With the rapid development of nanomaterials, the interactions of plants with nanoparticles (NPs) have attracted increasing attention. Zinc oxide (ZnO) nanoparticles (nZnO) are among the most commonly used nanoparticles, and they have been shown to have harmful effects on plants. However, the molecular mechanisms underlying NP tolerance, root NP sensing and responses to NP stresses have not been elucidated. 

Researchers from Xishuangbanna Tropical Botanical Garden (XTBG) compared the differential toxic effects of nZnO and Zn²⁺ on plants using a combination of transcriptomic and physiological analyses using agar media. 

They got their study published in Environmental Science & Technology.  

Arabidopsis thaliana Columbia wild-type plants (Col-0) and the ROS-deficient mutants respiratory burst oxidase homolog D (rbohD) and rbohF were used in the experiments. 

To elucidate the underlying mechanisms of nZnO-mediated root growth inhibition, they measured the growth of elongation zones (EZ) and meristem zones (MZ). 

They found that although nZnO almost completely stopped primary root (PR) growth, it did not lead to root death. Zn²⁺ toxicity had a stronger toxicity effect on meristem cell activity than did nZnO. 

Both nZnO and Zn²⁺ disrupted auxin accumulation and induced reactive oxygen species (ROS) accumulation in root tips. Increased ROS accumulation in Zn²⁺- or nZnO-treated roots was responsible for the PR growth inhibition 

Moreover, nZnO induced endocytosis and rearrange microfilaments in Arabidopsis roots. nZnO inhibited transition zones elongation by inducing endocytosis and rearranged microfilaments in the epidermal cells of root transition zones. 

Transcriptomic analysis revealed differential responses to nZnO and Zn²⁺ toxicity during exposure and recovery. nZnO induced greater ROS accumulation than that Zn²⁺ toxicity. 

Although nZnO had a stronger inhibitory effect on PR growth than Zn²⁺, nZnO-treated plants recovered more rapidly from stress than Zn2+-treated plants after transfer to normal conditions. nZnO had a greater effect than Zn²⁺ toxicity on metal ion accumulation, especially Fe accumulation. 

"This finding may help us broaden the application of NPs as nanocarriers in plant genetic engineering", said Prof. XU Jin, principal investigator of the study.