EIN3 and RSL4 interfere with an MYB–bHLH–WD40 complex to mediate ethylene-induced ectopic root hair formation in Arabidopsis

Abstract

The alternating cell specifications of root epidermis to form hair cells or nonhair cells in Arabidopsis are determined by the expression level of GL2, which is activated by an MYB–bHLH–WD40 (WER–GL3–TTG1) transcriptional complex. The phytohormone ethylene (ET) has a unique effect of inducing N-position epidermal cells to form root hairs. However, the molecular mechanisms underlying ET-induced ectopic root hair development remain enigmatic. Here, we show that ET promotes ectopic root hair formation through down-regulation of GL2 expression. ET-activated transcription factors EIN3 and its homolog EIL1 mediate this regulation. Molecular and biochemical analyses further revealed that EIN3 physically interacts with TTG1 and interferes with the interaction between TTG1 and GL3, resulting in reduced activation of GL2 by the WER–GL3–TTG1 complex. Furthermore, we found through genetic analysis that the master regulator of root hair elongation, RSL4, which is directly activated by EIN3, also participates in ET-induced ectopic root hair development. RSL4 negatively regulates the expression of GL2, likely through a mechanism similar to that of EIN3. Therefore, our work reveals that EIN3 may inhibit gene expression by affecting the formation of transcription-activating protein complexes and suggests an unexpected mutual inhibition between the hair elongation factor, RSL4, and the hair specification factor, GL2. Overall, this study provides a molecular framework for the integration of ET signaling and intrinsic root hair development pathway in modulating root epidermal cell specification.

Ethylene signaling increases reactive oxygen species accumulation to drive root hair initiation in Arabidopsis

“…These findings contrast with other processes in which higher doses of ACC can affect development without conversion to ethylene, especially when the ACC oxidase enzyme is limited (Li et al, 2021; Polko and Kieber, 2019; Van de Poel, 2020; Vanderstraeten et al, 2019). We also do not see ectopic root hair formation in response to these short term and low dose treatments with ACC and ethylene, as reported recently when seedlings were treated with 10ppm ethylene for 2 days (Qiu et al, 2021), which is a 12-fold longer and 20-fold higher concentration.…”

“…Downstream of RHD6, the RSL4 TF acts to define final root hair length based on its level of expression (Datta et al, 2015; Mangano et al, 2017). Previous work has shown that EIN3 physically interacts with RHD6 to form a transcriptional complex that coactivate RSL4 to promote root hair elongation (Feng et al, 2017), and that EIN3 and RSL4 both act to downregulate GL2 expression to drive ectopic root hair formation with long term treatments with high levels of ethylene (Qiu et al, 2021). It has also been shown that auxin treatment results in increased RSL4 transcript abundance, which binds to the promoters of RBOH and four class III peroxidase ( PRX ) genes, including PRX44 , to drive ROS synthesis in root hair cells.…”

“…This allows for cleavage and translocation of the EIN2 C-terminus into the nucleus (Ju et al, 2012; Wen et al, 2012) where it stabilizes the EIN3/EIL1 TFs leading to ethylene responsive gene expression. Previous work has shown that ethylene regulates root hair elongation through the EIN3 transcription factor (Feng et al, 2017) and that EIN3 also controls the formation of ectopic root hairs when ethylene reaches very high levels (Qiu et al, 2021). EIN3 physically interacts with RHD6, a positive regulator of root hair development, to form a transcriptional complex that binds to and induces expression of RSL4 resulting in increased root hair elongation (Feng et al, 2017).…”

Regulation of Phytohormones on the Growth and Development of Plant Root Hair

“…Moreover, exogenous ACC treatment was also able to induce the formation of ectopic root hairs where non-treatment plants produced non-hair cells ( Tanimoto et al, 1995 ). Recent study indicates that ETH induces the formation of ectopic root hairs by activating the transcription factor of ethylene signaling ETHYLENE INSENSITIVE 3 (EIN3)/EIN3-LIKE1 (EIL1) and reducing the transcription level of root hair growth inhibitor GL2 ( Qiu et al, 2021 ). EIN3 affects the formation of the WER-GL3-TTG1 complex by competing with GL3 for binding to TTG1, which in turn reduces the transcription of GL2 by decreasing the formation of the WER-GL3-TTG1 complex ( Qiu et al, 2021 ).…”

“…Recent study indicates that ETH induces the formation of ectopic root hairs by activating the transcription factor of ethylene signaling ETHYLENE INSENSITIVE 3 (EIN3)/EIN3-LIKE1 (EIL1) and reducing the transcription level of root hair growth inhibitor GL2 ( Qiu et al, 2021 ). EIN3 affects the formation of the WER-GL3-TTG1 complex by competing with GL3 for binding to TTG1, which in turn reduces the transcription of GL2 by decreasing the formation of the WER-GL3-TTG1 complex ( Qiu et al, 2021 ). These results suggest that ETH is involved in regulating the fate determination of root hair cells, but the underling molecular regulation mechanism remains unclear.…”