Affects of Exudates on Plant Communications

The roles exudate root secretions play in the communication and interaction of plants is a wide and varied area of study. These exudates can be used as a type of defense against other plants and in this form act allopathically. They inhibit the growth, establishment and affect survival rates of neighboring competing plants and root systems and therefore defend against their resources being taken away (Bais et al 2006).

The compounds in which plants use to defend themselves are usually in the form of phytotoxins but these vary considerably in composition and function between plants. Exudates need to be at certain concentration if they are to affect the growth of neighboring plants. Plants such as Centaurea maculosa and C. diffusa produce their phytotoxins in high concentrations. These phytotoxins can often work in the favor of invasive weeds species as in their new environment the native plants may have never encountered their damaging exudates and therefore no forms of resistances have evolved. This then gives the weeds an upper hand in taking over these environments and expanding their distribution. The sensitivity of native plants to unknown phytotoxins may also explain why invasive species haves such success in new environments (Bais et al 2006).

Exudates do not only serve as a purpose in plant defense interactions but also have use for positive plant to plant interactions. Although these positive interaction are less frequently documented some root exudates increase herbivore resistance in neighboring plants which is highly fascinating because it gives a sense of altruism in plants communities. This is highly debated as it is assumed that in the natural world organism are generally in competition with each other (Bais et al 2006).

Reference

Bais H. P., Weir T. L., Perry L. G., Gilroy S. and Vivanco J. M. (2006) THE ROLE OF ROOT EXUDATES IN RHIZOSPHERE INTERACTIONS WITH PLANTS AND OTHER ORGANISMS, Annual review of Plant Biology 57.  233-266p

The Use of Exudates in Plant Communication

I have talked in previous blogs about how plants use chemical signals or VOCs volatile organic compounds in order to communicate messages to each other through the air. But one of the newer and less researched forms or plant communication are those types in which happen under the ground. Within the rhizosphere which is the densely populated area in the soil in which plant root systems must compete with other organisms for access to nutrients (Bais et al 2004).

So how exactly do plants communicate through this densely populated network? We know that mycorrhizal fungi have an impact on plant communication but other research suggest that root exudates also play are large role in the function of plant communication. Root exudates are the chemicals secreted by root systems into the soil and they are known to perform a multitude of functions. One reason why plants release these exudates is as a defence mechanism against pathogens. Most root secreted compounds belong to the chemical class of phenolics, and it is the terpenoids that have antibacterial and antifungal properties. It has been founds that not only do plant roots release exudates into the rhizosphere but they also they release VOCs when defending themselves against predators, it was previously thought that VOCs where only released above ground by plants (Baetz 2014)

This is just another example of how plants communicate. By releasing these chemicals into the soil from their roots systems they are able to ward off any pathogens and defend themselves. Some exudate chemicals even provide benefit by luring certain microbes to them that they may get an advantage from such as mycorrizae.

References

Baetz U. & Martinoia E. (2014) Root exudates: the hidden part of plant defence, Trends in Plant Science 2. 90-98.

Bais P.H., Park S. W., Weir L. T., Callaway M. R and Vivanco M. J. (2004) How plants communicate using the underground information superhighway. Trends in Plant Science 1. 26-32

Roots Communicating

My previous blogs have mainly focused on how plant communicate and respond when something negative is happening within their environment, such as the attack of herbivores or drought like conditions occurring. 

Generally when plants detect and respond to neighbours under competition their method of preservation is avoidance by attempting to grow away from the competition. Through their root systems plants are constantly interacting with each other but by which mechanisms they use is still poorly understood (Callaway 2002).

In a recent study performed by Gersani et al (2001) he compared the growth of root systems in soy beans that had sole possession of growth space with those that were sharing space and resources. The plants with more space to grow produced 85% larger root mass then the sharing plants. Although the plants that were under competition showed increase root growth, even though it was at a reproductive cost for them. This over production can be explained by the fact that shared resources are more likely to be lost to competition than resources to which an organism has exclusive rights. Their studies also suggested that roots have the ability to perceive roots from that same species that are non-self and react aggressively to increase their root density in contest for resources. 

Unfortunately the ways in which roots communicate aren’t well known. What is also an interesting topic is how roots are able to identify root systems from their own species and from other species. This is hopefully a topic that there will be more research invested into it. 

References

 Gersani, M, et al . (2001) Tradegy of the commons as a result of root competition, Journal of Ecology. 89, 660-669 pp.

Callaway. M., R. (2002) The detection of neighbors by plants. Trends in Ecology & Evolution 3. 104-105.