Plant root exudates

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Plant root exudates are fluids emitted through the roots of plants. These secretions influence the rhizosphere around the roots to inhibit harmful microbes and promote the growth of self and kin plants.

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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, [1] presence of harmful microbes, and metal toxicity. The species [2] 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[ clarification needed ] both positive and negative plant-plant [2] 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 [1] 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[ clarification needed ] for a stress or defense response.[ clarification needed ] [1] 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. [1] 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. [1] The exudate secretion is then able to elicit a defense response against harmful microbes within the soil.

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. Quantifying how much photosynthetically fixed carbon is transferred to soil via plant root exudates is difficult, but 5% can be considered a rough estimate. [3] Root exudates are seen as key mediators in the interaction between plants and soil microbiota. [4]

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.[ citation needed ] Exudates influence several factors within the soil such as nutrient availability, soil pH, and recruitment of bacteria and fungi. [4] 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. [4] Mycorrhizae are known to promote plant growth and increase water use efficiency. [4] 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. [5] 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. [4]

The plant family (Asteraceae, Brassicaceae, Fabaceae, and Poaceae) is the most important source of variation in exudation rates and microbial community structure between plant species. Root symbiotic associations impacts the rate of sugar exudation in the rhizosphere. [6] Root exudation impacts microbial activities as well as the diversity of active microbiota involved in root exudate assimilation. [7] 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.

Mechanism and structure

Plants have developed various advantageous mechanisms to manipulate their habitats. This is important as a plants’ habitat is crucial towards their growth as it dictates energy, water intake, nutrient intake and others [1]. Thus, a mechanism known as exudation that has been used by plants to possibly manipulate its surroundings, has been found to be useful although it is not fully understood how plants utilize it. Nor is it understood if the process of exudations is truly advantageous or how it is controlled by plants. An example of this would be the maize species which is grown as an agricultural staple and thus is located in close proximity to other species of plants. [8] The maize plant releases exudates to deter herbivore attacks from pest by reducing its leaf nutrient value and as well repressing its size. [8] While a defensive mechanism this action can be problematic for farmers because repression of growth affects their bounties. A number of proposals to explain this mechanism have been offered however, they are just proposals and have not been fully developed and tested to assert their claims. One such claim is that root exudates are beneficial for defense. [1] While another claims that exudates can also recognize who is related to the plant and who is a stranger which promotes friendly competition. [1] Another claim asserts that plants are able to possibly adjust their source-sink allocation of resources and the process of exudation which promote positive effects for the plant's growth[2].

Primary metabolites that are released into the soil by plants consist of: amino acids, organic acids and sugars. These primary metabolites are thought to be primarily released through the root tip when the rhizosphere is negatively affected by stressors such as being nutrient poor. [1] This environmental sense of surroundings allows the plant to dictate when these metabolites should be released. [9] The mechanism described for this process is illustrated by facilitated diffusion from the root tip, this process requires the possible adjustment of the source sink conserves and this creates a pressure driven mechanism through the phloem. [10] Traveling through the simplistic pathway is the most common method as they can travel freely however while nearing their journey, they have to pass through a plasma membrane and to do this they need a transmembrane protein to complete the trip. [10] “The phloem unloads the primary metabolites through the plasmodesmata using both facilitated diffusion and pressure flow mechanics to push release at the root tip”. [10]

Another possible mechanism of release of exudates would be the plants ability to control “efflux of primary metabolites is controlled through distinct channels and carriers which in turn allow down regulation in response to gene expression and or post translational modifications” examples of such transporters are GDU, SWEET, and CAT transporters. [10] This mechanism also allows the reuptake of metabolites which requires active transport to bring them against the concentration gradient It is also thought that through the rhizosphere microorganisms can trigger root exudations by changing the gradient of the soil, causing a response from the plant to trigger root exudation. [9] While the study of primary metabolites still needs more work, the proposals displayed due seem to provide a logical explanation for the mechanism driving root exudation.

Secondary metabolites consist of a variety of small molecules, they are vast in the rhizosphere of plants and are used in numerous ways to benefit them.  Since these small molecules come in a variety of forms, they are able to pinpoint diverse targets in microorganisms, other plants and even animals. [11] An example of a secondary metabolite would be Flavonoids which have been noted to provide an important role in movement of auxin for growth, the development of shoots and roots, and in certain species of plant promotes communication between plants and symbiotic bacteria. [11] Another example would be long-chain hydroquinone who are transported through the plant via passive exudation. [11] Specifically, these metabolites are released using the simplistic and apoplastic pathways and then finally “through the pores in the tips of root hairs where they bind to soil particles and organic matter”. [11] They are also known to be involved with germination stimulation in certain species. [11] All in all, the role of secondary metabolites is still in need of research as their fate in the rhizosphere remains unknown after several days have passed.

