Hormones respond to the presence or absence of water, allowing roots to stop growing when they lose contact with it
Roots are central to plant growth and recent research has shown just how efficient they are and how precisely they forage to find water, minimize water stress and adapt their shape while branching out to secure moisture.
Now, researchers at the University of Nottingham in the United Kingdom have discovered how roots pause their branching when they lose contact with water and only resume when they reconnect. The research underscores plant roots’ plasticity in their ability to change branching patterns to find soil water.
This water sensing by roots has been termed hydro-signalling by the researchers and their study has shown how hormone movement responds to the presence or absence of water.
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“We are fascinated by how plant roots are able to sense and respond to changing environmental conditions like water stress, enabling them to modify their shape to adapt and thrive under these new conditions,” said Malcolm Bennett, professor in plant sciences at the University of Nottingham.
“A Belgian colleague originally observed that roots are able to sense when they lose contact with water and then cease to branch until they reconnect with moist soil but (he) did not understand exactly how this took place.”
Working with colleague Poonam Mehra, postdoctoral fellow in biosciences, the researchers used X-ray micro-CT imaging and were able to show that roots alter their shape in response to external moisture availability by linking the movement of water with plant hormone signals that control root branching.
“Our X-ray imaging set-up works on the exact same principle as medical X-ray scanners except we use much higher levels of X-rays to reveal very high-resolution features in soil like roots, soil particles and water,” said Bennett.
“To image root responses to soil moisture, we designed roots to grow through a small (two centimetre) air-filled gap, which temporarily exposed the root tip to no moisture until it reached the other side of the gap and touched moist soil.”
This transient exposure is enough to cause roots to stop branching, which makes sense from a plant’s point of view since roots can only take up nutrients if they are soluble.
“This simple yet elegant mechanism enables plant roots to fine-tune their shape to local conditions and optimize foraging,” said Mehra.
The hydro-signalling mechanism refers to how plants appear to sense water, not only by measuring moisture levels directly but by sensing other soluble molecules that move with the water within plants.
“This is only possible because (unlike animal cells) plant cells are all interconnected by small pores that enable water and small soluble molecules, including hormones like auxin and ABA, to move together,” said Bennett.
He said that, when water is being absorbed by the plant root, it is taken up by the outermost cells that contain the hormone auxin. The hormone is mobilized inward to tissues, triggering root branching.
But even when water is no longer available, for instance when a root grows through a dry, air-filled space in the soil, it still needs water to grow. Roots then rely on a water supply from their own veins deep inside the root. This water comes from the vascular tissue that distributes nutrients in the plant.
When this happens, water must flow toward the root tissues. This reverse flow carries the hormone ABA, which closes off the small pores connecting root cells and blocks the growth hormone auxin from initiating root branching.
Auxin itself normally moves inward with water, moving from the outermost layer of the root called the epidermis. The movement normally triggers lateral root branching in inner tissues called the pericycle.
But when ABA closes off those small pores, it blocks the ability of auxin to reach the pericycle, causing root branching to cease. Then, when roots reconnect with moist soil and restore inward water flow, ABA weakens, allowing auxin to resume root branching.
“Our discoveries have identified the key genes, signals and processes that enable roots to respond to one form of water stress,” said Bennett. “Remarkably, these genes, signals and processes also appear to be important during other water stress responses like hydrotropism, when roots sense where water is available and grow toward the source. They also sense soil compaction where roots cease growing when they encounter hard soils, making it difficult to extract water.”
Bennett termed the ability of plants to cease root branching in the absence of moisture as “xero-branching.”
“I came up with the name xero-branching after living in Tuscon, Arizona, where gardens are xero-scaped using native plants that do not require watering,” he said.
“When we observed what happens when plant roots are no longer in contact with soil moisture and cease branching, the term xero-branching made sense particularly as, phonetically, it explains the root branching response perfectly.”
He said that understanding how plants sense and respond to xero-branching has provided new insights and tools into other more challenging water stress responses that are critical for crops and for securing water in stressful field conditions.
“We are very keen to understand the earliest stages of the xero-branching response,” he said. “What triggers water (and ABA) flowing outwards from the phloem? Is this triggered after the root tip ‘senses’ water stress? If so, how does the plant sense this? We have several ideas that we are actively researching.”
While drought is a natural part of the climate cycle, climate change is causing droughts to become more frequent, severe and pervasive. The past two decades have seen exceptional droughts across the Prairies and some of the driest conditions on record in the western United States. Protecting crops from further environmental stress while maximizing crop yields will become more challenging.
“Our plant research is vitally important for understanding how we can futureproof crops and find ways to ensure successful crop yields even in the most challenging climates,” said Bennett. “We are already experiencing a hotter climate and designing plants that can still access water in these conditions is vital. This research is an all-important step in understanding how to do this.
“These new discoveries were only possible because of the cutting-edge tools and collaborative approaches of the authors, which involved an international team of scientists based in the U.K., Belgium, Sweden, U.S.A. and Israel.”
The research was published recently in the journal Science.
