Neofunctionalisation of basic helix-loop-helix proteins occurred when embryophytes colonised the land

Area of Science:

• Plant developmental biology
• Evolutionary genetics

Background:

• ROOT HAIR DEFECTIVE SIX-LIKE (RSL) genes are crucial for cell differentiation in land plants, forming structures like root hairs.
• RSL proteins belong to the basic helix-loop-helix (bHLH) transcription factor family.
• Charophyceae algae possess tissue-like structures and rhizoids, suggesting a potential ancestral link.

Purpose of the Study:

• To investigate if the cell differentiation function of RSL genes in embryophytes was inherited from a streptophyte algal ancestor.
• To characterize the homologous gene in Chara braunii and assess its functional conservation.

Main Methods:

• Identification of the single class VIII bHLH gene in Chara braunii (CbbHLHVIII).
• Phylogenetic analysis placing CbbHLHVIII sister to RSL proteins.
• Functional assays involving expression of CbbHLHVIII in Marchantia polymorpha and Arabidopsis thaliana to test for rescue of RSL loss-of-function phenotypes.
• Analysis of CbbHLHVIII expression patterns in Chara braunii.

Main Results:

• CbbHLHVIII is phylogenetically sister to land plant RSL proteins, forming a monophyletic group.
• CbbHLHVIII expression did not rescue the loss-of-function mutations in RSL genes in either Marchantia polymorpha or Arabidopsis thaliana.
• In Chara braunii, CbbHLHVIII is expressed at sites of morphogenesis but not in rhizoids.

Conclusions:

• The function of RSL proteins in controlling cell differentiation at the plant surface appears to have evolved through neofunctionalization within the land plant lineage.
• This evolution occurred after the divergence from the common ancestor with Chara braunii, likely preceding or coinciding with the colonization of land by embryophytes.