Insect Counter Adaptations to Plant Defense Mechanisms

Introduction

Co-adaptations, co-evolution, and co-speciation between phytophagous insects and their host plant have been the main subjects of interest for very long (Scriber 2002). The struggle of these subjects was to figure out the relationships, physiological, biochemical and ecological adaptations of the phytophagous insects. According to Scriber (2002), the topic of insect-plant relationships was defined as the evolutionary changes between plants and insects where the plants have evolved physical and chemical defence mechanisms and the insects evolved mechanisms to overcome these defences. This led to a statement that plants and herbivorous insects continue in a silent war for many years and will always stay in this war (War et al. 2018).

Plants are full of nutrients and that is because herbivorous insects include them in their diet, but for these meals to continue being successful the insects need to become specialized in consuming the plants (Alba et al. 2011). In other words, these chemicals can act as both weapons and messenger which emphasize on the co-evolution of insect-plant interaction (War et al. 2018) where.

The capacity of rapid movement between the variability of herbivorous insects that occur independently of the host plant can be seen in the detoxification abilities of the insect (Scriber 2002). This suggest that plants always look for new tactics to sidestep insect pests and insects, in turn, are always prepared to develop counter-adaptations (War et al. 2018) and where the speed of this resistance is depending on the interactions between the host plant and herbivorous insect (Gardner and Agrawal 2002). According to Gardner and Agrawal (2002), there are two types of host defences namely constitutive and induced defences and these defences play a role in the population dynamics and evolutionary time.

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Induced plant defences can regulate herbivorous insect population and affect the evolution of resistance in herbivores meaning they show the ability to overcome plant defences whereas constitutive defences do not have this potential (Gardner and Agrawal 2002). According to Gardner and Agrawal (2002), if herbivorous insects develop the ability of resistance and cause more damage, natural selection favour or maintain inducible defence. The discussion will lead to an understanding of how insects develop counter-adaptations towards plant defence mechanisms.

Insect Adaptations

Just like plant defences that were divided into induced and constitutive defences (Gardner & Agrawal 2002), is insect resistance against plant defences also divided into constitutive and induced responses where it may overlap with the plant defence mechanisms (Gatehouse 2002). According to War et al. (2018), a further consumption can be made regarding herbivorous insect feeding habits, where they are separated from generalists and specialists herbivores.

Specialists are less affected by the plants defence mechanisms meaning that they evolve in such way that they use the same traits in host finding or protection from predators whereas generalists have common mechanisms to withstand the array of plant defences and hold mechanisms to manipulate plants via highly conserved plant pathways (Ali & Agrawal 2012). According to Gatehouse (2002), the main advantages gained by specialist herbivores is the skill to sequester plant secondary compounds as a defense against their predators where they can store these compounds, or metabolized it to insect-specific compounds where the generalists have they skills to deal with many insecticidal compounds under appropriate conditions (Gatehouse 2002).

Specialists seem to have fewer adaptations to deal with plant defences but their detoxification abilities seem to be excellent (Lankau 2007) and examples of enzymes caple of doing so are P- 450 monooxygenases and Glutathione S-transferase (Gatehouse 2002).

Olfactory Systems

Insects developed counter-adaptations to protect them from plant defence mechanisms through modification in morphological, behavioural and biochemical traits (War et al. 2018).

A well developed olfactory system contributes to an insect’s successful life. Antennae, proboscis and/or maxillary palps are the important organs that insects use to resist or adapt to the plant defences for oviposition and feeding (Bruce & Pickett 2011). Proteins such as odorant-binding proteins (OBPs), olfactory receptors (ORs) and gustatory receptors (GRs) are solubilized and transported by the OBPs which leads to the activation of chemosensory neurons (Leal 2013).

