Plants have colonized most of the Earth’s surface. So what is the key to their success?
People often think of plants as simple, meaningless life forms. They can live rooted in one place, but the more scientists learn about plants, the more complex and sensitive we realize they are. They are excellent at adapting to local conditions. Plants are connoisseurs who get the most out of those close to where they germinate.
Learning the intricacies of plant life is more than just arousing human curiosity. Studying plants is also about making sure we can still grow crops in the future, as climate change is making our weather increasingly extreme.
Environmental signals shape the growth and development of plants. For example, many plants use day length as a cue to trigger flowering. Roots, the hidden half of plants, also use cues from their environment to ensure their shape is optimized to seek water and nutrients.
Roots protect their plants from stresses such as drought by adapting their shape to find more water (for example, by branching to increase their surface area). But until recently, we didn’t understand how roots sense when there is water in the surrounding soil.
Water is the most important molecule on earth. Too much or too little can destroy an ecosystem. The devastating impact of climate change (as recently seen in Europe and East Africa) is making both floods and droughts more common. As climate change makes precipitation patterns increasingly erratic, learning how plants respond to water shortages is vital to making crops more resilient.
Our team of plant and soil scientists and mathematicians recently discovered how plant roots change their shape to maximize water uptake. Roots normally branch horizontally. However, when they lose contact with water (such as growing out of an air-filled space in the soil), they stop branching and the roots continue to branch only after they reconnect with the moist soil.
Our team found that plants use a system called hydrosignaling to manage where roots branch in response to water availability in the soil.
Hydrosignaling is the way plants sense where water is, not by directly measuring moisture levels, but by sensing other soluble molecules in plants that move with the water. This is possible only because (unlike animal cells) plant cells are connected to each other by small pores.
These pores allow water and small soluble molecules (including hormones) to move together between stem cells and tissues. When water is taken up by the plant root, it passes through the outermost epidermal cells.
External stem cells also contain a branching-promoting hormone called auxin. Water uptake mobilizes auxin into the inner root tissues, triggering branching. When outside water is no longer available, for example when a root grows out of an air-filled space, the root tip still needs water to grow.
So when roots can’t get water from the soil, they have to rely on water from their own veins deep in the root. This changes the direction of the water movement, allowing it to move outward which now disrupts the flow of auxin, the branching hormone.
The plant also produces an anti-branching hormone called ABA in the root veins. ABA also moves in the opposite direction of auxin with the flow of water. Thus, while the roots draw water from the veins of the plant, the roots also draw the anti-branching hormone towards themselves.
ABA stops root branching by closing all the tiny pores that connect the stem cells – just like blast gates on a ship. This seals the stem cells apart and stops the auxin from moving freely with the water, preventing root branching. This simple system allows plant roots to adjust their shape to local water conditions. This is called xerobranching (pronounced zero branching).
Our study also found that a plant’s roots use a similar system to reduce water loss as does its shoots. Leaves stop water loss by closing micro pores called stomata on their surfaces in drought conditions. Stoma closure is also triggered by the hormone ABA. Similarly, in roots, ABA reduces water loss by closing nanopores called plasmodesmata that connect each stem cell.
This is how tomato, cress, corn, wheat and barley roots respond to moisture, although they have evolved in different soils and climates. For example, tomatoes come from a South American desert, while thale cress comes from the temperate regions of Central Asia. This suggests that xerobranching is a common feature in flowering plants that are less than 200 million years younger than non-flowering plants such as ferns.
The roots of ferns, an early-growing type of land plant, don’t respond to water that way. Its roots grow more evenly. This suggests that the blooming species are better at adapting to water stress than previous land plants such as ferns.
Flowering plants can colonize a wider range of ecosystems and environments than non-flowering species. Given the rapid changes in precipitation patterns around the world, the ability of plants to sense and adapt to a wide variety of soil moisture conditions is now more important than ever.
This article has been republished under a Creative Commons license from The Conversation. Read the original article.
Malcolm Bennett receives funding from the UK Research Council BBSRC and is hosted by EU MCSA and EMBO member Poonam Mehra.
Poonam Mehra receives funding from EMBO and Horizon 2020