Curr. Biol. 26, 93–99 (2016)

Epidermal structures, such as unicellular rhizoids and multicellular propagules, played important functions in facilitating colonization of the land by plants. They promoted traits associated with land adaptation, such as nutrient acquisition, desiccation tolerance and reproduction. However, the genetic basis bolstering these epidermal structures remains largely unknown. Hélène Proust, of the University of Oxford, UK, and colleagues now find that the ROOTHAIR DEFECTIVE SIX-LIKE class 1(RSL1) transcription factors positively regulate the development of these epidermal structures.

Credit: JOHN BAKER / UNIV. OXFORD

Wild-type liverwort, Marchantia polymorpha, is abundant in epidermal structures, but two mutants showed no cells expanding out of the epidermis. The researchers identified an MpRSL1 gene whose expression was disrupted in the mutants and whose overexpression restored the epidermal structures. Five gain-of-function mutants with T-DNA inserted upstream of MpRSL1 all developed ectopic rhizoids. Gene expression analysis confirmed that MpRSL1 was specifically expressed in epidermal cells that developed into the epidermal structures.

The researchers showed that RSL1 genes also controlled the development of rhizoids and axillary hairs in mosses. Moreover, transforming MpRSL1 into the Arabidopsis mutant with defective RSL1 function restored the root hairs, suggesting that the RSL1 genes of Arabidopsis and liverwort are functionally conserved. It would seem that RSL1 genes once regulated epidermal structures in the common ancestors of land plants, and retain that important function to this day.