Plant biology

Same Players, Different Roles

Science Signaling  17 May 2011:
Vol. 4, Issue 173, pp. ec143
DOI: 10.1126/scisignal.4173ec143

Bryophytes such as mosses and liverworts are anchored to and obtain nutrients from the substrates on which they grow through structures called rhizoids, which are functionally similar to but developmentally and evolutionarily distinct from the root hairs of angiosperms. In the moss Physcomitrella patens, rhizoids develop from epidermal cells, which requires the hormone auxin and the transcription factors PpRSL1 and PpRSL2. Jang et al. report that PpRSL1 and PpRSL2 not only were expressed in cells that gave rise to rhizoids and are necessary for rhizoid formation but also were sufficient to transform epithelial cells into rhizoids. Transgenic coexpression of the two transcription factors, but not overexpression of one or the other, usually resulted in the development of rhizoids in place of buds, thus preventing the plants from generating a leafy shoot. Shoots overexpressing both PpRSL1 and PpRSL2 that did develop formed more rhizoids than wild-type plants. PpRSL1 and PpRSL2 expression was limited to rhizoid-forming epidermal cells in untreated plants, but auxin treatment induced both genes throughout the rhizoids. Auxin treatment induced rhizoid formation in wild-type plants and PpRSL1 or PpRSL2 single mutants, but not in PpRSL1;PpRSL2 double mutants, indicating that auxin acts through these transcription factors to promote rhizoid development. AtRHD6 and AtRSL1 are the closest homologs of PpRSL1 and PpRSL2 in the seed plant Arabidopsis thaliana and are required for root hair development. Auxin treatment of wild-type Arabidopsis did not induce AtRHD6 or AtRSL1, but it did induce expression of a root hair reporter and rescue the hairless phenotype of AtRHD6;AtRSL1 double mutants, indicating that auxin acts independently of these transcription factors in Arabidopsis. However, auxin does regulate expression of other, more distantly related RSL genes that are downstream of AtRHD6 or AtRSL1 in root hair development. Thus, RSL proteins and auxin regulate the development of both rhizoids and root hairs, which are analogous but nonhomologous structures, though the topology of the regulatory network has undergone important changes in the 420 million years since the bryophyte and angiosperm lineages diverged.

G. Jang, K. Yi, N. D. Pires, B. Menand, L. Dolan, RSL genes are sufficient for rhizoid system development in early diverging land plants. Development 138, 2273–2281 (2011). [Abstract] [Full Text]