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Mechanisms Behind Interplant Communication - ‘How Do Plants Talk?’

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Mechanisms Behind Interplant Communication - ‘How Do Plants Talk?’ Mechanisms behind interplant communication - ‘how do plants talk?’ TOWARDS AN INTEGRATION OF THE MECHANISMS OF INTERPLANT COMMUNICATION MASTER THESIS Veronika Ramovs Mentor: Prof. Dr. Corné Pieterse Introduction 2 Abstract When plants are under herbivore or pathogen attack, or experiencing abiotic stress conditions, they can emit signalling molecules that can be received and perceived by their neighbouring plants. Receiving plants can respond to such signals by inducing defence responses, changing growth or priming their defences for possible future attack. Such transfer of information between plants, also named ‘interplant communication’, can take place above or below ground. Above ground signalling molecules are transported through air as blends of volatile compounds of specific concentrations. Below ground, plants can send and perceive signals through roots or common mycorrhizal networks. Numerous studies performed over the last three decades brought better understanding of the identity of signaling molecules and mechanisms behind the emission and perception of them. However there is still a lack of information, especially on the molecular mechanisms of signalling pathways involved in the signal emission, perception and processing. Furthermore, most of the published studies have so far focused on the specific types of signalling taking place either above or below ground, considering plant tissues as independent systems. This thesis presents review of all currently known mechanisms of interplant communication, the circumstances under which these mecahnsims are triggered and the signal molecules that are involved. Different mechanisms of interplant communication are discussed in integrated manner, taking into account plants as whole organisms that are able to perceive and simultanesously respond to several stimuli. Table of contents I. INTRODUCTION II. AIRBORNE VOLATILE SIGNALLING AS MECHANISM OF INTERPLANT COMMUNICATION A. Starting the conversation A.1. Cues for interplant signalling, their recognition and signalling pathways leading to VOCs synthesis A.2. Identity of signalling molecules A.3. Release of VOCs B. Receiving and responding to the message III. MECHANISMS OF UNDERGROUND INTERPLANT COMMUNICATION A. Signalling through roots B. Signalling through underground common mycorrhizal network (CMN) IV. PLANTS AS WHOLE ORGANISMS – INTEGRATED VIEW ON THE INTERPLANT COMMUNICATION V. CONCLUSIONS VI. REFERENCES Introduction 1 Introduction another. In this thesis I first review all currently known mechanisms of interplant communication, The ability of plants to emit and intercept signalling the circumstances under which these mecahnsims molecules from other plants has been one of the are triggered and the signal molecules that are developing research topics in molecular and plant involved. This is essential to establish where the data ecology over the last three decades. Ever since is lacking, which questions still need to be answered 1 2 Baldwin and Schultz as well as Rhoades provided and how to proceed. Secondly, I compare and first indications of interplant communication, discuss currently known data in the integrated 3 questions about community evolution , manner, placing different types of interplant circumstances under which plants exhibit such communication in the same context and trying to behaviour and questions on physiological and discuss their impact on each other, taking into molecular mechanisms behind it arose. With more account plants as whole organisms that are able to and more evidence that plants are able to perceive perceive and simultanesously respond to several and respond to the signals released from stimuli. neighbouring plants also in the natural environment4–8, interplant communication is Volatile signalling as mechanism becoming a widely recognised and accepted phenomena. Many of its aspects however remain of interplant communication somewhat controversial. It is still debated whether With more than 1700 currently known compounds11, plants deliberately send messages to their volatiles play an important role in the mechanisms of neighbours, or whether interplant communication is plant interaction within the environment. They are a side effect of other functions of signalling part of plant defence and reproductive mechanisms, molecules. Furthermore, it is unknown what this involving interactions of plants with organisms of means from an evolutionary perspective9. Current higher trophic levels (Fig. 1). The first implications of general opinions lean toward the idea of plants interplant communication, in which herbivore ‘eavesdropping’ on volatile organic compounds resistance was shown to increase in