Mechanochemical Activation of Polymers by Plant Growth

Abstract

Rising global food demand in combination with the steady decline of arable land highlight the need for innovative ways to improve agricultural efficiency. Current methods for delivering bioactive compounds, including pesticides, hormones, or metabolic regulators are often poorly controlled in the space and time domain, leading to low efficiencies, environmental contamination, and limited control over plant metabolic output. We translate the pro-drug concept from medicine to agriculture, by covalently conjugating plant bioactives to mechanoresponsive polymer networks to achieve external stimulus-free, plant growth-triggered release. Advances in polymer mechanochemistry and sonopharmacology inspire our approach in which mechanical force is used to trigger covalent bond scission in mechanophores and thereby release and activate drugs. Instead of relying on ultrasound, we explore mechanochemical activation of bioactives by plant-generated mechanical stress, such as growth-induced strain and turgor changes. To achieve this, we design stimuli-responsive smart materials (three-dimensionally crosslinked polymer networks) incorporating mechanolabile crosslinkers that covalently attach small molecules via self-immolative carbonate linkers. As a proof of concept, we first employ a disulfide without cargo, and subsequently the model dye (umbelliferone) and the bioactive (estradiol) as cargos to optimize the synthesis, network architecture, and force responsiveness of the material. We then extend the platform to further cargos such as tryptophol, (Z)-3-hexenol, and methyl salicylate, which modulate growth, defence, and volatiles. Ultimately, the carrier systems developed here are intended to be diversified towards pesticides, phytohormones (jasmonates), and small interfering nucleic acid (siRNA). This interdisciplinary work thus seeks to establish a force-gated, polymer-based pro-agrochemical platform for seed and plant application, with the long-term aim of improving plant health, reducing agrochemical inputs, and unlocking on-demand release of bioactives by the plant itself.