The broad and flat shape of many plant leaves optimizes light capture and the exchange of gas and water. During development, the dorsal and ventral sides of the developing leaf juxtapose and then grow out away from the stem and, to make a wide leaf, mediolateral growth accompanies proximodistal growth. However, the mechanisms by which leaves become broad are still incompletely understood, and the subject of Michael Scanlon and colleagues’ new paper. They focus on the maize homeobox transcription factor NARROWSHEATH (NS1); as the name implies, NS1 is required for mediolateral leaf expansion, and localizes to the pre-primordial and primordial leaf margins. The authors first demonstrate that auxin signaling is upstream of NS1 in maize. They then identify 52 genes that are both bound (assayed by ChIP-seq) and transcriptionally modulated (assayed by laser microdissection RNA-sequencing) by NS1. Two of these genes, ARF10 and ARF25, are expressed in leaf primordia and, in Arabidopsis, mutating arf2 (the ARF10/25 ortholog) is able to rescue leaf phenotypes of prs1/wox3 (an NS1 homolog). Maize ns mutants display increased proliferation in the leaf margins, overexpression of NS1 leads to leaf margin ruffling in the distal leaf blade, and NS1 acts cell autonomously to promote division. Thus NS1, downstream of auxin, upstream of ARF10/25, promotes leaf margin cell divisions to help leaves grow wide.

© 2020. Published by The Company of Biologists Ltd
2020