Can Plants Think?
The definition of the Rhizoshere is as follows: The rhizosphere is the narrow region of soil that is directly influenced by root secretions and associated soil microorganisms. Soil which is not part of the rhizosphere is known as bulk soil. The rhizosphere contains many bacteria that feed on sloughed-off plant cells, termed rhizodeposition, and the proteins and sugars released by roots. However, there is an incredible story behind that definition…
The rhizosphere is a densely populated area where the roots must compete with the invading root systems of neighboring plant species for space, water, mineral nutrients, and soil-borne microorganisms, including bacteria, fungi, and insects feeding on an abundant source of organic material. In this environment, root-root, root-microbe, and root-insect communicationsare likely continuous occurrences in this biologically active soil zone, but due to the underground nature of roots, these intriguinginteractions have largely been overlooked. Root-root and root-microbecommunication can either be positive (symbiotic) to the plant,such as the association of epiphytes (plants that grow on other plants), mycorrhizal fungi, and nitrogen-fixingbacteria with roots; or negative to the plant, including interactions with parasitic plants, pathogenic bacteria, fungi, and insects. So if plant roots are in constant communication with symbiotic and pathogenic organisms, how do roots effectively carry out this communication process within the rhizosphere?
Survival of any plant species in a particular rhizosphere environment depends primarily on the ability of the plant to perceivechanges in the local environment that requires an adaptive response.Local changes within the rhizosphere can include the growth and development of neighboring plant species and microorganisms. When the plant detects a challenge, roots typically respond by secreting certain small molecules and proteins . Root secretions may play symbiotic or defensive roles as a plant responds to positive or negative communication,depending on the other elements of its rhizosphere. In contrast to the extensive progress in studying plant-plant, plant-microbe,and plant-insect interactions that occur in leaves and stems, very little research has focused on root-root, root-microbe, and root-insect interactions in the rhizosphere. The following sections will examine the communication process between plant roots and other organisms in therhizosphere.
To recognize and prevent the presence of invading roots through chemical messengers. Allelopathy (organisms that promote growth) is mediated by the release of certain secondary metabolites (metabolism) by plant roots and plays an important rolein the establishment and maintenance of terrestrial plant communities.It also has important implications for agriculture; the effects may be beneficial, as in the case of natural weed control, ordetrimental, when allelochemicals (compounds produced by plants to prevent bacteria and fungi damage) produced by weeds that affect the growth of crop plants.
Further information suggests that certain plants such as the knapweed can also be used to study the way plants can be invasive by sending messages to other plants to stay away, or to make the surrounding area uninhabitable to competing plants.
The above example demonstrates how plants use root-secreted secondary metabolites to regulate the rhizosphere to the detrimentof neighboring plants. However, parasitic plants often use secondarymetabolites secreted from roots as chemical messengers to initiatethe development of invasive organs (haustoria) required for heterotrophicgrowth. Some of the most devastating parasiticplants of important food crops such as corn , sorghum, millet , rice ,and legumes belong to the Scrophulariaceae, which typically invadethe roots of surrounding plants to deprive them of water, minerals,and essential nutrients. It has been reported thatcertain allelochemicals such as flavonoids, p-hydroxy acids, quinones,and cytokinins secreted by host roots induce haustorium formation, but the exact structuralrequirements of the secreted compounds for haustorium inductionis not fullyunderstood.
Root-microbe communication is another important process. Some compounds identified in root exudates (secretions) have been shown to play an important role in root-microbe interactions. Although the studies are not yet conclusive, these compounds may also be responsible for vesicular-arbuscular mycorrhiza colonization. In contrast,survival of the delicate and physically unprotected root cells under continual attack by pathogenic microorganisms depend ona continuous “underground chemical warfare” mediated by secretion of phytoalexins, defense proteins, and other as yet unknown chemicals.
The study of plant-insect interactions mediated by chemical signals has largely been confined to leaves and stems, for now,the study of root-insect communication has remained largely unexplored due to the complexity of the rhizosphere and a lack of suitable experimental systems.
There is an amazing world that exists under our feet and the greater our knowledge of this world, the greater, or lessor our impact on the planet could be. If we understood the symbiotic relationship between plants and how they work with one another, we could better understand companion planting,(plants that work together to help each other), how we can use microbes, fungi and bacteria to our advantage in crop planting. We could also understand how the use of chemicals adversely affects the natural abilities of plants to work for themselves. We have a long way to go to understand our plant world, but it is a fascinating world and it is amazing to be part of it.
source: Plant Physiology/Root Exudation and Rhizosphere biology
Lisa’s Landscape & Design
“Saving the Planet One Yard at a Time”
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