Root Meristem Maintenance Mechanisms are Key to Plant Defense Against Nanoplastics.

The pervasive prevalence of nanoplastics in environment poses a challenge that threatens ecosystem and agricultural production. Despite their ubiquity, the determinants of nanoplastics phytotoxicity and the mechanisms through which plants defend against this phytotoxicity remain poorly understand. In this study, it is demonstrated that the phytotoxicity of nanoplastics is inversely correlated with particle size. Specifically, polystyrene-nanoplastics sized at 20 nm dramatically inhibit root growth in Arabidopsis, while larger particles (100 to 1000 nm) have minimal effects. Mechanistically, these small nanoplastics primarily target the root meristem (RM), disrupting cell integrity and inhibiting cell division, which impairs root development. Plants employ two key defense strategies to counteract this toxicity: i) upregulating genes associated with RM maintenance and ii) accumulating auxin in the roots by inhibiting the auxin efflux transporter PIN2-dependent efflux of auxin, thereby reducing upward transport. However, this defensive response comes at a cost, as it also impairs root gravitropism, a critical process for plant adaptation to environmental changes. These findings provide valuable insights into the mechanisms of nanoplastic-induced phytotoxicity and plant defense, establishing a foundation for the development of biosafe plastic products and strategies to genetically enhance plant resistance to tiny nanoparticle exposure by optimization of intrinsic detoxification pathways.