The role of isoprene in insect herbivory

Several hypotheses have previously been proposed to explain the function of isoprene in plants, including its ability to protect the leaf metabolic machinery from transient high temperature^1,2 and from oxidative stress.³ Isoprene may also serve as a metabolic overflow mechanism for carbon or photosynthetic energy^4–6 and may promote flowering in neighbouring plants.⁷ We have reported recently that isoprene can be detected by a herbivore, Manduca sexta, and that it directly deters them from feeding, with an isoprene emission threshold level of <6 nmol m⁻² s⁻¹.⁸ We demonstrated this using both in vivo experiments, using isoprene-emitting transgenic tobacco plants (Nicotiana tabacum cv. Samsun) and non-emitting azygous control plants, and in vitro experiments, using an artificial (isoprene-emitting and non-emitting control) diet. Here we discuss the potential role of isoprene in plant-herbivore interactions and the possibility that isoprene actually serves multiple purposes in plants.Key words: multiple functions, deterrence, signal, Manduca sextaIsoprene (C₅H₈; 2-methyl 1,3-butadiene) is a volatile organic compound that is produced in many, but not all, plant species.⁹ Because of its high volatility, once produced, isoprene is rapidly released from the leaf surface into the atmosphere, the chemical and physical properties of which can be altered due to the high chemical reactivity and large mass flux rate of isoprene.^10,11 While isoprene production in plants consumes considerable amounts of energy,¹² its role in plants has not yet been fully explained. Isoprene production and emission may confer significant benefits for plants that balance or outweigh its high production costs,¹³ but why only some plants produce the compound and why several beneficial effects have been demonstrated is not clear.Isoprene has recently been reported to directly deter M. sexta caterpillars from feeding. This is supported both by in vivo and in vitro studies and we propose that isoprene may confer competitive advantage to plant against herbivory.⁸ The reason why isoprene functions as a deterrent is not clear. The very obvious explanation is that isoprene is a harmful substance acting, for example as a digestibility reducing substance or toxin, and that it might operate together with other types of plant defences. However, another possibility is that isoprene might be a harmless deterrent as, for example, in the case of the alkaloid gramine in grasses. A grasshopper (Locusta migratoria) normally avoids this compound, but when force fed, habituation occurs and chronic intake does not affect herbivore fitness.¹⁴ This type of substance may be a chemical mimic or be associated with other more toxic compounds.¹⁵ It is also possible that the avoidance behaviour towards harmless deterrents may not be associated at all with toxicity but rather with the avoidance of non-host plants,¹⁶ or avoidance of unsuitable food sources to reduce the risk of feeding on a toxic plant.¹⁷Using nutritional indices, it is possible to investigate the effect of a compound on food consumption and utilisation.¹⁸ We therefore investigated the effect of isoprene on food consumption and utilization by M. sexta using nutritional indices. In this experiment, third instar caterpillars were force fed on isoprene-emitting or non-emitting artificial diets for 24 hour. There were no significant differences (p > 0.05, n = 27) in relative growth rate, consumption index, approximate digestibility, efficiency of conversion of ingested food to body mass and efficiency of conversion of digested food to body mass between the caterpillars that fed on the isoprene emitting-diet and those that fed on the non-emitting diet (Fig. 1). This indicates that isoprene does not directly influence food uptake and utilization by the caterpillars, and that the observed avoidance behaviour⁸ may be related to avoidance of isoprene behaving as a ‘non-host plant’ signal, or isoprene might act in association with other defence mechanism in plants.Open in a separate windowFigure 1Quantification of food consumption and utilization of third instar M. sexta larvae. Larvae fed on isoprene-emitting (55 nmol m⁻² s⁻¹) or non-emitting artificial diet for 24 hours. Larval weight gained, frass produced and leaf weight consumed were measured and analysed according to Waldbauer nutritional indices (1968). ANOVA was used to analyse the data at 95% confidence and no significant confidence was found (n = 27). RGR, relative growth rate; CI, consumption index; AD, approximate digestibility; ECI, efficiency of conversion of ingested food to body mass; ECD, efficiency of conversion of digested food to body mass.It should be noted that this experiment was only performed over 24 hours and it therefore does not probe the longer term effect of isoprene on caterpillar fitness. The function of isoprene in plant-herbivore interactions should therefore be investigated further by extending the experimental period and measurement of caterpillar development, both by using in vitro (isoprene-emitting and non-emitting artificial diet) and in vivo (isoprene-emitting and non-emitting transgenic plants) model systems. The recent availability of transgenic plants, including (a) one that does not normally emit isoprene but is induced to produce isoprene^19,20 and (b) one that normally produces isoprene but has its isoprene emission suppressed,² can facilitate this research and provide insights into the function of isoprene in planta.Various adaptation strategies for reducing the production costs of terpenes have been proposed, including sharing biosynthetic enzymes among multiple pathways, minimizing enzyme turnover rate, using a single enzyme to generate multiple products and using products for more than one function.²¹ The possibility of isoprene being a compound with multiple functions has been hypothesized,⁸ as isoprene has been shown, by inhibitor studies and genetic manipulation, to be able to defend plants from high temperature episodes^1,2 and against ozone³ and now also to protect against herbivory.⁸ It has also been shown to promote flowering in neighboring plants.⁷ Further investigations are, however, required to determine if isoprene emission can deter herbivore insects from a wide range of plant species, or whether this and other effects are specific to plant species. The multiple functions of isoprene may explain how the benefits of isoprene production by plants can outweigh the costs of its production, and why some, but not all, plants produce this reactive and volatile compound.