In one study the scientist studied how plant root exudates can manipulate the environment of the rhizosphere which in turn causes feedback in the soil to the plant [1]. The plants they studied for this experiment were wheat and maize species which release a secondary metabolite known as Benzoxazinoids which is a defensive metabolite. [8] Benzoxazinoids can change the relationship in fungal and bacteria for roots, as well as repress plant growth, and cause an increase in signaling for defensive purposes such as preventing herbivory attacks. [8] The experiment also included mutant types of the wheat and maize families to test the lack of BX which is an activator for the response of signaling through Benzoxazinoids. What they found was that the rhizosphere was very different between mutant and wild type plants. The next experiment they did was to test benzoxazinoids to increase plant defense, to do this they measured growth and defense against herbivory in maize plants while using both mutant and wild type maize plants. [8] The results for this test indicated that there were less herbivory attacks and increased defense mechanisms through increased signal Salicylic Acid, Jasmonic acid and others. [8] There was also a decrease in sugar content in leaves and decreased size of stems. The conclusion for this experiment was that through the breakdown of products using Benzoxazinoids they can stimulate the soil to promote beneficial changes in the soil environment. [8]

Plants are extremely versatile and have been able to develop advantageous mechanisms to overcome the constant environmental changes throughout time. Despite being unable to move and flee like animals they are able to utilize other skills to gain nutrients, water, enact defensive mechanisms and possibly communicate with fellow kin. [1] This has been linked to a process known as root exudation which is a product of plants that is released from the root tips through the root hairs. [10] This can be in the form of primary metabolites which are directly related to plant growth or in the form of indirect secondary metabolites. Either way these have been shown to impact the way the plant interacts with its rhizosphere through manipulation of the rhizosphere environment, which enables symbiotic relationships as well as causing feedback in the plant to repress growth, promote friendly competition with kin, or aggressive behavior with unrelated plants. [10] The mechanism powering this response has many proposals, one being that exudate secretion is controlled through redistribution of concentration gradients by manipulating the source-sink in plants. [10]  It is also thought to be controlled by down regulation through distinct channels utilizing specific transporters which also allow reuptake of metabolites through active transport. [10] In order to focus on the ability of plants to regulate their secretion of exudates in the rhizosphere, scientists studied maize and wheat plants and their mutants to see how missing the activator for the respective metabolite [8] affected the release of exudates. They found that those missing the activator were more likely to be eaten by pests while showing normal growth. [8] Those plants with the activator showed repressed growth and lower nutrient uptake as a defense mechanism against the pest. This experiment displays that root exudates are capable of providing a beneficial response for a plant by providing a number of reactions to deter pests through defensive mechanisms and promotion of beneficial symbiotic relationships. More research is needed to specify the exact mechanisms and consequences of root exudation.

Kin recognition

Arabidopsis seedlings grown in non-sibling exudates produced more lateral roots compared to those grown in exudates of kin or self-origin. [12] Additionally, roots appeared to grow shorter when grown in non-sibling exudates. The Arabidopsis seedlings are capable of detecting the exudates around them and responding accordingly. [12] This ability may be beneficial for fitness, allowing plants to allocate fewer resources to competition when grown amongst kin.

Related Research Articles

<span class="mw-page-title-main">Root</span> Basal organ of a vascular plant

In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the surface of the soil, but roots can also be aerial or aerating, that is, growing up above the ground or especially above water.

1-Triacontanol (n-triacontanol) is a fatty alcohol of the general formula C30H62O, also known as melissyl alcohol or myricyl alcohol. It is found in plant cuticle waxes and in beeswax. Triacontanol is a growth stimulant for many plants, most notably roses, in which it rapidly increases the number of basal breaks. 1-Triacontanol is a natural plant growth regulator. It has been widely used to enhance the yield of various crops around the world, mainly in Asia. Triacontanol has been reported to increase the growth of plants by enhancing the rates of photosynthesis, protein biosynthesis, the transport of nutrients in a plant and enzyme activity, reducing complex carbohydrates among many other purposes. The fatty alcohol appears to increase the physiological efficiency of plant cells and boost the potential of the cells responsible for the growth and maturity of a plant.

