of plant survival

Faced with an unstable climate, nature is looking for a way to survive. For plants, this often means that they have to extend and deepen their new roots in search of water, especially in times of drought. While scientists have recognized the process of root emergence for decades, it was not previously known how intercellular communication could be at the origin of this phenomenon.

Today, Jung-Youn Lee, professor of plant molecular and cellular biology at the University of Delaware, and Ross Sager, a graduate student and alumnus of the university, have identified the hormones and proteins that interact to regulate the process of root emergence.

The study, the first of its kind, conducted by their team was recently published in the journal Nature Communications.

Plant communication

Plasmodiums are the main communication pathway within a plant, sending messages through virtually every cell, from root to shoot. Often, each cell receives new information and this intercellular communication is essential for the plant's survival.

"Imagine a brick wall and this is what the surface of a root looks like. Each brick is an individual cell. The cement that binds them together is the cell wall. But unlike a brick wall, plant cells are also connected by thin-wire nanotunnels called plasmodesmata, through which the cells transmit various signals and messages to be shared. Obviously, when the channels are closed, no signals are transmitted, which prevents the cells from receiving messages from their neighbours," Lee said.

When lateral roots, or secondary roots, need to emerge from the primary root, the cells directly above the emerging root need to separate from each other to make room. To do this, the plasmodesmata connecting these soon-to-separate cells must be closed so that the new root emerges at a normal rate. If the plasmodesmata remain open, the new root emerges at a faster rate, which can compromise the root's vitality or immunity and make the plant vulnerable to threats from various soil pathogens.

Regulating root emergence

While studying the expression pattern of PDLP5 - a plasmodesmate-associated protein - in Arabidopsis seedlings, Sager noticed an unexpected pattern in the roots. Closer examination revealed that the pattern involved cells covering the emerging lateral roots.

"I had designed this particular experiment to study the expression of PDLP5 in young seedling leaves," said Sager, "but when I noticed this pattern in the roots and showed it to Dr. Lee, we agreed that it was unique for a plasmodesmatous protein and warranted further investigation.

A microscopic cross-section of the primary cells in the roots illustrates the cell separation during lateral emergence of the roots. Highlighted dots show accumulations of PDLP5 that close the plasmodetamal connections of superimposed primary root cells. Credit: Monica Moriak/ University of Delaware
The pursuit of this intriguing scheme led Sager and Lee to discover a critical feedback loop that appears to allow small subsets of cells to regulate their plasmodem connections via PDLP5, allowing them to function independently of the rest of the plant as the lateral root develops and emerges.

When auxin, the hormone that directs the formation of lateral root tissue, signals plant cells that a new root is ready to form and emerge, it also signals the cells directly overlying the newly formed root to begin producing PDLP5. As this protein accumulates, Sager and Lee say it closes the plasmodesmata connections, ensuring that these overlapping cell layers are able to function autonomously when they separate and allow the lateral root to pass. When the process is complete, this research suggests that PDLP5 sends a feedback signal that represses the auxin. Once the new lateral root has fully emerged, the overlying cells reopen plasmodesmata connections, effectively reconnecting to the plant's communication pathways.

"Although our research suggests that plasmodial closure in these cells is important for lateral root emergence, we still don't really know why," said Dr. Sager. "Does it alter the movement of key signalling components? Does it prevent harmful soil factors from entering the cell? I look forward to other scientists using our paper as a springboard to answer these questions.

Opportunity due to climate change

According to Lee, this signaling mechanism and feedback loop could pave the way for revolutionary advances in plant and crop engineering.

"Understanding the individual components that regulate lateral root emergence, both sequence and timing, opens up many possibilities," said Dr. Lee. "When there is a drought, plants and crops die because they can't find water quickly and efficiently. One of the mechanisms they use to survive the drought is to put down more roots. With this discovery of the communication loop that regulates lateral roots, we may one day be able to control when and how many more roots a plant can form".

Crops are often adapted to the environments in which they grow. But as climate change continues to make patterns more erratic, such as longer dry seasons, plant adaptability will be vital for agricultural production and the survival of ecosystems. Roots may need to grow at different rates or at different times. Dr. Lee notes that there is not yet any information on engineered crops that can germinate such roots, but it is important first to determine precisely how the roots emerge.

Exploring communication between plants at the molecular and cellular levels continues to be the main focus of Lee's laboratory at the Delaware Institute of Biotechnology. As a result of this and previous research on cellular communication, Lee and his team are now further exploring PDLP5 and similar proteins.

"PDLP5 was our chance," Lee noted. "This protein has opened up so many new avenues for us and newcomers to explore. It has also become a fantastic bridge to great research collaborators, including Dr Malcolm Bennett of the University of Nottingham, the world's leading expert on root branching.

"What's next?" Lee continues. "We are currently conducting interdisciplinary research with Dr. Li Liao in Computational Science and Engineering at UD to find out how PDLP5 and its family members find and anchor themselves to plasmodesmata, which is generously funded by the National Science Foundation. We are already so amazed at the path that PDLP5 is taking us.