Phosphosite charge rather than shootward localization determines OCTOPUS activity in root protophloem

Polar cellular localization of proteins is often associated with their function and activity. In plants, relatively few polar-localized factors have been described. Among them, the plasma membrane-associated Arabidopsis proteins OCTOPUS (OPS) and BREVIS RADIX (BRX) display shootward and rootward polar localization, respectively, in developing root protophloem cells. Both ops and brx null mutants exhibit defects in protophloem differentiation. Here we show that OPS and BRX act genetically in parallel in this process, although OPS dosage increase mends defects caused by brx loss-of-function. OPS protein function is ancient and conserved in the most basal angiosperms; however, many highly conserved structural OPS features are not strictly required for OPS function. They include a BRASSINOSTEROID INSENSITIVE 2 (BIN2) interaction domain, which supposedly mediates gain-of-function effects obtained through ectopic OPS overexpression. However, engineering an increasingly positive charge in a critical phosphorylation site, S318, progressively amplifies OPS activity. Such hyperactive OPS versions can even complement the severe phenotype of brx ops double mutants, and the most active variants eventually trigger gain-of-function phenotypes. Finally, BRX-OPS as well as OPS-BRX fusion proteins localize to the rootward end of developing protophloem cells, but complement ops mutants as efficiently as shootward localized OPS. Thus, our results suggest that S318 phosphorylation status, rather than a predominantly shootward polar localization, is a primary determinant of OPS activity.