Dose-dependent sensitivity of Arabidopsis thaliana seedling root to copper is regulated by auxin homeostasis

- Three CuSO4 concentrations have distinguishable impacts on the growth characteristics of Arabidopsis roots.
- Only excess Cu reduces the root meristematic activity by interfering auxin homeostasis in Arabidopsis.
- The decreased endogenous auxin level in Arabidopsis root tip is due to the transcriptional repression of genes involved in the IPyA auxin biosynthesis pathway by excess Cu.

Plants need copper for normal growth and development; however, excess Cu interfere numerous biological functions in plants. To investigate the complex network controlling Cu sensitivity and adaptability, a wide range of Cu concentrations were tested for their differential impacts on primary root length of Arabidopsis thaliana seedlings. Three CuSO4 concentrations (5, 20 and 60 μm) were further analyzed in parallel due to their highly distinguishable impacts on multiple morphogenetic characteristics of Arabidopsis roots. Excessive (Cu60) but not moderate amount of Cu (Cu5 and Cu20) reduced the meristem size by affecting meristematic cell division potential and stem cell niche activity. As revealed by the DR5:GUS marker line, the Cu-triggered root phenotype was correlated well with the change in the levels of endogenous auxin in Arabidopsis. The role of auxin homeostasis in regulating the differential Cu responses was verified by exogenous NAA application and in Arabidopsis mutants defective in auxin production, signaling sensitivity or transport. This study also presents evidences for the involvement of an auxin/cytokinin crosstalk in regulating auxin biosynthesis upon Cu treatment and demonstrated that the decreased level of auxin is achieved by the repressed expression of TAA1 and YUCCA family members in the indole-3-pyruvic acid pathway.

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