Pesticides are routinely applied onto crops to protect them from invasive pests that would otherwise impair crop yields. Pesticides however, have failed to efficiently treat crops infested by root-dwelling parasites due to their low stability and poor soil mobility. For these reasons, contemporary strategies rely on flushing fields with large doses of pesticides, with the goal and hope that 10% will leach through the soil to target the roots. The remainder 90% of the applies pesticides, essentially toxins, are lost in the environment. So, it is clear that that contemporary pesticide delivery is inefficient and results in undesired consequences for the eco-system and human health. Nanotechnology presents a promising solution for a dirty problem. 

Nanoparticles can be engineered as smart agrochemical delivery systems to promote the accumulation of pesticides at the root level, where the pests reside. This targeted precision farming enables the reduction of the pesticide dose that must be applied on the field.  In recent years, precision farming has been explored using a number of inorganic and organic polymers, metallic compounds, as well as clay or silica nanoparticles. Nevertheless, these delivery systems eventually accumulate in the environment and therefore there is a need for eco-friendly delivery systems, ideally materials already found in soil. 

Therefore, we turned toward the study of nanoparticles from plant viruses as biopesticides. While it may seem counterintuitive at first, the idea to repurpose plant viruses as agrochemical designer nanoparticles makes sense:  several plant viruses are soil-borne, so they are already part of the ecosystem. From a human health perspective, plant viruses are considered safe; they are non-infectious to humans. In fact, we eat and pass plant viruses. To avoid plant infection, virus-like particles that are non-infectious, inactivated viruses, or candidates that do not infect the target crop could be used. Notably, tobacco mild green mosaic virus (TMGMV) is already approved by the Environmental Protection Agency (EPA) as a bioherbicide in the state of Florida for the treatment of tropical soda apple weed, thus laying the ground for plant viral agrochemicals.

Regardless of the nanoparticle origin, a successful nanopesticide must be able to transport through soil and deliver its cargo to the root level. Not all nanoparticles may be good candidates for deep soil penetration and time-consuming trial-and-error experimentation is needed to screen for suitable materials. In our recent Nature Nanotechnology publication, we have coupled a computational method with a few simple soil column experiments that allows to predict the behaviour of a nanoparticle and its cargo in soil. Furthermore, we assessed, both experimentally and computationally, the soil transport properties of plant viruses, plant virus-like particles as well as virus-like particles from a bacteriophage. These biologics of different shape, charge, and surface chemistry were then compared to mesoporous silica nanoparticles (MSNPs) and poly(lactic-co-glycolic acid) (PLGA) nanoparticles; the latter have already been developed as synthetic nanopesticides. We are excited to report that the plant viruses outperformed the synthetic counterparts and literally got to the root of the problem! 

Written by Paul L. Chariou (pchariou@ucsd.edu) and Nicole F. Steinmetz (nsteinmetz@ucsd.edu)

Read more: https://www.nature.com/articles/s41565-019-0453-7#Sec23