Odor receptors open a new front in pest management

Recent advances in visualizing insect olfactory receptors have created a molecular basis for new strategies for managing agricultural pests. A scientific review compiles data on the structure, evolution, signal transduction, and applications of odor receptors, called ORs, proteins linked to the detection of food, sexual partners, oviposition sites, and natural enemies.

The work highlights a central point: structures obtained by cryo-electron microscopy in the last three years have changed the understanding of the OR-Orco complex. The data indicate a functional ion channel formed by an odorant-specific OR subunit and three Orco subunits. This 1:3 ratio has appeared in distantly related insects, such as aphids, mosquitoes, and flies.

Molecular interfaces

Odor receptors (ORs) act as molecular interfaces. They convert chemical signals from the environment into electrical impulses in olfactory neurons. Volatile molecules enter through the pores of the sensilla, cross the sensillary lymph with the aid of odor-binding proteins, and reach the dendritic membrane of the neurons. There they interact with OR-Orco complexes. Activation then travels to the antennal lobe and subsequently to higher brain centers.

The review describes insect odor receptors as proteins with seven transmembrane domains. They have a topology that is the inverse of that of vertebrate G protein-coupled receptors. The N-terminus is located on the intracellular side, while the C-terminus is located on the extracellular side. This characteristic makes ORs similar to ion channels.

The orc receptor plays a structural role. It does not bind directly to odorants. It acts as an obligatory partner of fine-tuning olfactory receptors (ORs). It also participates in the formation of the ion-conducting pore and contributes to the correct trafficking of receptors to the olfactory neuron membrane. The review reports high conservation of the orc receptor among insects, in contrast to the great diversity of specific ORs.

High resolution

High-resolution structures allowed for the localization of the odorant binding pocket in the aphid's Acyrthosiphon pisum ApOR5-Orco receptor.The alarm pheromone (E)-beta-farnesene binds to the OR subunit. This binding moves the S7b helix and promotes asymmetrical opening of the canal. In mosquitoes Aedes aegypti and Anopheles gambiae OR-Orco structures confirmed the same architectural principle.

The review also proposes an integrated model for signal transduction. The OR-Orco complex operates via a fast ionotropic pathway, with direct channel opening after odorant binding. This opening allows the flow of cations such as sodium, potassium, and calcium. The system also triggers a metabotropic pathway mediated by IP3. This pathway amplifies the signal by releasing calcium from intracellular stores.

evolutionary field

In the evolutionary field, scientists point to the origin of ORs as stemming from taste receptors. This transition would have accompanied the increasing importance of detecting volatile compounds during insect evolution. After diverging from taste receptors, ORs underwent specific expansions in different lineages.

The diversity of OR genes varies according to the ecology of each group. Social insects, such as ants and bees, may have broad repertoires associated with complex communication systems. Other groups maintain smaller repertoires. The review attributes this variation to the gene "birth and death" model, in which duplications create new copies. Some acquire useful functions. Others lose function.

Semiochemical design

For agriculture, the most direct application involves the rational design of semiochemicals. The three-dimensional structure of receptors allows for virtual screenings of molecules capable of activating or blocking ORs. This approach reduces reliance on trial and error in the search for compounds with behavioral effects.

Scientists cite reverse chemical ecology as a way to accelerate the discovery of ligands. In this method, predicted receptor sequences and structures guide in silico or in vitro tests. The goal involves finding compounds capable of attracting, repelling, or disrupting the behavior of pest insects.

Another axis involves genetic interference. Reducing Orco expression through RNAi and editing via CRISPR-Cas9 caused olfactory deficits in different species. In some cases, there was also altered mating behavior and reduced fitness. These results position Orco and specific ORs as targets for functional validation and future management strategies.

Further information at doi.org/10.3390/insects17050496