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Manganese (Mn) is an essential microelement for plant growth but can be toxic when in excessive. Mn toxicity-inhibited root growth possible involves in auxin biosynthesis and transport in plants, but the mechanism remains elusive. Our results have suggested that Mn-inhibited root growth was possible associated with a reduced auxin levels in roots, as shown in experiments with (1) transgenic DII-VENUS and DR5rev::GFP ; (2) the application of L-kynurenine (an inhibitor of TAA1 activity), 1-naphthylacetic acid (NAA), 1-naphthylphthalamic acid (NPA) to manipulate auxin levels; (3) using taa1 (defection in auxin biosynthesis) mutants; and (4) determining IAA by enzyme-linked immunosorbent assay. Those results were explained via down-regulated transcriptional levels of auxin biosynthesis-related genes in plants, and reduced abundance of PM-localized auxin transport carriers in roots. Combination of auxin transport-related transgenic pPINs-GFP or pAUX1::AUX1-YFP lines and a real-time in vitro observation of PM-localized PIN2/3/4 carriers using transgenic Dendra2 lines have revealed that excessive Mn induced degradation of these proteins, which might involve in Mn toxicity-elicited up-regulation of partially ubiquitin-, clathrin-meidiated endocytosis (CME)-, and endosomal sorting complex required for transport (ESCRT)-related genes. Overall, our results suggest that Mn toxicity reduces auxin level in the root apex possible via down-regulation of auxin biosynthesis-related genes and post-translational stimulation of the degradation of auxin transport proteins, leading to root growth inhibition caused by Mn toxicity. [ABSTRACT FROM AUTHOR]