Family Ties

Imagine if one of your siblings were adopted and you were never told. If you bumped into this long lost sibling on the street, would you be able to discern if they were a stranger, or if  they were your biological brother or sister? Does your behaviour change when you recognise a person walking by? Like us, some plants can recognize their relatives, reacting and responding to their presence.
 

Kin and stranger recognition has been demonstrated in some species including legumes (Dudley & File 2007). However, the extent of this phenomena in the plant kingdom is unknown. A species from the northern hemisphere, the North American Impatiens pallida, has demonstrated kin recognition. I.pallida or Yellow Jewelweed are herbaceous annual plants found in moist, woody and shady areas. Subjects were exposed to aboveground light quality cues of competition, and to the presence of root neighbours to determine whether responses were influenced by the presence of neighbours or kin. (Murphey & Dudley 2009).

Figure 1. Impatiens pallida  commonly known as Yellow Jewelweed, growing in the woods in North America. Photographer: Christopher C.N C.

The study found that I.pallida were capable of kin recognition below ground and could respond above ground if below ground cues are present. That is, they could recognise the presence of strangers and relatives using their roots. Individual plants increased stem elongation and branchiness in response to kin. On the other hand, if a stranger was present, I.pallida responded by increasing leaf to root allocation (Murphy & Dudley 2009). Another species that has been observed to increase root allocation when it shares a pot with strangers is the American searocket, Cakile endulata. C.edulata is found to grow in sandy beaches where light is plentiful and belowground resources are limited. A very different habitat compared with I.pallida. (Dudley & File 2007)

Figure 2. American searocket , Cakile endulata has demonstrated kin recognition and can be made into salads. Photographer: Sunny Savage.

Kin recognition is advantageous in competitive situations. It can reduce costly competitive behaviour and promote cooperation among siblings. Looking at the life history of plants, it can be said that these traits were likely to occur given that dispersal often results in highly related groups of neighbours (O’Brien et al.2005).

References: 

Dudley, S.A., File, A.L. 2007. Kin recognition in annual plant. Biology Letters. 3: 435-438.

O’Brien, E.E., Gersani, M., Brown, J., S. 2005. Root proliferation and seed yield in response to spatial heterogeneity of below – grown competition. New Phytologist. 168: 401-412.

Murphey, P. G., Dudley, S.A.2009. Kin Recognition: Competition and cooperation in Impatiens (Balsaminaceae).American Journal of Botany. Vol. 96. 11: 1990-1996. 

Figure 1. Impatiens pallida  commonly known as Yellow Jewelweed, growing in the woods in North America. Photographer: Christopher C.N C.  http://outsideclyde.blogspot.com.au/2013/05/weed-it-and-they-will-come.html Image retrieved 20/04/15.

Figure 2. American searocket , Cakile endulata has demonstrated kin recognition and can be made into salads. Photographer: Sunny Savage. http://wildfoodplants.com/2007/08/sea-rocket-rocks/ Image retrieved 20/04/2015.

Beat of the drum: Plant Bioacoustics.

I will scratch your back if you scratch mine . Co-evolution between fungi and land plants.

The majority of terrestrial (land) plants associate with mycorrhizal fungi ( Cairney et al 2000).This association can connect the roots of neighbouring plants into common mycelium networks (CMN's). We touched on this topic last week looking at the chemical secretion between plants in response to drought.However, there are many other uses for these underground highways of information. In fact, to answer some of your questions, it is thought that 90% of land plants have co -evolved with fungi in a mutualistic symbiosis. These mutualisms allow the plants to connect with each other, as well as performing a range of other functions (Babikova & Johnson et al 2013). 

 

Figure 1. Aphid species attached to plant stem. Photographer: Yongxin zhang (2012).

So as we have seen, plants can communicate through the air with the release of volatile compounds in response to herbivory. This type of communication has been observed between roots as well. It has been observed that insect herbivory can trigger the release of the volatile methyl salicylate. In the bean plant, Vicia faba, methyl salicylate production can repell aphids. Not only do they repell; they  also attract aphid enemies! Such as parasitoids. This particular study demonstrates that plants are not only able to  protect themselves through CMN cables, but they are directly aiming to communicate and therefore influence the behaviour of insects (Babikova & Gilbert et al 2013).

References:

Babikova, Z., Gilbert L., Bruce, T., J., Birkett, M., Caulfield J., C., Woodcock., C., Pickett., J., A., Johnson., D. 2013. Underground signals carried through mycelial networks warm neighbouring plants of aphid attack. Ecology Letters, Vol 16. 7: 835-843.

Babikova, Z., Johnson, D., Bruce, T., Pickett, J., Gilbert., L.2013. Underground allies: How and why do mycelial networks help plants defend themselves? Bioessays, vol 36, 1 - 21-26.

Cairney, J., W., G. 2000. Evolution of mycorrhiza systems. Naturwissenschaften. 87L 467-475.
Zhang, Y. (2012). Aphid species attached to plant stem. http://www.gardeningknowhow.com/plant-problems/pests/insects/homemade-aphid-control.htm; retrieved 02/04/ . 

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.