Role of Fe-responsive genes in bioreduction and transport of ionic gold to roots of Arabidopsis thaliana during synthesis of gold nanoparticles

• Higher concentrations of KAuCl4 (≥25 ppm) revealed inhibition of root growth.
• Monodisperse AuNPs of different shapes and sizes detected in root of seedlings grown in medium supplemented with 10 ppm KAuCl4.
• KAuCl4 -mediated increase in expression of Fe-responsive genes (FRO2, IRT1, and IRT2).
• KAuCl4treatment under Fe-deprived conditions increased Au content in root and shoot.

Several studies have shown potassium chloroaurate (KAuCl4)-mediated synthesis of gold nanoparticles (AuNPs) by using extracts of different parts of diverse plant species. However, the mechanism underlying the formation of AuNPs in planta has far from being elucidated. Here, we report the molecular evidence towards the role of genes involved in iron (Fe) homeostasis during in planta synthesis of AuNPs in roots of Arabidopsis thaliana. Firstly, we examined the dosage-dependent effects of KAuCl4 treatment on primary root length (PRL), and meristematic activity of roots in transgenic CycB1;1::uidA. Compared to control seedling (0 ppm KAuCl4), PRL and meristematic activity of primary and lateral roots showed progressive attenuation in seedlings treated with higher concentrations of KAuCl4 (25 ppm or above). Therefore, subsequent studies on in planta synthesis of AuNPs, and molecular responses were carried out in roots of the seedlings treated with 10 ppm KAuCl4 for 7 d. TEM of KAuCl4-treated seedlings showed the presence of monodisperse AuNPs of different shapes and sizes in root biomatrix. There was a significant induction of FRO2 in KAuCl4-treated roots, and therefore its likely involvement in bioreduction of Au3+ could be assumed. Elevated expression levels of Fe transporters IRT1 and IRT2 further suggested their potential role in transport of bioreduced Au3+ across root membrane. Expression levels of other genes involved in Fe homeostasis, and also different members of zinc (Zn), phosphate (Pi), and potassium (K) transporter families remained unaffected by KAuCl4 treatment. An increased Au content in Fe-deprived roots further provided evidence towards the specific role of a subset of Fe-responsive genes during in planta synthesis of AuNPs.