Effects of the ionic liquid 1-hexyl-3-methylimidazolium bromide on root gravitropism in Arabidopsis seedlings

• Ionic liquid [C6mim]Br inhibited root growth and gravity response in Arabidopsis.
• [C6mim]Br affects the amount and distribution of amyloplasts in root cap.
• The auxin distribution in root apex was altered by IL exposure.
• The signals and mRNA level of PIN2 and PIN3 were decreased in IL-treated seedlings.
• The asymmetric DR5 signals under gravity stimulus was disturbed by IL exposure.
• [C6mim]Br affects the deployment of PIN2 and PIN3 in response to gravity stimulus.

The toxic effects of ionic liquids (ILs) have attracted increasing attention in recent years. However, the knowledge about the toxic effects of ILs on tropism in organisms remains quite limited. In this study, the effects of 1-hexyl-3-methylimidazolium bromide [C6mim]Br on root gravitropism were evaluated using Arabidopsis seedlings. Our results showed that the root growth and gravity response were significantly inhibited with increasing IL concentration. [C6mim]Br treatment affected the amount and distribution pattern of amyloplasts in root cap compared with controls. The auxin distribution marked with DR5rev::VENUS was altered in IL-treated seedlings. The signal intensity and gene expression of auxin efflux carriers PIN2 and PIN3 were obviously decreased by IL stress. Moreover, as consequences in response to gravity stimulus, the asymmetric DR5 signals in control root apex were impaired by IL treatment. The predominant PIN2 signals along the lower flank of root and PIN3 polarization in columella cells were also significantly reduced in seedlings exposed to IL. Our results suggest that the ionic liquid [C6mim]Br affects the amount and distribution of amyloplasts and disturbs the deployment of PIN2 and PIN3, thus impairing auxin flows in response to gravity stimulus and causing deficient root gravitropism in Arabidopsis seedlings.