Plant communication: the big question
Plants can communicate too. They can detect other plants next to or around them and respond accordingly. They can do this by a number of ways, one way is secreting certain chemicals into the soil to affect how their neighbouring plants grow and behave, depending on how crowded they find their neighbourhood to be. As plants can’t simply run away, they have acquired a number of strategies to help them cope with their neighbours. They can detect other plants surrounding them in a number of ways, such as using signals like far-red light reflection where the photoreceptors in the plant can detect both red and near infra-red light. Plants rely on direct sunlight for photosynthesis, which is the most important chemical reaction in plants needed for producing carbohydrates from inorganic substrates, which is vital for survival. This direct sunlight is abundant in red-light, whereas light can be reflected from other plants around them and this causes it to be less red and more far-red. This can cause a major decrease in the rate in growth of the surrounding plants, which is much less useful than the rich red-light of direct sunlight which encourages fast growth, being more beneficial for survival (Devlin, 2015). Plants produce chemical signals to manipulate their environment, and they can share those signals with each other plants through the air or the soil. The ability of plants to inhibit or encourage growth of their roots or leaves when detecting either members of the same species or different can be explained by the plant needing its own space.
Background: previous research
A large number of observations show that plants can detect each other at root-level, which is likely controlled by chemicals emitted through the roots. Research by Barto et al. (2012) suggests the signals that are emitted by the plant below the ground may be conducted by physical contact of the root hairs or tips. Competition between plants may involve a negative interaction, thereby causing reduced growth or reduced survival of its neighbours. Plants that are growing together in close proximity may depend upon the same nutrients in the soil and therefore these interactions can cause them to adapt. This ensures they have a better survival advantage over others, where the resources of either light or nutrients in the soil are limited. (Casper and Jackson, 1997). Plants may also be able to recognise their own species from strangers using kin recognition, and this suggests that roots can sense when other roots are in close proximity and is also vital in altering the growth of the plant root to beat its competitors (Dudley and File (2007). Kin recognition of other species is said to occur by different mechanisms compared to that of the same species. It was found during a study by Crepy and Casal (2014) that in Arbidopsis, photosensor receptors can differentiate between the different light signals emitted by plants of both the same and different species, allowing the plant to alter the placing of its leaf to decrease the competition for light. The communication by plants belowground in the soil is guided by the trading of substances discharged by one plant and detected by another. For two species that are existing in close proximity, plants may firstly detect and perhaps recognise their neighbours which will then commence allelopathic interference to control both inter or extra-specific interactions. These two processes become hand in hand when one or more plants are coexisting and are interacting together. A study by Meiners et al. (2012) found that this pattern may come to the surface through the production and consequential release of signalling chemicals in the soil, which in turn prompt the all-important production of the defensive allelochemicals that neighbouring plants are detecting. Ideally, the chemicals used in the signalling would be familiar to the fewest possible competitors, however the manufacture of allelochemicals may be produced even when there are no neighbours nearby, misusing valuable resources required for by the plant.
Additions to the field
Writing in Nature Communications, Kong et al. (2018) aimed to investigate how plants communicate belowground, which is an area of plant communication research where there are still many unanswered questions. They focused on assessing how wheat detected and responded to 100 other species of plant by underground signalling and found that wheat can detect the presence of their neighbours both heterospecific and conspecific and responds to heterospecific neighbours by the increase of allelochemical production. It was also noticed that two specific chemicals, (-)-loliolide and jasmonic acid were present in the roots of a diverse range of species that were included in the study, and these two chemicals are able to trigger allelochemical production by wheat, perhaps suggesting that these two root-secreted chemicals may be the key signals involved in plant neighbour detection, which remained poorly defined before this study was carried out. It could also mean that they may be the universal mediators of plant neighbour interactions in the soil. Biochemical responses could be related to which neighbour is coexisting with them and could be detected and controlled through the presence of detection cues. The results of this study could suggest that other plants, as well as wheat, can hinder the growth of their neighbours by producing secondary metabolites in response to the biotic and abiotic factors in the surrounding environment. By noticing the presence of (-)-loliolide and jasmonic acid in the roots of a variety of species included in the study, it adds to the current field by helping us understand plant neighbour behaviour and the response strategies that some may use to aid growth and survival.
Now that chemicals involved in neighbour detection and response have been identified, this opens up the door for further studies on the behaviour of the chemicals in the soil involved and its molecular constitution. This could lead to new perspectives into how plants sense other plants. In the study by Kong et al. (2018) there was no specific response found to (-)-loliolide or jasmonic acid, or a common way that different species behaved once detecting it. The potential of how the signalling chemical leads to responses to other plant systems also needs to be understood, to test if they respond in a similar way. It is likely that there may be more signalling mechanisms involved in the soil that could be contributing to plant-plant responses.