Role of the Physcomitrella patens PpMAX2 gene.

International audience

English. Bryophytes are non vascular plants and represent the extant-living descendants of the earliest plant species that colonized land 470 million years ago. Most genes involved in Strigolactone (SL) biosynthesis and signalling pathways are found in the genome of the bryophyte model Physcomitrella patens. Still, canonical homologs for the biosynthetic MAX1 gene and the recently characterized D14 SL receptor gene were not found, but at least 13 PpD14-like paralogs are present in P. patens (moss) genome. Homologs for CAROTENOID CLEAVAGE DIOXYGENASE 7(CCD7), and CCD8 SL biosynthesis genes as well as homolog for the MAX2 F-box protein gene, that mediates SL perception in vascular plants, are found in moss genome. Previously, we demonstrated that the CCD8 homolog in moss is implicated in SL biosynthesis as in vascular plants. Still, the SL signalling pathway in bryophytes is unknown. Therefore, to gain insights into SL signalling pathway evolution, we obtained and analysed P. patens max2 mutants, taking advantage of the high homologous recombination rate in this model. Unexpectedly, the Ppmax2 KO mutant shows an opposite phenotype to the SL biosynthesis Ppccd8 mutant. Phenotypic analyses show that fewer filaments are produced in Ppmax2 with respect to the WT and Ppccd8. Ppmax2 more rapidly differentiates into gametophores, the reproductive structures. Importantly, the Ppmax2 mutant does respond to low concentrations of SL, as shown by phenotypic analysis and molecular markers expression. In addition, constructs expressing the moss PpMAX2 protein do not complement the Arabidopsis max2 mutant phenotype. Thus, the PpMAX2 F-box protein may not be involved in SL perception. Since the MAX2 protein has many other roles in vascular plants, e.g. in senescence, photomorphogenesis, drought tolerance and karrikin signalling, we analysed the Ppmax2 mutant response for some of these traits. Interestingly, the Ppmax2 mutant appears hyposensitive to red light. Our results suggest that the MAX2 protein may have retained and diversified its roles during land plant evolution. They support the hypothesis that MAX2 function in photomorphogenesis would be more ancient, while the role as SL co-receptor would have appeared later. Whether or not another F-box protein plays a role in moss SL perception remains to be discovered.