Silver nanoparticles inhibit root growth and promote root waving by inhibiting polar auxin transport and local auxin biosynthesis in Arabidopsis root tips

Silver nanoparticles (AgNP) are incorporated into numerous consumer products for their antimicrobial and conductive properties. Despite the widespread use, the environmental implications of AgNP leakage, particularly on plant growth, remain underexplored. This study examined the effects of AgNP on root growth. Arabidopsis seedlings grown on vertical agar plates supplemented with AgNP showed a wavy root phenotype, which is caused by asymmetric growth at the root tips. The results showed that AgNP inhibited primary root growth and induced root waving in a dose-dependent manner; such effects were absent in seedlings treated with equivalent concentrations of silver ions (Ag⁺), indicating the unique impact of AgNP. Using auxin signaling mutants, we demonstrated that AgNP-induced root waving depends on functional auxin signaling. Analysis of auxin reporter lines revealed that AgNP disrupted normal auxin distribution and induce asymmetric auxin accumulation by interfering with polar auxin transport, specifically through downregulation of auxin efflux and influx carrier expression in the root tip —except for PIN2, which was upregulated in the epidermis and cortex. Furthermore, inhibition of TAA1-mediated local auxin biosynthesis using kynurenine, as well as mutation of the TAA1 gene, exacerbated the root waving phenotype under AgNP treatment. Together, these findings reveal that AgNP modulates root growth and waving by interfering with auxin homeostasis and transport, highlighting a potential ecological risk posed by nanoparticle contamination in the environment.