1. Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 11724, USA
2. Graduate Program in Genetics, State University of New York, Stony Brook, New York 11794, USA
3. The Watson School of Biological Sciences, Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 11724, USA

Correspondence to: Marja C. P. Timmermans¹ Email: timmerma@cshl.org

Abstract

In both animals and plants, many developmentally important regulatory genes have complementary microRNAs (miRNAs), which suggests that these miRNAs constitute a class of developmental signalling molecules¹. Leaves of higher plants exhibit a varying degree of asymmetry along the adaxial/abaxial (upper/lower) axis. This asymmetry is specified through the polarized expression of class III homeodomain/leucine zipper (HD-ZIPIII) genes^{2, 3, 4}. In Arabidopsis, three such genes, PHABULOSA (PHB), PHAVOLUTA (PHV) and REVOLUTA (REV), are expressed throughout the incipient leaf, but become adaxially localized after primordium emergence. Downregulation of the HD-ZIPIII genes allows expression of the KANADI and YABBY genes, which specify abaxial fate^{5, 6, 7, 8}. PHB, PHV and REV transcripts contain a complementary site for miRNA165 and miRNA166, which can direct their cleavage in vitro^{9, 10, 11}. Here we show that miRNA166 constitutes a highly conserved polarizing signal whose expression pattern spatially defines the expression domain of the maize hd-zipIII family member rolled leaf1 (rld1). Moreover, the progressively expanding expression pattern of miRNA166 during leaf development and its accumulation in phloem suggests that miRNA166 may form a movable signal that emanates from a signalling centre below the incipient leaf.

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