Reducing eggs on eggplant: a common naturally emitted plant volatile could replace insecticides in the ‘king of vegetables’

Abstract

The development of insecticidal chemicals (commonly termed pesticides) has revolutionized the process of cultivation in agriculture; yet, similarly to the development of antimicrobial resistance in pathogens, insects can rapidly develop resistance to these chemicals (Alyokhin & Chen, 2017). Pesticides can also negatively affect beneficial insects such as pollinators and natural enemies of herbivorous insects (Bourguet & Guillemaud, 2016). Extensive pesticide use also poses risks to farmers and growers who apply the pesticides, as well as consumers who eat the resulting produce (Del Prado-Lu, 2015; Bourguet & Guillemaud, 2016). Additionally, pesticides are not always cheap, increasing the economic burden on farmers and consumers alike (Bourguet & Guillemaud, 2016). As a result, alternative strategies are needed to control major crop pests whose damage affects yield and crop quality. A key component of integrated pest management (IPM) is the identification of extant crop varieties carrying resistance phenotypes against pest insects (Stenberg, 2017), in particular, the identification of varieties or lines that emit deterrent volatile organic compounds (VOCs), which can stop pest insects at the source by preventing physical contact, oviposition, and feeding on vulnerable crops. However, rather than killing the insects once a plant is infested, or in the early stages of infestation, why not just keep the insects from infesting in the first place? An exciting study by Ghosh et al., published in this issue of New Phytologist (2023, 1259–1274) identifies a variety of eggplant (aubergine/brinjal) (Solanum melongena L., Solanaceae) resistant to the eggplant/brinjal shoot and fruit borer (Lucinodes orbonalis Guenée, Lepidoptera: Pyralidae), which infests both the vegetative and fruit tissues of the plant (Fig. 1).

This eggplant variety, which originates in the eastern Himalaya region, shows nearly complete resistance to infestation by the adult moth of L. orbonalis, with a complete lack of infested fruits and shoots, and very limited presence of moth eggs on the leaves of the plant – the moth's usual oviposition site. The identification of a naturally resistant variety of eggplant is exciting news, as the pest moth is found world-wide and can cause the loss of 45–100% of marketable fruit (Reshma et al., 2019). As a result of this heavy infestation and loss potential, eggplant receives some of the heaviest pesticide burdens of any cultivated species, with plants sprayed up to 20 times per month in some locations (Del Prado-Lu, 2015). The presence of these pesticides affects not only the moths, but also potentially beneficial insects such as pollinators and parasitoid wasps. Eggplant is largely self-pollinated but benefits from pollination for seed set and fruit production (Pessarakli & Dris, 2004). Pollinators and other beneficial insects are also likely to be affected by heavy spraying due to pesticide drift onto adjacent crops and uncultivated natural areas. However, while identification of a single resistant variety is a key step forward in IPM of eggplant crops, understanding the mechanism of resistance to the moth, and perhaps even identifying ways to protect other crops with similar risk of infestation, is a key next step. Ghosh et al. address this in their study in extensive and rigorous detail.

The authors selected seven varieties of eggplant, including both the resistant strain and six popular cultivated Indian varieties, and tested their susceptibility to oviposition and damage by the moth and its caterpillar larvae in field conditions. These tests identified the variety RC-RL-22 (hereafter RL22) as an outlier, with very limited oviposition and no damage found compared with the six popular varieties. Using solid-phase microextraction (SPME) and gas chromatography-coupled mass spectrometry (GC–MS), the authors identified a total of 21 foliar volatiles emitted across the seven varieties, mainly benzenoid and fatty acid-derived (FAD) compounds. Their identified volatiles include common compounds such as geraniol, (Z)-3-hexen-1-ol, phenylacetaldehyde, and methyl salicylate, some of which have been shown to play a role in host choice in herbivorous insects (Theis, 2006; Knauer & Schiestl, 2017). Interestingly, variety RL22 had a completely distinct foliar volatile profile from the other selected varieties, showing decreased emission of benzenoid compounds and a highly increased level of FADs, in addition to its emission of the common monoterpenoid alcohol geraniol which was absent in the other varieties. The authors next tested moth preferences in a controlled environment using both real and artificial plants. The artificial plants were supplemented with dichloromethane extracts of the leaf volatiles of their respective varieties to demonstrate the role of leaf volatile emissions, rather than texture or visual cues in attracting the moths and driving oviposition. Recapitulating the field studies, RL22 was again not preferred for oviposition by female moths in either test, showing equivalent levels of oviposition to an artificial plant with no volatiles added.

