ChEBI: April's entity of the month

Plant defence against herbivory defines a cascade of morphological, biochemical, and molecular alterations in plants to improve their survival and reproduction by reducing herbivore preference and performance [1]. Plant resistance against herbivores can be either constitutive (expressed irrespective of the external stimuli) or induced in response to an attack by the predator. Inducible plant defence is a paradigm of phenotypic plasticity that elicits physiological or behavioural changes in the invader [2]. In turn, the impact of plant resistance on the herbivores can be either direct or indirect. The resistance factors for direct plant defence include morphological features such as thorns, spines, and prickles, trichomes [3], wax-coated cell walls, as well as secretions like gummosis or sap that trap insect predators [4]. Plants also produce toxic chemicals such as insect repellents or feeding deterrents that can adversely affect the growth and development of herbivores [5]. Indirect methods of defence includetritrophic interactions, whereby plants protect themselves by attracting natural enemies of the herbivores such asparasitoids and predators. This is mediated via a release of volatile cues, commonly known as herbivore-induced plant volatiles (HIPVs), by the plants when attacked [6].

HIPVs primarily comprise of terpenoids, fatty acid derivatives and phenylpropanoids and can be emitted either from injured plant tissues, or systematically from uninjured tissues. Also known as infochemicals, the HIPVs communicate between the infested plant and the natural carnivorous enemies of the attacking herbivores as well as neighbouring plants and different parts of the damaged plants (inter- and intra-plant signalling respectively) [7].

According to a recent paper published on the study of mutualistic interactions in the Southern Ocean by Matthew Savoca and Gabrielle A. Nevitt from the Department of Neurobiology, Physiology, and Behavior at University of California, Davis, dimethyl sulfide (DMS, CHEBI:17437), widely studied in the context of regulation of the global climate (the ‘CLAW’ hypothesis – the acronym is made up from the first letter of the surnames of its proponents), mediates tritrophic interactions between phytoplankton (a group of photosynthesising microorganisms including diatoms, dinoflagellates, cyanobacteria and algae; the primary producers) and procellariiformes (an order of seabirds which include albatrosses, petrels and shearwaters; top predators) [8].

DMS, produced mainly from the algal metabolite dimethylsulfoniopropionate (DMSP) during lysis of the algal cell triggered by grazing crustaceans such as krill (primary consumers), is known to attract some species of procellariiform seabirds. These carnivorous species in turn selectively forage on krill. Additionally, the defecation by marine top predators serves as a potential source of recycled iron for the phytoplankton. The challenge now is to understand the response of such marine ecosystems to the possible extinction of marine top predators – of the 21 albatross species on the International Union for Conservation of Nature IUCN Red List, 19 are threatened and the other two are "near threatened" – and the subsequent loss of their contribution to the trace-nutrient recycling [9].

The background image is a Creative Commons licensed picture showing a Northern krill (Meganyctiphanes norvegica).

Reference

1. 1. Cory, J.S. and Hoover, K. (2006) Plant-mediated effects in insect-pathogen interactions. Trends Ecol. Evol., 21(5), 278–286.
   2. War, A.R., Paulraj, M.G., Ahmad, T., Buhroo, A.A., Hussain, B., Ignacimuthu, S. and Sharma, H.C. (2012) Mechanisms of plant defense against insect herbivores. Plant Signaling Behav., 7(10), 1306–1320.
   3. Riddick, E.W. and Simmons, A.M. (2014) Do plant trichomes cause more harm than good to predatory insects? Pest Manage. Sci., doi: 10.1002/ps.3772, published online 1 March 2014.
   4. Skrzypek, E., Miyamoto, K., Saniewski, M. and Ueda, J. (2005) Identification of jasmonic acid and its methyl ester as gum-inducing factors in tulips. J. Plant Res., 118(1), 27–30.
   5. Ibanez, S., Gallet, C. and Després, L. (2012) Plant insecticidal toxins in ecological networks. Toxins, 4(4), 228–243.
   6. War, A.R., Sharma, H.C., Paulraj, M.G., War, M.Y. and Ignacimuthu, S. (2011) Herbivore induced plant volatiles: their role in plant defense for pest management. Plant Signaling Behav., 6(12), 1973–1978.
   7. Arimura, G., Matsui, K. and Takabayashi, J. (2009) Chemical and molecular ecology of herbivore-induced plant volatiles: proximate factors and their ultimate functions. Plant Cell Physiol., 50(5), 911–923.
   8. Savoca, M.S. and Nevitt, G.A. (2014) Evidence that dimethyl sulfide facilitates a tritrophic mutualism between marine primary producers and top predators. Proc. Natl. Acad. Sci. U. S. A., doi: 10.1073/pnas.1317120111, published online 18 March 2014.
   9. Fitzgerald, K.T. (2013) Longline fishing (how what you don't know can hurt you). Top. Companion Anim. Med., 28(4), 151–162.