Chinese whispers.

It's not just through the air that plants can send chemical signals to each other. In fact, they are talking to each other right under your feet in an area under the soil known as the rhizosphere. The rhizosphere is derived from the Greek word rhiza - meaning root. This is broadly defined as the area around a plant root that is inhabited by microorganisms, which are influenced by the plant and the chemicals it releases. As different plants and roots cannot be defined as one shape, the rhizosphere refers to the gradient of chemical, physical and biological processes that occur either longitudinally or radially along the root. (Hartmann et.al 2008).

 It is here that scientists have witnessed the secretion of soluble chemicals into the rhizosphere, which are then transported by the mycelium, which are in turn received by neighbouring plants. The mycelium is formed by fungi. It consists of a mass of thread - like hyphae, which acts as a branching network. The mycelium has been referred to as the plant worlds’ version of the internet.

A team in Israel recently demonstrated one particular way in which this method of communication can benefit plants. They found that when placed under stress, unstressed plants are able to respond to stress cues which are emitted from their stressed neighbours. Interestingly, once they receive this cue, they will pass it onto unstressed neighbours which are further away from the original signaler. The experiment was carried out by blocking or allowing root and shoot communication.  After exposing the initial plant to drought - like conditions, it only took 15 minutes to initiate a response – a neighboring plant closed its stomata. The stomata allows for exchange of gases. Plants can reduce water loss by closing the stomata, which are found on the leaf surface. Like a game of dominos, after one hour, all unstressed neighbours had closed their stomata. The plants that were not allowed root contact had left their stomata open. This has indicated that plants are able to anticipate hazard and stress and thus prepare themselves. (Falik et al 2011). This is not the only way that plants have been found to communicate through the ground. Another mechanism, Common Mycelium Networks (CMN’s) will explored next week. Understanding these networks can be of great significance to agriculture. These communication systems could lead to new methods of preparing crops for stresses such as an influx of predation or like in this study, drought.

Falik, O., Mordoch, Y., Quansah, L., Fait, A., Novoplansky, A. 2011. Rumor has it: Relay communication of stress cues in plants. PloSONE 6(11):e23625, doi:10.1371/journal.pone.0023625

Hartmann, A., Rothballer,M., Schmid, M.2008. Lorenz Hiltner, a pioneer in rhizosphere microbial ecology and soil bacteriology research. Plant Soil, 312: 7-14.

Volatile Organic Compounds

This week we will look briefly at one well known form of plant communication; volatile organic compounds, (VOCs). Some plants respond to herbivory by releasing chemical cues. Many plants do this to prepare themselves for the onset attack on other parts of their structure. This means they can release a volatile organic compound at the site that is under attack in an effor to deter predation. A nearby branch can detect the compound and begin to increase its production of the same compound (Karban et al. 2014).

Similarly, separate individuals have the ability to detect these chemicals and they too can coordinate their system to increase their own defence.  In one particular study, sagebrush and wild tobacco plants were grown in close proximity to each other. The sagebrush leaves were clipped and began to release a volatile signal ( methyl jasmonate).  Some of the wild tobacco plants were exposed to the air filled with the VOCs emitted by the sagebrush. The exposure induced the wild tobacco to produce a chemical defence ( polyphenol oxidase). Subsequently, these wild tobacco plants significantly reduced the level of leaf damage caused by grasshoppers and cutworms. (Karban et al. 2000).

Figure 1. Levels of VOCs (polyphenol oxidase) activity for tobacco plants near clipped or unclipped sagebrush. (Karban et al. 2000).

The outcome of this adaptation has been demonstrated to increase fitness for the emitting plant in cases where it has emitted to prepare other parts of its own structure for attack. With intra - plant communication, it has been shown to increase fitness in the same regard. It has also been found that lima beans exposed to VOCs were able to defend existing leaves against herbivory to a greater degree than the controls in the experiment. They were also able to produce more new leaves than the controls after exposure. (Kost & Heli , 2006). Further research may identify more ways in which plants can benefit from this form of communication.


