Resumen de Origin & evolution of the C3HDZ-ACl5-SACL regulatory module in land plants
The correct development of vascular tissues depends on the precise adjustment between vascular cell proliferation and cell differentiation. In Arabidopsis thaliana, vascular cell proliferation in the cambium is enhanced by cytokinin, whose synthesis is promoted by the auxin-dependent activity of a transcription factor (TF) heterodimer formed by LONESOME HIGHWAY (LHW) and TARGET OF MONOPTEROS 5 (TMO5). As a safety mechanism, auxin also deploys a negative feedforward regulatory module which involves the precise induction of the Thermospermine (Tspm) synthase ACAULIS5 (ACL5) in proliferating vascular cells by the joint action of auxin and the class-III HD-ZIP (C3HDZ) AtHB8 TF. Tspm then allows the cell-autonomous translation of the SACL proteins, which impair the activity of LHW.
However, the observation that these elements are present in the genomes of all land plants -and not only vascular plants- poses two questions from an evolutionary perspective: (i) what is the function of these genes in non-vascular land plants? and (ii) when was the full regulatory module assembled? In this Thesis, through the combination of phylogenetic, cellular, and molecular genetic analyses with the liverwort Marchantia polymorpha, we propose that auxin and C3HDZ are ancestral regulators of ACL5 expression, and that this connection is maintained in extant tracheophytes and bryophytes. On the contrary, thermospermine-dependent translation of SACL seems to be specific of tracheophytes, based on the appearance of a conserved uORF in the 5' leader sequence of SACL transcripts and on experimental evidence using transient assays for SACL translation. In agreement with these observations, the functions of MpACL5 and MpSACL are different in M. polymorpha. MpACL5 is expressed in apical notches and modulates meristem bifurcation. On the other hand, MpSACL expression is mostly excluded from apical notches and its activity negatively affects gemmae and rhizoid production through the interaction with MpRSL1. Finally, in situ RNA hibridization of C3HDZ, ACL5 and SACL orthologs in the gymnosperm Ginkgo biloba, the fern Ceratopteris richardi and the lycophyte Selaginella kraussiana indicates that the expression of the three genes overlaps in vascular tissues. Our results suggest that the function of C3HDZ, ACL5 and SACL followed divergent evolutionary trajectories in bryophytes and tracheophytes, to ultimately control different lineage-specific functions. Only in tracheophytes was the regulatory module assembled and associated with the restriction of vascular cell proliferation.