After identifying seven key volatiles in RL22, the authors assessed their individual roles in driving moth oviposition behaviour using artificial plants. Of these seven volatiles, only geraniol showed an effect on oviposition, severely decreasing moth egg-laying on artificial plants. When the other six (non-RL22) susceptible varieties' leaves were complemented with geraniol, oviposition again decreased significantly, showing that geraniol alone is sufficient to drive oviposition repellence behaviour. Although geraniol has previously been shown to be a natural fungicide (Chen et al., 2023) and insecticide (Reis et al., 2016) exhibiting many other biological effects (Chen & Viljoen, 2010), in this case, it acts at an earlier point by strongly deterring oviposition and preventing infestation and damage. The identification of geraniol at this stage would suggest that it could be used in an IPM context and that spraying it on the leaves of popular susceptible varieties in the field would most likely decrease oviposition and damage. However, the authors went further and identified the genetic basis of geraniol emission in RL22 plants.

Using sequence similarity to a known Petunia × hybrida (Solanaceae) geraniol synthase, the authors identified a putative geraniol synthase (SmGS) in eggplant and expressed it in a heterologous system, demonstrating that it is sufficient to catalyse the conversion of geranyl pyrophosphate (GPP, the common monoterpenoid precursor) to geraniol. In planta confirmation with elegant, target-specific virus-induced gene silencing (VIGS) demonstrated that SmGS is necessary for geraniol production by RL22. Behavioural assays with female moths using the VIGS plants and wild-type controls showed that the loss of geraniol in RL22 through silencing of SmGS strongly affected moth behaviour and oviposition, with 98% of eggs being laid on the silenced plants over the wild-type RL22. Complementing the silenced plants with geraniol to normal RL22 levels restored their repellency.

The identification of SmGS and its function both in vitro and in planta not only paves the way for genetic screening of eggplant varieties for geraniol synthase function, but also identifies a route forward for the selective breeding or gene editing of other crop species where geraniol might play a role in deterring insect oviposition and damage. Rather than relying on costly and time-consuming field trials to identify resistant varieties of eggplant in the future, breeders can simply screen for the presence of geraniol in leaf volatile emissions and use this as a ‘first pass’ to screen varieties for resistant phenotypes. The authors did not sequence and determine the functionality of SmGS in other eggplant varieties, and this seems to be an obvious next step in the development and assessment of pest-resistant varieties in the future. It will also be interesting to determine whether the production of geraniol by RL22 has off-target effects such as influencing pollinator visitation, natural enemy attraction, or affecting eggplant fruit flavour profile. If not, this positions geraniol as an ideal biological deterrent to infestation by pest moths and their larvae. This work should also inspire future work on the identification and incorporation of naturally resistant varieties into agriculture and also serve as a template for how plant secondary metabolites responsible for resistance can be identified in field and controlled conditions and their genetic basis understood.

In summary, the authors identified geraniol, a common monoterpenoid found in at least 31 plant families (Schiestl, 2010), and one catalysed by a single step from a common precursor found across all land plants, as an oviposition deterrent for a serious pest species in eggplant. This represents an exciting step forward in IPM in eggplant and other important crop species. It opens new doors for research into how common plant volatiles can deter pest species, as well as their better-known role in attracting pollinators. In addition, as geraniol is generally a safer alternative to traditional pesticides, and can be readily manufactured in vitro by bacterial expression systems, application of geraniol to nonresistant varieties represents a potential step forward in utilizing plant natural products in a sustainable fashion in agriculture and beyond. Rather than relying on traditional pesticides, which have significant off-target effects, the discovery of a variety of eggplant naturally resistant to the shoot and fruit borer moth, and the chemical and genetic basis of its resistance, provides a way forward for IPM. By combining ecological studies (the identification of geraniol as a basis for deterrence of oviposition and herbivory) with genetic studies (the identification and validation of geraniol synthase), Ghosh et al. tell us a complete, rigorously tested, and highly applicable story from field trials to the genetic basis of resistance to herbivory in eggplant, the ‘king of vegetables’.