Looking at the abundance of fruits and vegetables on the supermarket shelves, have you ever thought about how much water or fertilizers were used to produce them? Nowadays we are lucky to have such availability of products. But what would happen if, for example, wheat or tomato fields no longer received enough water for a long period of time? 

Prolonged water deficiency would impact the seed germination of these crops or the production of fruits and grains [1]. 

In fact, water is fundamental for many plant metabolic processes, such as photosynthesis, multiplication and growth of cells, and uptake of nutrients [1]. Some plants, such as succulents – the ones that usually live the longest in our apartments -, have evolved strategies to retain more water in their tissues against drying out [2]. However, many crops which are important for our diet did not adapt to live in very dry environments, and are currently threatened by drought events, especially in warm and arid regions [3,4]. The current situation and the prediction that drought will be more severe and frequent in the future due to climate change [4], not only pose a threat to farmers’ profits but also to the quality and availability of the food we consume. 

One might think: “why not just apply more water to the fields?”. This approach could work temporarily in certain contexts but not in regions where water availability is already limited. Therefore, to avoid depleting important resources, scientists and farmers have been looking for sustainable alternatives to grow crops under precarious availability of water. One way is to explore the interactions that plants establish with microorganisms belowground [3]. 

Plants interact with many different microbes, namely bacteria, archaea, fungi and protists, which can be beneficial or pathogenic and can be found on both above- and below-ground plant organs [5]. Plants influence the communities of microbes living underground by releasing characteristic molecules called “exudates” through their roots. Microbes are attracted by these molecules and establish interactions with plants in the endosphere (i.e., inside the root tissues), in the rhizosphere (i.e., the soil surrounding the roots) or at a distance by exchanging volatile molecules that travel through the soil pores [5,6]. These interactions have evolved over time and contribute significantly to many vital processes for both plants and microbes [5]. For example, some microorganisms help to break down complex organic compounds in the soil to make nutrients more available to plants, such as ammonium (NH4+) and nitrate (NO3–) that plants use to obtain nitrogen, a fundamental element to build their organs [7]. 

When plants grow under drought, they not only activate their own internal mechanisms to resist water deficiency, but also change the interaction with microbes in the soil. Studies have shown that plants start releasing more or specific exudates containing carbon and other elements, which are substrates that microbes feed on. It is as if plants start “crying for help” to attract microbes that can aid them to survive under stress [8,9]. Microbes do so through various mechanisms, such as promoting the solubilization and exchange of nutrients with plants or stimulating the production of plant compounds that improve tolerance to drought [8].

For example, some fungi establish symbiotic interactions with plants by growing inside the root cells. Attracted by carbon and specific plant molecules produced under drought, these fungi extend their filamentous body structures, called “hyphae”, even more into the roots to improve the direct exchange of water and nutrients with the plant [10]. Bacteria play an important role under drought as well. For example, the Actinobacteria group can promote the absorption of water and nutrients by roots, and influence the production of specific hormones involved in plant development [11]. Other bacteria belonging to the group Bacillus, can induce the production of specific compounds, such as proline, that protect the activity of proteins and the structure of plant cells under drought [12].

Over the past decades, researchers identified various microbial mechanisms and types of interactions that help plants to grow under limiting conditions, including drought. Specific microbes and molecules have been isolated and supplied to plants to test how much they could help plants to tolerate water scarcity [13-15]. For example, a study showed that providing maize plants with a mix of two specific fungi and one bacterium can improve plant growth under drought thanks to the production of antioxidant compounds [16]. In fact, experiments in the greenhouse and in the field showed that beneficial microbes supplied to plants growing under drought, could increase their yield and biomass by 40-45% [17]. For this reason, identifying and exploiting beneficial plant-microbe interactions is one way to more sustainably approach the reduction of yield that we are facing worldwide due to drought. 

Biological products based on microorganisms have been developed and successfully commercialized as biofertilizers in some countries, such as Brazil [18]. Other combinations of soil microorganisms have also shown to be effective under semi-arid conditions in the field [19]. However, there are still challenges to face. First of all, many studies have tested isolated beneficial microbes in controlled settings. When these microbes are introduced in the field, they have to adapt to new conditions, interact with other plants and co-exist with microorganisms which already inhabit that soil. Thus, microbial products that work in the greenhouse might not work in the field, and products that work in a field in Switzerland, might not work in the same way in a field in India. Hence, it is important to perform more studies in the field [14] and identify beneficial microbial communities within the specific site of interest. Secondly, plant-microbe interactions cannot be an isolated solution. This approach needs to be combined with good agricultural practices that help plants to deal better with water scarcity and that maintain healthy communities of microbes in soil [20]. 

Therefore, the next time you are in a supermarket, try to guess how much water and nutrients were used to produce those beautiful apples or tomatoes you put in your cart. And imagine how different the situation could be if important resources were depleted and no sustainable solutions were available.