Agri-inputs & biocontrol
Peptide crop protection
Insecticidal spider-venom peptides, cationic antimicrobial peptides and plant elicitor peptides — three different targets and one shared constraint: delivery and proteolysis.
A peptide active gives what is hard to get from a small molecule: high selectivity for its target and rapid breakdown in the environment to amino acids. Both follow from the same property — it is a protein. So do the constraints.
Three targets
Insecticidal peptides come from venoms, where selection has already done the optimisation against arthropod ion channels. The best-developed example is ω/κ-HXTX-Hv1a from the venom of the Australian funnel-web spider Hadronyche versuta, which acts on the insect nicotinic acetylcholine receptor at a site distinct from those used by synthetic insecticides. IRAC assigned it its own group, 32 — formal recognition that there is no cross-resistance with existing classes, and simultaneously a reminder that this is a single target and rotation still has to be planned.
Antimicrobial peptides work physically. A short cationic amphipathic chain binds the negatively charged surface of a microbial membrane — phosphatidylglycerol, cardiolipin, lipopolysaccharide, teichoic acids — then inserts into the bilayer and forms a pore or disrupts it in carpet fashion. Selectivity comes from the outer leaflet of plant and animal membranes being largely zwitterionic and sterol-containing. Membrane disruption is not an enzymatic target, so resistance to this mechanism accumulates slowly.
Elicitor peptides are the plant’s own signalling molecules: systemin, and the Pep family perceived by the PEPR receptor kinases. Applied externally they amplify the jasmonate branch of defence — inducing resistance rather than killing a pathogen. The price is the growth–defence trade-off: a defence response held on costs the plant assimilate.
Why the field problem is delivery
A peptide does not cross the insect cuticle, so an insecticidal peptide has essentially no contact activity: it has to be eaten and absorbed into the haemolymph. That immediately bounds the target range — chewing pests, lepidopteran larvae above all, rather than sucking pests whose stylet bypasses the treated surface. On the way it passes a gut full of active proteases, and the alkaline lepidopteran midgut is a hostile place for a protein; survival there comes from a compact disulfide-braced fold, not from chemical protection.
On the leaf a second constraint applies: residuality. The peptide is degraded by proteases from the epiphytic microflora and damaged by ultraviolet light, so the protective window is measured in days. That is an advantage for residues in the harvested crop and a disadvantage for spray interval.
The third constraint is manufacturing economics, and it is physical rather than commercial. The active is produced by secretory expression in yeast, and the rate is set in grams of protein per hectare. Fermentation titre and recovery yield decide whether a product lands in a cost range broadacre crops can carry, or stays in glasshouses and high-value horticulture.