plants growing (VOCs) of their neighbours and consequently in close range to herbivore-attacked plants1,2, was responding by adapting their phenotypes and suggested to be mediated through air-borne therefore enhancing their fitness. Supporting such a signals12. These pioneer findings caused mixed conceptfurther is also recently elucidated within- responses in public and the topic was popularly plant signalling through aerial space, mediated by named ‘talking trees’. similar compounds as found to induce response in Since then, new evidence has been gathered under neighbouring plants10. laboratory, as well as field conditions. This evidence At the same time there are also an increasing has confirmed that plants respond to volatile number of reports on the different mechanisms of airborne molecules emitted from other plants and interplant communication. Besides well established identified to some extend identity of molecules and airborne communication other mechanisms of mechanisms of action involved. The emission and interplant communication have been observed perception of VOCs are currently one of the most below ground. Modern approaches and techniques researched and acknowledged types of interplant are enabling better understanding of the identity of communication. signalling molecules, as well as molecular mechanisms leading to their release, their STARTING THE CONVERSATION perception and formation of the response by the receiving plant. Wide transcriptional analyses can be Cues for interplant signalling, their used to connect biochemical, ecological and recognition and signalling pathways physiological data within a genetic framework. leading to VOCs synthesis Most of the published data on interplant What are the cues causing plants to release VOCs communication has so far focused on specific types whichare able to affect their neighbouring plants of signalling taking place either above or below and what is the mechanism behind their perception ground. However, since plant tissues do not act as and recognition? As sessile organisms, plants had to independent organisms, an integrated approach is develop diverse and numerous defensive needed to fully understand how plants signal to one Volatile signalling as mechanism of interplant communication 1 Fig. 1: Roles of the plant volatile organic compounds (VOCs) in interaction of plants within the environment. VOCs play part in defence responses, such as a ‘cry for help’ and protection under abiotic or biotic stress conditions, transfer of information within and between plants and in reproductive mechanisms, such as attraction of pollinators and seed dispersal. mechanisms in order to survive constant attacks by Several experiments, where lima bean plants herbivores and pathogens. Therefore it is not very (Phaseolus lunatus) were infestated with spider surprising that most plants shown to be emitters of mites (Tetranychus urticae, Acari), showed that airborne VOCs involved in the interplant receiving lima beans responded to volatile communication are responding to damage caused by compounds from attacked plants13–15. HIPVs released attackers. from lima bean plants that were infested with the leaf miners caused a defence response in exposed RESPONSE TO HERBIVORY 16 Arabidopsis thaliana plants . Damage caused by Herbivore feeding has been known to invoke several species of beetles (Coleoptera) was also defence responses in the neighbours of attacked shown to be able to induce release of HIPVs that plants since the first observations of interplant resulted in defence responses in receiving plants; communication. In 1983 David F. Rhoades, one of cereal leaf beetle (Oelema melanipus) in the the pioneers in the field of plant-plant signaling, common wheat (Triticumae stivum), common oat reported that Sitka willow (Salix sithensis) trees (Avena sativa) in barley plants (Hordeum vulgare)17 respond to their neighbouring willow trees, attacked and Gynandrobrotica guerreroensis and Cerotoma by tent caterpillars (Malacosomacalifornicum- ruficornis beetles in the lima bean plants10. Other pluviale). Because no root connections between herbivorous arthropods, reported to be involved in trees were found, Rhoades assumed that interplant communication, include aphids neighbouring trees are able to sense airborne signals (Nasonovia ribis-nigri)18,19, whiteflies (Trialeurodes emitted from the damaged willows2. Subsequently, vaporariorum)20 and armyworms (Spodoptera numerous researchers have shown that plants can exigua, Mythimna pseudaletia)14,21 . intercept and respond to herbivore-induced plant Tissue damage is the most obvious consequence of volatiles (HIPVs) in response to attacks by an herbivore attack. It has been shown that herbivorous arthropods. mechanical wounding, clipping or defoliation of some plant species can be sufficient to induce a Volatile signalling as mechanism
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