cGMP is involved in Zn tolerance through the modulation of auxin redistribution in root tips

Excess zinc (Zn) inhibits primary root (PR) growth but induces lateral root (LR) formation. Both auxin and cGMP play a role in controlling root growth in plants. However, whether and how their interaction is involved in Zn-regulated root development remain unclear. Here, we reported that excess Zn leads to auxin accumulation in root tips, as indicated by DR5:GUS expression. Further study showed that excess Zn represses PIN4:GFP abundance in root tips and that PR elongation and LR formation in the pin4 mutant is insensitive to excess Zn. Excess Zn also elevates cyclic guanosine monophosphate (cGMP) production in seedlings. Supplementation with the exogenous cGMP donor 8-bromoguanosine 3′,5′-cyclic guanosine monophosphate (8-Br-cGMP) increased PR elongation and LR formation in Zn-treated seedlings, whereas the guanylate cyclase (GC) inhibitor LY83583 decreased these processes. Additional physiological and genetic analyses indicated that PIN4 is involved in cGMP-modulated root development in Zn-treated seedlings. Taken together, these results indicate that Zn-regulated cGMP production plays an important role in modulating root development by maintaining PIN4 abundance in excess Zn-treated roots and subsequent adaptation to Zn toxicity.