Regulation of LANCEOLATE by miR319 is required for compound-leaf development in tomato

Plant leaves show pronounced plasticity of size and form. In the classical, partially dominant mutation Lanceolate (La) 1 ,t he large compound leaves of tomato (Solanum lycopersicum )a re converted into small simple ones. We show that LA encodes a transcription factor from the TCP family 2,3 containing an miR319-binding site 4 . Five independent La isolates are gain-offunction alleles that result from point mutations within the miR319-binding site and confer partial resistance of the La transcripts to microRNA (miRNA)-directed inhibition. The reduced sensitivity to miRNA regulation leads to elevated LA expression in very young La leaf primordia and to precocious differentiation of leaf margins. In contrast, downregulation of several LA-like genes using ectopic expression of miR319 resulted in larger leaflets and continuous growth of leaf margins. Our results imply that regulation of LA by miR319 defines a flexible window of morphogenetic competence along the developing leaf margin that is required for leaf elaboration. The tomato leaf provides a sensitive system to explore mechanisms underlying variations in leaf shape, size and complexity, owing to the presence of a broad range of leaf features among cultivars, mutant lines and crossable sibling species 5–7 . Indeed, a recent saturation mutant screen in tomato revealed that leaf shape is one of the most variable morphological traits 8 . The classical, partially dominant mutation Lanceolate (La) causes the dose-dependent, gradual conversion of compound tomato leaves into small, simple ones. Homozygous mutants show meristem-maintenance defects, and when viable they produce very small leaves with entire