Upregulation of a <i>KN1</i> homolog by transposon insertion promotes leafy head development in lettuce

Abstract

Leafy head is a unique type of plant architecture found in some vegetable crops, with leaves bending inward to form a compact head. The genetic and molecular mechanisms underlying leafy head in vegetables remain poorly understood. We genetically fine-mapped and cloned a major quantitative trait locus controlling heading in lettuce. The candidate gene (<i>LsKN1</i>) is a homolog of <i>knotted 1</i> (<i>KN1</i>) from <i>Zea mays</i> Complementation and CRISPR/Cas9 knockout experiments confirmed the role of <i>LsKN1</i> in heading. In heading lettuce, there is a CACTA-like transposon inserted into the first exon of <i>LsKN1</i> (<i>LsKN1</i>▽). The transposon sequences act as a promoter rather than an enhancer and drive high expression of <i>LsKN1</i>▽. The enhanced expression of <i>LsKN1</i>▽ is necessary but not sufficient for heading in lettuce. Data from ChIP-sequencing, electrophoretic mobility shift assays, and dual luciferase assays indicate that the LsKN1▽ protein binds the promoter of <i>LsAS1</i> and down-regulates its expression to alter leaf dorsoventrality. This study provides insight into plant leaf development and will be useful for studies on heading in other vegetable crops.