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      Nectar in Plant–Insect Mutualistic Relationships: From Food Reward to Partner Manipulation

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          It has been known for centuries that floral and extra-floral nectar secreted by plants attracts and rewards animals. Extra-floral nectar is involved in so-called indirect defense by attracting animals (generally ants) that prey on herbivores, or by discouraging herbivores from feeding on the plant. Floral nectar is presented inside the flower close to the reproductive organs and rewards animals that perform pollination while visiting the flower. In both cases nectar is a source of carbon and nitrogen compounds that feed animals, the most abundant solutes being sugars and amino acids. Plant–animal relationships involving the two types of nectar have therefore been used for a long time as text-book examples of symmetric mutualism: services provided by animals to plants in exchange for food provided by plants to animals. Cheating (or deception or exploitation), namely obtaining the reward/service without returning any counterpart, is however, well-known in mutualistic relationships, since the interacting partners have conflicting interests and selection may favor cheating strategies. A more subtle way of exploiting mutualism was recently highlighted. It implies the evolution of strategies to maximize the benefits obtained by one partner while still providing the reward/service to the other partner. Several substances other than sugars and amino acids have been found in nectar and some affect the foraging behavior of insects and potentially increase the benefits to the plant. Such substances can be considered plant cues to exploit mutualism. Recent evidence motivated some authors to use the term “manipulation” of animals by plants in nectar-mediated mutualistic relationships. This review highlights the recent background of the “manipulation” hypothesis, discussing it in the framework of new ecological and evolutionary scenarios in plant–animal interactions, as a stimulus for future research.

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              The evolutionary context for herbivore-induced plant volatiles: beyond the 'cry for help'.

              Attacks by herbivores elicit changes in the bouquet of volatiles released by plants. These herbivore-induced plant volatiles (HIPVs) have been interpreted as being indirect defenses. However, given that no studies have yet investigated whether HIPVs benefit the fitness of a plant, their defensive function remains to be established. Moreover, herbivores, pathogens, pollinators and competitors also respond to HIPVs and, in addition, neighbouring plants in native populations also emit volatiles that provide a background odour. These considerations enrich the evolutionary context of HIPVs and complicate predictions about their adaptive value. Molecular advances in our understanding of HIPV signaling and biosynthesis is enabling the creation of HIPV-'mute' and possibly HIPV-'deaf' plants. As we discuss here, such plants could be used for unbiased examination of the fitness value of HIPV emissions under natural conditions. Copyright 2009 Elsevier Ltd. All rights reserved.

                Author and article information

                Front Plant Sci
                Front Plant Sci
                Front. Plant Sci.
                Frontiers in Plant Science
                Frontiers Media S.A.
                19 July 2018
                : 9
                1Department of Life Sciences, University of Siena , Siena, Italy
                2Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma , Parma, Italy
                3Department of Agrifood Production and Environmental Sciences, University of Florence , Florence, Italy
                Author notes

                Edited by: Lukasz Lech Stelinski, University of Florida, United States

                Reviewed by: Martin Heil, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (CINVESTAV-IPN), Mexico; Quint Rusman, Wageningen University & Research (WUR), Netherlands

                *Correspondence: Massimo Nepi, massimo.nepi@

                This article was submitted to Functional Plant Ecology, a section of the journal Frontiers in Plant Science

                Copyright © 2018 Nepi, Grasso and Mancuso.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                Figures: 2, Tables: 1, Equations: 0, References: 147, Pages: 14, Words: 0
                Plant Science


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