Identification of an ancient plant Armadillo-protein signalling module

physcomitrella-sporeThe moss Physcomitrella, showing the leafy gametophyte and spore capsule

Recent work in the Coates lab has shown that plant Armadillo proteins have very ancient, conserved functions. Laura Moody, Younousse Saidi and others characterised Armadillo proteins in the early-evolving plant, the moss Physcomitrella patens, in a paper just published in New Phytologist.

This new research not only gives us the first molecular information on how early-evolving plants were able to distribute themselves across the land, but also highlights co-option of ancient proteins for different developmental processes throughout plant evolution. 

Moss Armadillo proteins regulate spore germination in response to an ancient hormone, abscisic acid (ABA), which is found across the eukaryotic tree of life, including in plants, animals and parasites such as Toxoplasma.

The ABA-Armadillo signalling module is conserved in germination processes in flowering plant (Arabidopsis) seeds. This indicates that the module was co-opted into both halves of the land plant life cycle very early during land plant evolution, as spores and seeds are not developmentally equivalent structures. 
 

physcomitrella-leafy-tissuePhyscomitrella leafy tissue with chloroplasts (cyan) and nuclear-localised Armadillo-GFP fusion protein (magenta)

Dan Gibbs et al. had previously shown that flowering plant Armadillo proteins also have a later-evolving function, regulating root branching.   

Since mosses do not have multicellular roots, this function of plant Armadillos is not ancient, and in fact we know that Arabidopsis Armadillos regulate root branching by interacting with flowering plant root-specific MYB transcription factors.

Read the paper AtMYB93 is a novel negative regulator of lateral root development in Arabidopsis