Spotlight
Nightshade Wound Secretion: The World's Simplest Extrafloral Nectar? Martin Heil1,z,* Wounded nightshade leaves secrete a sugary liquid that, like extrafloral nectar (EFN), attracts ants as defence against herbivores. The secretion of these droplets requires no detectable nectary tissue, showing how little it takes to produce a functioning nectary. Easy de novo formation of extrafloral nectaries explains their ‘scattered’ phylogenetic distribution.
evolutionary emergence of a functioning secretion once the leaves sustained damextrafloral nectary. age [6]. Moreover, wound secretion by S. dulcamara represents a specific trait that Lortzing et al. [6] describe sugar-contain- could not be observed in the congeneric ing droplets that appear on the leaves of species S. nigrum (T. Lortzing and A. bittersweet nightshade (Solanum dulca- Steppuhn, personal communication). mara) when these are damaged by herbivores. These droplets attract native ants, In short, the wound secretion of bitterwhich successfully defend the plant sweet nightshade can be considered against two important groups of natural EFN with respect to all of its functional enemies: flea beetle larvae and – some- characteristics, but its production and what surprisingly – slugs. Extrafloral nec- secretion is independent of any specific taries are known from Solanum nigrum, nectary. The authors could even exclude where they release EFN via constitutively the existence of ‘gestaltless’ nectaries, as open stomata. By contrast, Lortzing and described for Brassica juncea [7]. This colleagues could not detect any visible discovery paves the way for a mechanistic structures such as nectary parenchyma, understanding of the repeated gains and secretory trichomes, or stomata or spe- losses of extrafloral nectaries over the cific vascular connections, as typical for course of plant evolution – a phenomenon nectaries, and they report that the drop- that has challenged biologists since the lets can appear at any place around the earliest research into the natural history wound site (Figure 1A). Nevertheless, we of EFN [2,3]. It is tempting to speculate do not simply see passive bleeding of that most, if not all, elements of the core phloem or xylem here; the chemical com- set of genes and structural predisposiposition of the wound secretion of S. tions that are required to form a functiondulcamara clearly differs from the compo- ing extrafloral nectary are conserved sition of phloem or xylem sap and among plants and require only spatiotempretreatment with methyl-jasmonate poral coordination to allow controlled almost doubled the amount of wound secretion of EFN.
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Plants in more than 100 plant families, comprising ferns as well as angiosperms, respond to herbivory with the secretion of EFN [1–3]. The secretion of EFN is usually controlled by jasmonic acid (JA) and its chemical composition is adapted to the nutritional needs of ants and other predators and parasitoids, which are attracted as defenders of the plant against herbivores [1]. These observations indicate that considerable biosynthetic activities are required for the production of EFN and its controlled secretion. Nevertheless, the structural and functional diversity of extrafloral nectaries as well as the (A) chemical composition of EFN can strongly differ among closely related speCell cies [4,5] and even the extrafloral nectarSweet Sucrose wall ies per se represent an evolutionarily unstable trait that can be present or (B) absent in species within the same genus Sweet Invertase Chloroplast [2]. How can it be that extrafloral nectaries seemingly appear and disappear during the evolutionary trajectory of plants, sim- Figure 1. Wound Secretion by Solanum dulcamara. (A) Droplets of wound secretion on an herbivoreply as they are needed [3]? A recently damaged leaf. (B) Hypothetical mechanism of secretion. Sucrose is unloaded from the phloem or liberated from published study by Tobias Lortzing, Anke accumulated starch in the chloroplast by cell-wall invertase and transported into the apoplast (cell wall) of the wounded leaf by SWEETs. If tissue rupture exposes the cell wall to the external space, the apoplast might become Steppuhn, and colleagues [6] sheds light an ‘endless sink’ and increasing amounts of sucrose accumulate on the outer parts of the wound site. Photograph on how we can imagine the fast in (A) courtesy of Tobias Lortzing; (B) redrawn from Figure 2 in [1] and based partly on concepts in [12]. Sweet
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Which elements are most likely to be required for wound secretion by S. dulcamara? The cell-wall invertase CWIN4 has been reported as an absolutely required factor for floral nectar secretion in the Brassicaceae [8] and sucrose fluxes in the phloem and a JA-inducible cell-wall invertase were key factors in EFN secretion by Ricinus communis [9]. Invertases control three steps in nectar secretion: (i) the unloading of sucrose from the phloem into the nectary parenchyma; (ii) the creation of sinks that are required for the release of sucrose into the extracellular space; and (iii) the adjustment of the sucrose:hexose ratio in the secreted nectar [1,8,10]. A later study using Arabidopsis thaliana, Brassica rapa, and Nicotiana attenuata showed that the sucrose transporter SWEET9 and sucrose phosphate synthases [11] are also crucial for floral nectar secretion. Taken together, these findings make it likely that the secretion of nectar starts with synthesis of sucrose in the nectary parenchyma or its unloading from the phloem followed by SWEET9mediated secretion into the apoplasm and partial hydrolysis by cell-wall invertase to produce a mixture of sucrose, glucose, and fructose [12].
