Microplastics and nanoplastics have received much attention recently due to their damaging effects on aquatic and marine environments. Microplastics and nanoplastics can be produced either deliberately for use in certain products, or can result from degradation or larger plastics. Therefore, while efforts may be underway to reduce intentional manufacture of microplastics and nanoplastics, widespread use of larger plastics still presents a significant source for their introduction into the environment.
On top of this, little is known about the effects of microplastics and nanoplastics on terrestrial environments and the plants that grow there. This is of interest both when considering threats to current plant biodiversity, but is also of concern when considering potential detrimental effects on important food crops and potential introduction of microplastics and nanoplastics into food supplies. To highlight the possible impacts of nanoplastic contaminants on plant growth in terrestrial environments, Xiao-Dong Sun and colleagues in their recent paper in Nature Nanotechnology assess the impact of two different nanoplastics on growth of Arabidopsis and investigate whether these plastics are taken up by the plants.
Sun and colleagues use two types of nanoplastics: one with a positive surface charge and another with a negative surface charge. When mixed with soil or growth medium, both positively and negatively nanoparticles had detrimental effects on growth of Arabidopsis across a range of concentrations. When growth of primary roots was measured, the authors found that the positively charged nanoplastic was more detrimental to root growth than the negatively charged nanoplastic. Closer inspection revealed that both nanoplastics caused cell swelling in some parts of the root, as well as shorter cell length in multiple areas of the root.
To attempt to understand how nanoplastics may produce these plant growth defects, Sun and colleagues analysed changes in gene expression upon exposure of the plants to nanoplastics. Consistently with their greater impact on plant growth, the positively-charged nanoplastics had a greater effect on gene expression than the negatively-charged nanoplastics. The positively-charged nanoplastics particularly caused upregulation of genes associated with the synthesis of pigments known to protect against reactive oxygen species that are often produced in plants experiencing stress. Consistent with this, the authors report accumulation of hydrogen peroxide in root tips and root maturation zones in plants exposed to either the positively or negatively-charged nanoplastics.
So at least in the experimental setup of Sun and colleagues, nanoplastics can have a detrimental impact on plant growth. The authors also investigated whether these nanoplastics were capable of being taken up by the Arabidopsis plants, or whether they exerted detrimental effects merely by being in the surrounding soil. Using fluorescently-labelled nanoplastics, the authors found that both nanoplastics appeared to be taken up by plant roots to some degree. Further work using electron microscopy also showed that nanoplastic particles could be found in root epidermal cells and in the lumen of the xylem.
As Sun and colleagues concede, the nanoplastics and conditions used in this study may not necessarily apply to the diverse environments that plants grow in and the type of nanoplastics that they may encounter. However, it is an important indication that, in theory at least, we have reason to be concerned about the impacts of nanoplastics on the terrestrial environment and the organisms that live in it. The findings of this study also indicate that it is at least theoretically possible that nanoplastics may be present in important food crops. Further work is therefore needed to establish the extent to which nanoplastics impact plants in natural environments, and how concerned we should be about their impact on the land as well as in the water.