Karban, R., Baldwin, I.T., Baxter,K.J., Laue, G., Felton, G.W. 2000. Communication between plants: induced resistance in wild tobacco plants following clipping of neighboring sagebrush. Oecologia. 125, 1: 66 – 71.

Karban, R., Louie, H.Y., Edwards, K.F. 2014. Volatile communication between plants that affects herbivory: a meta – analyses. Ecology Letters. 17: 44 – 52.

Kost, C., Heli, M. 2006. Herbivore-induced plant volatiles induce an indirect defense in neighboring plants. Journal of Ecology, 94: 619 – 628.

A controversial science.

Let’s start with a bit of background.

Plants have evolved a variety of ways in which they communicate. Until relatively recently, communication in the plant kingdom has been a controversial topic. In 1983, two significant papers were published that seemingly advanced what we know about the world of plants. In one study it was found that poplar and sugar maple seedlings would pump out anti –herbivore toxins when placed in a chamber with shredded leaves of saplings (Baldwin and Schultz 1983). In another study, it was found that willows increased the production of a chemical defense when their neighbours were being attacked by webworms and caterpillars (Rhoades 1983).

Both studies were under heavy scrutiny by eminent ecologists who pointed out that there could be other explanations for the phenomen. At the time, scientists dabbling in this field faced another problem. During the early 80's the book: The Secret Life of Plants, was attracting criticism for making claims about the plant world.  The book was criticised for being as described by Daniel Chamovitz, ‘”scientifically anemic” (Chamovitz 2012). Skepticism from respected scientists resulted in the research slowing down for a short period. Luckily, scientists such as Ted Farmer ( considered a pioneer in the field) continued on. Today, the evidence is clear – at least on this aspect. Plants can release volatiles to deter herbivores, while other plants can detect these airborne signals and respond by increasing their own chemical defense. (Baluska & Ninkovic 2010).    

Figure 1. Front cover of the contraversial book, The Secret Life of Plants by Peter Tompkins and Christopher Bird. Cover illustration: Alan Aldridge (1973).

    

Since this time, an array of mechanisms of which plants can communicate have been identified. Such methods include territorial root communications, volatile signals that induce defense against herbivores, signals from ovules to mother plants, signals associated with root graft information, as well as male to female signals during pollen competition. These mechanisms assist the plants evolutionary success and accomplishes self/non-self kin recognition. From an evolutionary perspective, if the signalling plant derives a fitness benefit from conveying information to other plants, this signal can be classified as communication. (Baluska & Ninkovic 2010). 

A big question that stands today is; what is the plant trying to achieve? If a plant releases a signal, is it trying to help itself, or its neighbour? If it is trying to help its neighbour what does it get in return? Are these signals unintentionally being eavesdropped by surrounding organisms or are they sent with intent? (Baldwin et al. 2006)  These are some of the questions that will come up in the research we will look at over the next few weeks.

Check out my references for some fascinating reading:

Alderidge, A. 1973. The secret life of plants. Penguin. https://zeppelinruc.files.wordpress.com/2012/11/secret-life-of-plants-book.jpg; retrieved 12/03/2015.
Baldwin, I.T., Halitschke, R., Paschold, A., Von Dahl, C.C., Preston, C.A. 2006. Volatile signaling in plant-plant interactions: “talking trees” in the genomics era. Science,   311:812-815.

Baldwin, I.T., Schultz, J.C.1983. Rapid changes in tree leaf chemistry induced by damage: evidence for communication between plants. Science, 221: 277–279

Baluska, F., Ninkovic, V.2010. Plant communication from an ecological perspective. Springer –Verlag, Berlin Heidelberg. 

Chamovitz, D. 2012. What a plant knows. Griffin Press, Australia.Rhoades, D.F. 1983. Response of alder and willow to attack by tent catepilllars and webworms: evidence for phenomonal sensitivity of willows. American Chemical Society Symposium Series, 208: 55 – 68.