the chloroplast [1]. Furthermore, shading and structural elements that are required experiments indicated that wound secre- to produce a functioning nectary. tion by S. dulcamara depends quantitatively on photosynthetic activity in the Acknowledgments wounded as well as systemic leaves, The author thanks Susanne Brink for inviting this in the hours before wounding (T. Lortzing Spotlight, Anke Steppuhn and Tobias Lortzing for and A. Steppuhn, personal sharing unpublished data, and Alejandro de León communication). Thus, sucrose unloaded for drawing the figure. from the phloem appears to serve as a 1Departamento de Ingeniería Genética, CINVESTAV – source of the sucrose in the wound drop- Irapuato, Km 9.6 Libramiento Norte, 36421 Irapuato, Guanajuato, México lets, a process in which SWEETs might be crucially involved (Figure 1B). Major *Correspondence:
[email protected] (M. Heil). reallocation processes form part of the zTwitter: @martinplantecol general response to herbivory and com- http://dx.doi.org/10.1016/j.tplants.2016.06.004 prise the SWEET- and invertase-mediated References influx of carbohydrates into a wounded 1. Heil, M. (2011) Nectar: generation, regulation and ecological functions. Trends Plant Sci. 16, 191–200 leaf, to supply energy for the local synthe2. Weber, M.G. and Keeler, K.H. (2013) The phylogenetic sis of resistance compounds. distribution of extrafloral nectaries in plants. Ann. Bot.
Lortzing et al. reported that the wound secretion of S. dulcamara contains only sucrose but no hexoses, which shows that the hydrolytic activity of invertase plays no role in determining the composition of its carbohydrate fraction. Nevertheless, invertase is likely to play a role in the creation of sinks that are required for directed flow of sugars and the mobilisation of stored assimilates from
9. Millán-Cañongo, C. et al. (2014) Phloem sugar flux and jasmonic acid-responsive cell wall invertase control extrafloral nectar secretion in Ricinus communis. J. Chem. Ecol. 40, 760–769
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In summary, common elements of the carbohydrate metabolism of plants and its conserved response to herbivory might be sufficient to explain major aspects of wound secretion by S. dulcamara. However, future studies will be required to understand why wound droplets are produced by S. dulcamara but not S. nigrum or other related species. Similarly, it remains an open question where the ‘two nitrogen-containing and five unknown compounds that were more abundant in wound secretions than in phloem samples’ [6] stem from and how they are transported into the wound droplets. As discussed by Lortzing et al. [6], wound secretions and gestaltless nectaries are likely to be much more common than currently known. These very primitive nectaries represent an ideal basis for comparative studies aimed at understanding the minimum set of physiological activities
3. Marazzi, B. et al. (2013) The diversity, ecology and evolution of extrafloral nectaries: current perspectives and future challenges. Ann. Bot. 111, 1243–1250 4. Aguirre, A. et al. (2013) Morphological characterization of extrafloral nectaries and associated ants in tropical vegetation of Los Tuxtlas. Mexico. Flora 208, 147–156 5. Marazzi, B. et al. (2013) Diversity and evolution of a trait mediating ant–plant interactions: insights from extrafloral nectaries in Senna (Leguminosae). Ann. Bot. 111, 1263–1275 6. Lortzing, T. et al. (2016) Extrafloral nectar secretion from wounds of Solanum dulcamara. Nat. Plants 2, 16056 7. Mathur, V. et al. (2013) A novel indirect defence in Brassicaceae: structure and function of extrafloral nectaries in Brassica juncea. Plant Cell Environ. 36, 528–541 8. Ruhlmann, J.M. et al. (2010) CELL WALL INVERTASE 4 is required for nectar production in Arabidopsis. J. Exp. Bot. 61, 395–404
10. Heil, M. et al. (2005) Post-secretory hydrolysis of nectar sucrose and specialization in ant/plant mutualism. Science 308, 560–563 11. Lin, I.W. et al. (2014) Nectar secretion requires sucrose phosphate synthases and the sugar transporter SWEET9. Nature 508, 546–549 12. Eom, J-S. et al. (2015) SWEETs, transporters for intracellular and intercellular sugar translocation. Curr. Opin. Plant Biol. 25, 53–62