<span class="mw-page-title-main">Arbuscular mycorrhiza</span> Symbiotic penetrative association between a fungus and the roots of a vascular plant

An arbuscular mycorrhiza (AM) is a type of mycorrhiza in which the symbiont fungus penetrates the cortical cells of the roots of a vascular plant forming arbuscules.

<span class="mw-page-title-main">Root hair</span> Part of plant root

Root hair, or absorbent hairs, are outgrowths of epidermal cells, specialized cells at the tip of a plant root. They are lateral extensions of a single cell and are only rarely branched. They are found in the region of maturation, of the root. Root hair cells improve plant water absorption by increasing root surface area to volume ratio which allows the root hair cell to take in more water. The large vacuole inside root hair cells makes this intake much more efficient. Root hairs are also important for nutrient uptake as they are main interface between plants and mycorrhizal fungi.

<span class="mw-page-title-main">Rhizosphere</span> Region of soil or substrate comprising the root microbiome

The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microorganisms that feed on sloughed-off plant cells, termed rhizodeposition, and the proteins and sugars released by roots, termed root exudates. This symbiosis leads to more complex interactions, influencing plant growth and competition for resources. Much of the nutrient cycling and disease suppression by antibiotics required by plants, occurs immediately adjacent to roots due to root exudates and metabolic products of symbiotic and pathogenic communities of microorganisms. The rhizosphere also provides space to produce allelochemicals to control neighbours and relatives.

<span class="mw-page-title-main">Soil respiration</span> Chemical process produced by soil and the organisms within it

Soil respiration refers to the production of carbon dioxide when soil organisms respire. This includes respiration of plant roots, the rhizosphere, microbes and fauna.

The laimosphere is the microbiologically enriched zone of soil that surrounds below-ground portions of plant stems; the laimosphere is analogous to the rhizosphere and spermosphere. The combining form laim- from laimos denotes a connecting organ (neck) while -sphere indicates a zone of influence. Topographically, the laimosphere includes the soil around any portion of subterranean plant organs other than roots where exuded nutrients stimulate microbial activities. Subterranean plant organs with a laimosphere include hypocotyls, epicotyls, stems, stolons, corms, bulbs, and leaves. Propagules of soil-borne plant pathogens, whose germination is stimulated by a plant exudates in the laimosphere, can initiate hypocotyl and stem rots leading to "damping-off". Pathogens commonly found to cause such diseases are species of Fusarium, Phoma, Phytophthora, Pythium, Rhizoctonia and Sclerotinia.

<span class="mw-page-title-main">Rhizobacteria</span>

Rhizobacteria are root-associated bacteria that can have a detrimental, neutral or beneficial effect on plant growth. The name comes from the Greek rhiza, meaning root. The term usually refers to bacteria that form symbiotic relationships with many plants (mutualism). Rhizobacteria are often referred to as plant growth-promoting rhizobacteria, or PGPRs. The term PGPRs was first used by Joseph W. Kloepper in the late 1970s and has become commonly used in scientific literature.

<span class="mw-page-title-main">Microbiota</span> Community of microorganisms

Microbiota are the range of microorganisms that may be commensal, symbiotic, or pathogenic found in and on all multicellular organisms, including plants. Microbiota include bacteria, archaea, protists, fungi, and viruses, and have been found to be crucial for immunologic, hormonal, and metabolic homeostasis of their host.

<span class="mw-page-title-main">Biofertilizer</span>

A biofertilizer is a substance which contains living micro-organisms which, when applied to seeds, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of primary nutrients to the host plant. Biofertilizers add nutrients through the natural processes of nitrogen fixation, solubilizing phosphorus, and stimulating plant growth through the synthesis of growth-promoting substances. The micro-organisms in biofertilizers restore the soil's natural nutrient cycle and build soil organic matter. Through the use of biofertilizers, healthy plants can be grown, while enhancing the sustainability and the health of the soil. Biofertilizers can be expected to reduce the use of synthetic fertilizers and pesticides, but they are not yet able to replace their use. Since they play several roles, a preferred scientific term for such beneficial bacteria is "plant-growth promoting rhizobacteria" (PGPR).

Soil microbiology is the study of microorganisms in soil, their functions, and how they affect soil properties. It is believed that between two and four billion years ago, the first ancient bacteria and microorganisms came about on Earth's oceans. These bacteria could fix nitrogen, in time multiplied, and as a result released oxygen into the atmosphere. This led to more advanced microorganisms, which are important because they affect soil structure and fertility. Soil microorganisms can be classified as bacteria, actinomycetes, fungi, algae and protozoa. Each of these groups has characteristics that define them and their functions in soil.

