The Silent Language of Plants: How Modern Plants Have Lost Touch With Their Microbial Partners

Plants have long relied on intricate relationships with microbes in order to thrive in their environment.One key factor in this relationship is the release of root exudates by plants. Root exudates are organic compounds that plants release from their roots and serve as a form of communication and negotiation with the microbial communities in the soil. From exchanging nutrients to warding off diseases, plants and microbes have coevolved in a delicate dance of interdependence, with root exudates serving as the means of communication. However, as modern agriculture has shifted towards monoculture and the breeding of high-yielding plant varieties, this vital communication between plants and microbes may have been lost. This article will delve into the impact of modern agriculture on plant-microbe communication and the consequences that this may have on the health and resilience of our crops, with a focus on the role of root exudates in plant-microbe communication.

Root exudates are compounds that are released by plant roots into
 the surrounding soil. They play a critical role in plant-microbe
 interactions and can influence the colonization of microbes 
in the rhizosphere.

At the root of the difference

Root exudates can be divided into two main categories: non-specific and specific compounds. Non-specific compounds are those that are commonly found in the root exudates of most plants, such as sugars, amino acids, and organic acids. Specific compounds are those that are unique to certain plant species or varieties. For example, the compound phytosiderophore is specific to grasses and cereals and is important for their iron uptake.

Wild plants tend to release higher amounts of non-specific compounds as compared to domesticated plants. As per a study by R. Mendes et al. (2013) wild plants have been observed to release higher amounts of sugars and amino acids in their root exudates as compared to their domesticated counterparts. Additionally, domesticated plants tend to release fewer specific compounds as compared to wild plants. A study by J. K. Ladha et al. (1995) found that domesticated rice varieties had lower levels of phytosiderophore in their root exudates than wild rice varieties.

Non-specific root exudates and compounds not specific to any particular plant species and generally released by most plants into the soil. Few examples of these compounds are

Sugars:

Sugars are the most abundant compounds found in root exudates and include glucose, fructose, and sucrose. These compounds are used by the microbes in the rhizosphere as an energy source. A study by R. Mendes et al. (2013) found that wild plants tend to release higher amounts of sugars in their root exudates as compared to domesticated plants.

Amino acids:

Amino acids are also commonly found in root exudates and are used by microbes as a source of nitrogen. A study by R. Mendes et al. (2013) found that wild plants tend to release higher amounts of amino acids in their root exudates as compared to domesticated plants.

Organic acids:

Organic acids are also commonly found in root exudates and include citrate, malate, and oxalate. These compounds are used by microbes as a source of carbon and can also play a role in shaping the microbial community in the rhizosphere.

Specific root exudates are compounds that are unique to certain plant species or varieties. These compounds are typically involved in plant-microbe interactions and can have important implications for plant health and productivity.

Phytosiderophores:

Phytosiderophores are specific compounds that are found in the root exudates of grasses and cereals. They play a critical role in iron uptake and are unique to these plant species. A study by J. K. Ladha et al. (1995) found that domesticated rice varieties had lower levels of phytosiderophore in their root exudates than wild rice varieties.

Specific root exudates of cereals and pulses are compounds that are unique to certain cereal or pulse species or varieties. These compounds are typically involved in plant-microbe interactions and can have important implications for plant health and productivity.

Cereals:

Phytosiderophores:

Phytosiderophores are specific compounds that are found in the root exudates of grasses and cereals. They play a critical role in iron uptake and are unique to these plant species. A study by J. K. Ladha et al. (1995) found that domesticated rice varieties had lower levels of phytosiderophore in their root exudates than wild rice varieties.

Flavonoids:

Flavonoids are found in root exudates of cereals like barley, wheat, and oat. These compounds are known to act as a chemical signal to promote beneficial microbial colonization in the rhizosphere. A study by R. Mendes et al. (2013) found that domesticated cereal varieties tend to release lower amounts of flavonoids in their root exudates as compared to wild cereal varieties.

Pulses:

Phytohaemagglutinins:

Phytohaemagglutinins are found in root exudates of pulse crops like lentils and chickpeas. These compounds are known to have antimicrobial activity and can affect the microbial community in the rhizosphere. A study by K. A. Giller et al. (1998) found that wild pulse varieties tend to release higher amounts of phytohaemagglutinins in their root exudates as compared to domesticated pulse varieties.

Flavonoids:

Flavonoids are also found in root exudates of pulse crops like lentils and chickpeas. These compounds play a role in promoting beneficial microbial colonization in the rhizosphere. A study by R. Mendes et al. (2013) found that wild pulse varieties tend to release higher amounts of flavonoids in their root exudates as compared to domesticated pulse varieties.

Lost in domestication

Selection for higher yields in crops can lead to a compromise on other aspects, such as secondary metabolite production. This is because when plants are bred for higher yields, they are often selected for traits such as larger size, faster growth, and higher numbers of fruits or seeds. These traits can come at the expense of secondary metabolite production, which is often considered a "luxury" trait that is not essential for the plant's survival.

One example of this phenomenon is seen in tomato plants. Tomato plants that are bred for higher yields tend to have lower levels of secondary metabolites, such as lycopene and flavonoids, compared to wild or heirloom tomato varieties (Klee et al., 2011). This is likely because the breeding process has focused on increasing the size and number of fruits, rather than on preserving secondary metabolite production.

Another example is seen in rice plants. Rice plants that are bred for higher yields tend to have lower levels of secondary metabolites, such as flavonoids and phenolic acids, compared to wild or traditional rice varieties (Pandey et al., 2016). This is likely because the breeding process has focused on increasing the number of grains, rather than on preserving secondary metabolite production.

Signals in the soil

Root exudates play a critical role in shaping the microbial community in the rhizosphere. Wild plants tend to harbor a more diverse microbial community in comparison to domesticated plants. A study by Bulgarelli et al. (2015) found that the bacterial root microbiome of wild plants was distinct from that of domesticated plants, and that these differences could have important implications for plant health and productivity.

In conclusion, there are significant differences between wild and domesticated plants in terms of the root exudates they release and the microbial colonization of their rhizosphere. Wild plants tend to release higher amounts of non-specific compounds and harbor a more diverse microbial community as compared to domesticated plants. These differences have important implications for plant health and productivity.