Plant root exudates

Plant root systems can grow to be complex due to a variety of species and microorganisms existing in a common soil. Plants have adapted to respond to the soil conditions and presence of microbes through various mechanisms, one of which is the secretion of root exudates. This secretion allows plants to largely influence the rhizosphere as well as the organisms that exist within it. The contents of exudates and the amount of substance released is reliant on multiple factors, including the root system architecture, presence of harmful microbes, and metal toxicity. The species of the plant as well as its developmental stage can also influence the chemical mixture that is released through exudates. The contents may include ions, carbon-based compounds, amino acids, sterols, and many other chemical compounds. At sufficient concentrations, exudates are capable of mediating both positive and negative plant-plant and plant-microbe interactions.

The physiological mechanism by which exudates are released is not entirely understood and varies depending on the stimulus as well as the contents of the secreted exudate. Various types of root cells have been suggested to sense microbes or compounds in the soil and secrete exudates accordingly. One example of root exudation occurs when plants sense elicitors and prime for a stress or defense response. It is believed that elicitors, such as methyl jasmonate and salicylic acid, are sensed by receptors on root cap cells, often referred to as border cells. This induces a change in gene regulation, up-regulating specific defense or stress-response genes. This differential gene expression results in metabolic changes that ultimately result in the biosynthesis of primary and secondary metabolites. These metabolites exit cells in the form of exudates through transporters that vary depending on the chemical structure of the metabolites. The exudate secretion is then able to elicit a defense response against harmful microbes within the soil.

A specific experiment explored the role of root exudates in kin recognition in Arabidopsis thaliana. The researchers grew young seedlings in liquid media that contained exudates from either themselves, siblings, or non-siblings. They then assessed root length as well as the number of lateral roots that the seedlings produced in the different treatment groups. They found that Arabidopsis seedlings grown in non-sibling exudates produced more lateral roots compared to those grown in exudates of kin or self-origin. Additionally, roots appeared to grow shorter when grown in non-sibling exudates. These results suggest that the Arabidopsis seedlings were capable of detecting the exudates around them and responding accordingly. Researchers speculate that this ability may be beneficial for fitness, allowing plants to allocate fewer resources to competition when grown amongst kin.

While there is a legitimate current understanding of root exudates, further research and experimentation is still necessary to fully understand these mechanisms and how they may differ depending on the contents of exudates.

The Rhizosphere

The rhizosphere is the thin area of soil immediately surrounding the root system. It is a densely populated area in which the roots compete with invading root systems of neighboring plant species for space, water, and mineral nutrients as well as form positive and negative relationships with soil-borne microorganisms such as bacteria, fungi and insects. The rhizosphere is a very fruitful area since nearly 5% to 21% of all photosynthetically fixed carbon is transferred from plants to the rhizosphere via root exudates . Root exudates are seen as key mediators in the interaction between plants and soil microbiota.

Root exudates contain a wide variety of molecules released by the plant into the soil. They act as a signaling messenger that allows for communication between soil microbes and plant roots. Exudates influence several factors within the soil such as nutrient availability, soil pH, and recruitment of bacteria and fungi . All of these impact the relationships that plants have with each other as well as soilborne microorganisms. The most notable positive relationship is that of roots and mycorrhizae. It is estimated that 80-90% of plants are colonized by mycorrhizae in nature . Mycorrhizae are known to promote plant growth and increase water use efficiency. Plants establish these mutualistic relationships with bacteria and fungi by modulating the composition of the root exudates. While positive relationships like this do exist, it is worth noting that most microbes have incompatible interactions with plants. One of the main forms of negative relationships in the rhizosphere is allelopathy. This is the act of releasing phytotoxins into the rhizosphere that can influence neighboring plant’s growth, respiration, photosynthesis, metabolism, and water and nutrient uptake. Allelochemicals released by the roots do this by inducing changes in cell structures, inhibiting cell division and elongation, destabilizing the antioxidant system, and increasing membrane permeability .

One study examined the impact of sugar exudation rates from plant roots on bacterial and fungal symbiotic associations in the rhizosphere. They compared four plant families (Asteraceae, Brassicaceae, Fabaceae, and Poaceae) with different bacterial and fungal associations. They found that plant family was the most important source of variation in exudation rates and microbial community structure between plant species. These results allowed them to conclude that root symbiotic associations had great impacts on the rate of sugar exudation in the rhizosphere . Another study found that root exudation impacts microbial activities as well as the diversity of active microbiota involved in root exudate assimilation. While several relationships between root exudates and the rhizosphere have been observed and we see that root exudates play a major role in root-soil contact, the exact purpose of the exudates and the reactions they cause are still poorly understood.