A bioeffector is a viable microorganism or active natural compound which directly or indirectly affects plant performance (biofertilizer), and thus has the potential to reduce fertilizer and pesticide use in crop production.

<span class="mw-page-title-main">Root microbiome</span>

The root microbiome is the dynamic community of microorganisms associated with plant roots. Because they are rich in a variety of carbon compounds, plant roots provide unique environments for a diverse assemblage of soil microorganisms, including bacteria, fungi and archaea. The microbial communities inside the root and in the rhizosphere are distinct from each other, and from the microbial communities of bulk soil, although there is some overlap in species composition.

<span class="mw-page-title-main">Phycosphere</span> Microscale mucus region that is rich in organic matter surrounding a phytoplankton cel

The phycosphere is a microscale mucus region that is rich in organic matter surrounding a phytoplankton cell. This area is high in nutrients due to extracellular waste from the phytoplankton cell and it has been suggested that bacteria inhabit this area to feed on these nutrients. This high nutrient environment creates a microbiome and a diverse food web for microbes such as bacteria and protists. It has also been suggested that the bacterial assemblages within the phycosphere are species-specific and can vary depending on different environmental factors.

Root mucilage is made of plant-specific polysaccharides or long chains of sugar molecules. This polysaccharide secretion of root exudate forms a gelatinous substance that sticks to the caps of roots. Root mucilage is known to play a role in forming relationships with soil-dwelling life forms. Just how this root mucilage is secreted is debated, but there is growing evidence that mucilage derives from ruptured cells. As roots penetrate through the soil, many of the cells surrounding the caps of roots are continually shed and replaced. These ruptured or lysed cells release their component parts, which include the polysaccharides that form root mucilage. These polysaccharides come from the Golgi apparatus and plant cell wall, which are rich in plant-specific polysaccharides. Unlike animal cells, plant cells have a cell wall that acts as a barrier surrounding the cell providing strength, which supports plants just like a skeleton.

<span class="mw-page-title-main">Mycorrhiza helper bacteria</span> Group of organisms

Mycorrhiza helper bacteria (MHB) are a group of organisms that form symbiotic associations with both ectomycorrhiza and arbuscular mycorrhiza. MHBs are diverse and belong to a wide variety of bacterial phyla including both Gram-negative and Gram-positive bacteria. Some of the most common MHBs observed in studies belong to the phylas Pseudomonas and Streptomyces. MHBs have been seen to have extremely specific interactions with their fungal hosts at times, but this specificity is lost with plants. MHBs enhance mycorrhizal function, growth, nutrient uptake to the fungus and plant, improve soil conductance, aid against certain pathogens, and help promote defense mechanisms. These bacteria are naturally present in the soil, and form these complex interactions with fungi as plant root development starts to take shape. The mechanisms through which these interactions take shape are not well-understood and needs further study.

Disease suppressive soils function to prevent the establishment of pathogens in the rhizosphere of plants. These soils develop through the establishment of beneficial microbes, known as plant growth-promoting rhizobacteria (PGPR) in the rhizosphere of plant roots. These mutualistic microbes function to increase plant health by fighting against harmful soil microbes either directly or indirectly. As beneficial bacteria occupy space around plant roots they outcompete harmful pathogens by releasing pathogenic suppressive metabolites.

<span class="mw-page-title-main">Plant microbiome</span>

The plant microbiome, also known as the phytomicrobiome, plays roles in plant health and productivity and has received significant attention in recent years. The microbiome has been defined as "a characteristic microbial community occupying a reasonably well-defined habitat which has distinct physio-chemical properties. The term thus not only refers to the microorganisms involved but also encompasses their theatre of activity".

<span class="mw-page-title-main">Plant holobiont</span>

Since the colonization of land by ancestral plant lineages 450 million years ago, plants and their associated microbes have been interacting with each other, forming an assemblage of species that is often referred to as a holobiont. Selective pressure acting on holobiont components has likely shaped plant-associated microbial communities and selected for host-adapted microorganisms that impact plant fitness. However, the high microbial densities detected on plant tissues, together with the fast generation time of microbes and their more ancient origin compared to their host, suggest that microbe-microbe interactions are also important selective forces sculpting complex microbial assemblages in the phyllosphere, rhizosphere, and plant endosphere compartments.

Saprotrophic bacteria are bacteria that are typically soil-dwelling and utilize saprotrophic nutrition as their primary energy source. They are often associated with soil fungi that also use saprotrophic nutrition and both are classified as saprotrophs.

References

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