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

Significance The evolution of plant vasculature was a key event in earth history because it enabled plants to effectively colonize land. The phloem vasculature was particularly important in this process. In angiosperms, the dominant group of extant terrestrial plants, phloem is assembled from specific conductive cells, the sieve elements. Here we provide evidence that a master regulator of the commitment to sieve element fate in Arabidopsis is strongly conserved and already present in the most basal extant angiosperms. We demonstrate the exquisite dosage-sensitive action of this polar-localized protein and show that its activity is in part regulated through the charge in a phosphorylation site. Surprisingly, however, its shootward polar localization does not appear to be essential for its function.