Choline transporter-like 1 (CTL1) positively regulates apical hook development in etiolated Arabidopsis seedlings

Ethylene is a gaseous phytohormone that is perceived by two-component histidine kinase-type receptors. Recent studies identified choline transporter-like 1 (CTL1) essential for Arabidopsis growth and development, including apical hook development in the etiolated seedlings. Here, we report that CTL1 contributes to apical hook development by enhancing ethylene response. The expression of CTL1 was highly correlated with the intensity of ethylene response and was enriched in the apical hook, cotyledon tip and hypocotyl. Genetic analysis showed that the dark-grown ctl1 mutant displayed a defect in ethylene-induced apical hook development as compared with the wild type. Accordingly, the expression of ethylene signaling reporter EBS::GUS in ctl1 mutant was greatly reduced in leaves, apical hook, hypocotyl and root, suggesting that the disruption of CTL1 impairs the ethylene signaling. Furthermore, protein-protein interaction assays demonstrated that CTL1 may interact with ethylene receptors, including ETR1, ETR2, ERS1, ERS2. Importantly, the abundance of CTL1 was diminished when ETR1 was disrupted upon ethylene response. Taken together, our results suggest that CTL1 functions as a positive regulator in ethylene signaling which in turn contributes to apical hook development of etiolated plant seedlings.