Getting to the root of a developmental mystery


Researchers have revealed how two closely related proteins trigger opposing effects in developing roots


The formation of root epidermis in Arabidopsis thaliana, a popular plant research model, offers a valuable means for studying cell differentiation in developing tissues. During root development, progenitor cells yield two classes of epidermal cells, hair cells and hairless cells, which form in a fixed pattern along the root.

Previous research has identified factors that determine whether hair cells or hairless cells form. Two of the genes involved, CAPRICE (CPC) and WEREWOLF (WER), encode closely related transcription factors that exhibit notable functional differences, which piqued the interest of Takuji Wada, a researcher at the RIKEN Plant Sciences Center in Yokohama. “CPC activates root-hair cell differentiation whereas WER represses it, even though both belong to the same family of transcription factors,” explains Wada, “so I wondered why these two factors have opposite effects.”

CPC and WER belong to the MYB family of transcription factors, whose distinguishing characteristics include several domains with repeated amino acid sequences. Wada and his colleagues generated several CPC and WER variants, swapping different portions of one of these repeat domains (Myb R3) between the two proteins. These were expressed in plant strains that lack functional CPC or WER in order to understand the relevant regions that determine each protein’s function1.

Wada’s team found that WER only inhibited hair cell formation when its entire R3 domain was intact. On the other hand, most of CPC’s R3 domain could be replaced without impeding its activity (Fig. 1). Subsequent experiments showed that both proteins bind common targets—GL3 and EGL3, two proteins that induce hairless cell formation. Myb R3 substitutions had no effect on this activity, but did affect the ability of WER to bind DNA—a property absent in CPC. “The sequence of the WER MYB R3 domain is restricted—the equivalent domain of CPC cannot be substituted for it,” says Wada. “Therefore, these restricted sequences are necessary for binding to DNA.”

Wada’s group believes that both WER and CPC compete for binding GL3 and EGL3. When WER binds, its unique DNA-binding sequences allow it to recruit these proteins in order to regulate genes responsible for hairless cell formation. However, when CPC is present as a competitor, no DNA binding takes place and hair cells develop instead. Based on the findings from this study, Wada suggests that CPC probably originated from a duplicate copy of the WER gene, a truncated younger sibling that nevertheless evolved into an effective rival.
Reference

1. Tominaga, R., Iwata, M., Okada, K. & Wada, T. Functional analysis of the epidermal-specific MYB genes CAPRICE and WEREWOLF in Arabidopsis. Plant Cell 19, 2264–2277 (2007).