AUCSIA: an ancestral green plant miniprotein and the emergence of auxin transport.

Aucsia is a green plant gene family. In Angiosperms, Aucsia genes control several aspects of auxin biology, including polar auxin transport. AUCSIA miniproteins are produced via splicing of three exons. The first two exons span the conserved AUCSIA motif, while the third exon(s) encodes the more variable carboxyterminal end. AUCSIA presence in green algae indicates that the Aucsia gene family predated the emergence of land plants and the complex auxin biology of Angiosperms. In algae, however, AUCSIA might have been involved in a primitive auxin biology, when auxin was just a simple metabolite, probably noxious at high concentrations, and consequently pump out via the ancestral auxin exporters, i.e., ABCB1/19 homologs. This speculative scenario implies that in green algae AUCSIA is involved in controlling the ABCB-dependent efflux of noxious metabolites, including auxin. Such speculative hypothesis might be tested in living green algae.