Abstract
Angiosperm leaves generally develop as bifacial structures with distinct adaxial and abaxial identities. However, several monocot species, such as iris and leek, develop unifacial leaves, in which leaf blades have only abaxial identity. In bifacial leaves, adaxial-abaxial polarity is required for leaf blade flattening, whereas many unifacial leaves become flattened despite their leaf blades being abaxialized. Here, we investigate the mechanisms underlying the development and evolution of flattened leaf blades in unifacial leaves. We demonstrate that the unifacial leaf blade is abaxialized at the gene expression level and that an ortholog of the DROOPING LEAF (DL) gene may promote flattening of the unifacial leaf blade. In two closely related Juncus species, Juncus prismatocarpus, which has flattened unifacial leaves, and Juncus wallichianus, which has cylindrical unifacial leaves, DL expression levels and patterns correlate with the degree of laminar outgrowth. Genetic and expression studies using interspecific hybrids of the two species reveal that the DL locus from J. prismatocarpus flattens the unifacial leaf blade and expresses higher amounts of DL transcript than does that from J. wallichianus. We also show that leaf blade flattening is a trigger for central-marginal leaf polarity differentiation. We suggest that flattened unifacial leaf blades may have evolved via the recruitment of DL function, which plays a similar cellular but distinct phenotypic role in monocot bifacial leaves.
Highlights
A key question in biology is how diversity in organismal morphology arises and becomes established through evolution
We demonstrate that the unifacial leaf blade is abaxialized at the gene expression level and identify an ortholog of the DROOPING LEAF (DL) gene (Yamaguchi et al, 2004) as a strong candidate promoting flattened leaf blade formation in unifacial leaves
PRSa was expressed in the leaf margins of developing leaf sheaths but not in the unifacial leaf blade (Figures 7A to 7F). These results demonstrate that unifacial leaf blades do not differentiate a normal leaf margin identity, which further supports the abaxialization of unifacial leaf blades and indicates that PRSa is not involved in leaf blade flattening in unifacial leaves
Summary
A key question in biology is how diversity in organismal morphology arises and becomes established through evolution. The diverse leaf forms in angiosperms can be categorized as bifacial or unifacial Bifacial leaves, such as those of Arabidopsis thaliana, snapdragon (Antirrhinum majus), and maize (Zea mays), are the more typical form of leaves that differentiate adaxial-abaxial (upperlower) polarity with respect to the position of the shoot apical meristem (SAM) (Figures 1A and 1D). The establishment of adaxial-abaxial polarity in bifacial leaves is regulated by overlapping and antagonistic genetic interactions involving several distinct transcription factors and small regulatory RNAs (Husbands et al, 2009). In both eudicots and monocots, these include members of the Class III
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