Discover the biochemical mechanisms that alert plants to drought and salinity, and how TOLERATE studies them to promote more resilient agriculture
How does a plant know it´s thirsty?
Have you ever wondered how plants are able to sense that they need water before their leaves begin to wilt? The answer lies beneath the soil.
Plants have developed an alert system based in their roots. This system acts as a true environmental sensor. In today’s context, where droughts are becoming increasingly frequent and severe, understanding how plant-soil interactions function is vital. Population growth has made it urgent to develop crops with greater resilience and improved resource-use efficiency.
Projects such as TOLERATE, a European initiative, study how roots and the microorganisms that surround them communicate to withstand stress. In our article The rhizobiome microbes reinforce plants in times of stress we describe how the rhizobiome acts as a support network for plant survival. Today, we take a closer look at how this response is triggered.
Abiotic stress as an alarm signal
For a plant, abiotic stress refers to any environmental factor that limits its growth or survival. There are many types of stress, but drought and salinity are currently the main challenges for the agricultural sector.
When a plant detects that the soil is dry, it activates an emergency response by closing its stomata. These are tiny pores in the leaves that allow gas exchange. By closing them, the plant prevents water loss through evaporation, retaining moisture but slowing its growth. On the other hand, high soil salinity causes osmotic stress, making it difficult for roots to absorb water, and leads to toxicity due to excess ions.
In Europe, large areas of land are underutilized because of these two factors.
The root as the plant´s brain
Roots function as the plant’s control centre in response to stress. The rhizosphere (see previous articles) is influenced by root secretions and serves as the hub of communication. Roots can detect subtle changes in soil moisture, salinity, and nutrient concentration. Depending on what they sense, they adjust accordingly. They may grow deeper in search of moisture or expand laterally.
How do they communicate?
To transmit these stress signals to the rest of the plant, roots rely on complex chemical reactions. We can distinguish between internal and external messengers.
They use stress hormones such as abscisic acid. The plant synthesizes this hormone in the roots when drought is detected, and it quickly travels to the leaves to signal stomatal closure. This hormone belongs to a group of molecules that help balance growth and survival under pressure, such as ethylene and cytokinins.
In addition, roots exude compounds such as sugars and amino acids into the soil. These compounds act as signals that attract beneficial microbes which assist the plant. Another example is the release of volatile compounds such as terpenes, which can alert neighbouring plants or attract fungi that enhance water absorption.
The rhizosphere: the root microbiome
In previous articles, we have discussed the microbiome that surrounds plant roots, a community of microorganisms that communicates with the plant.
There are bacteria such as PGPR (Plant Growth-Promoting Rhizobacteria) that help dissolve nutrients locked in the soil and produce molecules that enhance drought resistance. In addition, certain fungi form networks that help absorb water and nutrients from areas inaccessible to roots.
A fascinating detail is that microbes can use plant signalling for mutual benefit. Some microorganisms produce their own phytohormones, helping the plant remain stable during episodes of mild stress, allowing it to continue growing. They act as modulators of signalling pathways.
TOLERATE: science for the future
The TOLERATE project represents the cutting edge in this field of study. Its approach of analysing ancient DNA is groundbreaking. It seeks answers in the past to solve future challenges.
By combining multiple analyses, it examines how rhizosphere biodiversity responded to past climate changes and extreme events. The aim is to identify how organisms survived and apply this knowledge through controlled bioaugmentation, using bacteria designed to express traits that benefit crops.
The future of agriculture
The knowledge generated by this project has concrete applications, such as the development of intelligent biofertilizers. Instead of conventional chemical fertilizers, specific microbial consortia are being developed to act when the plant requires them. Additionally, precision tools could be created to detect biochemical signals before stress becomes visible, for example, stress sensors that help optimize irrigation, fertilization, and other management practices.
These technologies are essential to meeting European goals for sustainable and resilient food production.
An ongoing conversation
Roots are not passive organs, but rather the centre of a constant dialogue between the plant, the soil, and the microorganisms with which it coexists. Every time a plant withstands drought or grows in saline soil, it is thanks to this complex system of biochemical communication.
Projects such as TOLERATE remind us that the most advanced technology can emerge from ancestral biological knowledge. The answers are often found in the past and learning how to apply them is key to our survival.
The resilience of our agriculture depends on whether we learn to listen to what roots can tell us.