1. Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK

Correspondence to: Miltos Tsiantis¹ Correspondence and requests for materials should be addressed to M.T. (Email: miltos.tsiantis@plants.ox.ac.uk).

Leaves of flowering plants are determinate organs produced by pluripotent structures termed shoot apical meristems. Once specified, leaves differentiate an adaxial (upper) side specialized for light capture, and an abaxial (lower) side specialized for gas exchange. A functional relationship between meristem activity and the differentiation of adaxial leaf fate has been recognized for over fifty years, but the molecular basis of this interaction is unclear. In Arabidopsis thaliana, activity of the class I KNOX (KNOTTED1-like homeobox) genes SHOOTMERISTEMLESS (STM) and BREVIPEDICELLUS (BP) is required for meristem function but excluded from leaves^{1, 2, 3}, whereas members of the HD-Zip III (class III homeodomain leucine zipper) protein family function to promote both meristem activity and adaxial leaf fate^{4, 5, 6}. Here we show that the zinc-finger protein SERRATE acts in a microRNA (miRNA) gene-silencing pathway to regulate expression of the HD-Zip III gene PHABULOSA (PHB) while also limiting the competence of shoot tissue to respond to KNOX expression. Thus, SERRATE acts to coordinately regulate meristem activity and leaf axial patterning.

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