An important protein is the ORs where it helps the insect to detect a diversity of chemicals and also observe airborne orders especially in insects with short contact and fast response. The contribution that OBPs and ORs/GRs bring to insect responses to stress have evolved in the regulation of genes, for example, OBP evolved rapidly in Drosophila sechellia and Drosophila erecta and shows physiological and behavioural adaptations especially towards Morinda citrifolia (Vieira et al. 2007). The GR gene reveals itself in the sensing of plant secondary chemicals during feeding as well in plant-specific oviposition (War et al. 2018). Some herbivorous insects uses down-and/or up-regulation where the cell decreases or increase respectively, the quality of a cellular component in response to external stimulus (Geisler et al. 2012) that can be seen in the cowpea weevil that uses up-regulation and at the turnip sawfly uses both in avoiding the plant defence and utilize the plant tissue (War et al. 2018).

Role of Protease Inhibitors (PIs) in Insect Adaptation

PIs are considered to be a important plant defence mechanisms against insects where they inhibit the activity of several digestive enzymes and affect the insects digestion, growth, and development (Furstenber-hagg et al. 2013). According to Warr et al. (2018), some insects are insensitive to PIs, hydroxylation, and detoxification through the production of proteases. The understanding of digestive proteases and/or isoforms in insect midgut allows them to deal with a diet of PIs. However, a majority of the gut proteinases still needs some attention and are yet to be identified and characterized, and further studies are needed to unravel the sequences, their expression and regulation in insect systems.

Plant Secondary Metabolites and Insect Detoxifying Enzymes

Plant secondary metabolites affect the growth and development of insects but through using detoxifying enzymes some insects developed resistance and later on adapted to it (War et al. 2018). Three detoxifying enzymes such as P450 monooxygenases (P450s), esterases (EST) and glutathione S-transferases (GSTs) occur either consistently in insects and/or in plant secondary metabolites. P450 is a multipurpose biocatalyst that catalyze regio- and stereospecific oxidation of non-activated hydrocarbons under mild conditions (Urlacher & Girhard 2012). Usually, these enzymes mentioned interact with some phytochemicals in insects such as the cotton bollworm and diamondback moth. An insect that shows increased activities of EST and P450 is the green peach aphid during the feeding on tobacco plants, increased activities of GSTs and ESTs in the fall armyworm moth and southern armyworm moth during ingestion of plant secondary metabolites (Heidel-Fischer & Vogel 2015).

Last but not the least, sequestration (Heidel-Fischer & Vogel 2015). Specialist insects can sequester cardenolides, glucosinolates and cyanogenic glucosides but first insects need to overcome toxic effects on physiology or development. A good example to illustrate evolutionary adaption in insect herbivores to plant toxins is to inspect the burnet moth that has developed mechanisms to overcome plant defence mechanisms. Convergent evolution in plants and insects has led to two multifunctional P450 enzymes and one GST which occur in sequence, catalyzing the formation of the cyanogenic glucosides which at the end prevent the activation by plant and insect beta-glucosidases.

Sequestering Pyrrolizidine Alkaloids

The invested species that sequester pyrrolizidine alkaloids (PAs) during their life stages, how they utilize it for pheromones as adults and use PSs for their defence against predator are the members of Arctiidae (Boppre 1990).

PA’s sequester from their diets are kept in various tissues (i.e. cuticle, spermatophores, exoskeleton) on which the concentration depends on species (Boppre & Schneider 1985). Senecio jacobaea which falls in the family of wildflowers contains PAs and is sequestered by larvae of the tiger moths (Arctiidae), which is transferred to the adults. Males produce aphrodisiacs from PAs, transferred to females by mating (nuptial gift) which results in females that are better protected against predators and then later on passes it to the eggs (Boppre 1990).

Conclusion

When plants develop defensive strategies, insects are just right behind them with developing counter-adaptations which is a complex topic regarding the challenges of plant defences and insect adaptations. Phytphygoes insects cope with the toxin plant secondary metabolites by expression of sensory genes, insect proteins, and detoxifying enzymes.

Further, identification of genes coding the target counter-adaptive enzymes in insects can be exploited for use in RNAi technology for silencing them. Also, the information on insect and plant genome sequences could provide a valuable understanding of the highly dynamic and ever-evolving insect–plants interactions.

References

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