SHORT-ROOT Controls Cell Elongation in the Etiolated Arabidopsis Hypocotyl.

Transcriptional regulation, a core component of gene regulatory networks, plays a key role in controlling individual organism's growth and development. To understand how plants modulate cellular processes for growth and development, the identification and characterization of gene regulatory networks are of importance. The SHORT-ROOT (SHR) transcription factor is known for its role in cell divisions in Arabidopsis (). However, whether SHR is involved in hypocotyl cell elongation remains unknown. Here, we reveal that SHR controls hypocotyl cell elongation via the transcriptional regulation of , , and , which encode cell wall remodeling enzymes called xyloglucan endotransglucosylase/hydrolases (XTHs). Interestingly, SHR activates transcription of the genes, independently of its partner SCARECROW (SCR), which is different from the known mode of action. In addition, overexpression of the genes can promote cell elongation in the etiolated hypocotyl. Moreover, confinement of SHR protein in the stele still induces cell elongation, despite the aberrant organization in the hypocotyl ground tissue. Therefore, it is likely that SHR-mediated growth is uncoupled from SHR-mediated radial patterning in the etiolated hypocotyl. Our findings also suggest that intertissue communication between stele and endodermis plays a role in coordinating hypocotyl cell elongation of the Arabidopsis seedling. Taken together, our study identifies SHR as a new crucial regulator that is necessary for cell elongation in the etiolated hypocotyl.