Social networking in plants: biodiversity as a selective force for inter-plant signalling

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There is now unequivocal evidence, mainly from laboratory experiments, that plants emit signals of insect herbivory that are detected by neighbouring plants. However, the significance of inter-plant signalling for ecological and evolutionary processes in nature is untested. Recent exciting findings have shown that plant to plant signalling occurs via common mycorrhizal networks (CMNs). Mycorrhizal fungi form symbiotic relations on the roots of at least 80% of the world's plant species, and they frequently connect individual plants simultaneously, thus forming a CMN. In nature, the consequences of the costs and benefits of inter-plant signalling via CMNs in communities are likely affected by i) the specificity of the signals (and the plant's response to the signals) in relation to plant species and herbivore feeding type (i.e. whether it is a phloem sucking insect or a chewing insect), ii) mycorrhizal fungal species diversity, and iii) the degree of specialisation of insect herbivores. In this adventurous proposal, we test the hypotheses that CMN-based signals and the receiver plant's response to signals is dependent on the feeding guild and host specificity of the insect herbivore. Moreover, because many mycorrhizal fungi exhibit host preferences, we also hypothesise that greater mycorrhizal fungal diversity will lead to more patchy fungal networks and therefore decrease the proportion of receiver plants that respond to infested plants in the community. In addition, fungi may be less likely to pass warning signals to plants that harbour competing fungi than to plants that are colonised only by themselves. We predict that this would also result in mycorrhizal fungal diversity decreasing the proportion of receiver plants receiving or responding to signals from plants under attack. Finally, we predict that there are costs to receivers in responding to signals that also impact the functioning of mycorrhizal fungi, but overall there is net fitness benefit to the plants. Our multidisciplinary project builds on past and current NERC funded research and will lead to a major step-change in understanding of how plant and fungal biodiversity shapes inter-plant signalling, community dynamics and multi-trophic interactions in nature.