Abstract
SummaryLeaf shape in Arabidopsis is modulated by patterning events in the margin that utilize a PIN‐based auxin exporter/CUC2 transcription factor system to define regions of promotion and retardation of growth, leading to morphogenesis. In addition to auxin exporters, leaves also express auxin importers, notably members of the AUX1/LAX family. In contrast to their established roles in embryogenesis, lateral root and leaf initiation, the function of these transporters in leaf development is poorly understood. We report that three of these genes (AUX1, LAX1 and LAX2) show specific and dynamic patterns of expression during early leaf development in Arabidopsis, and that loss of expression of all three genes is required for observation of a phenotype in which morphogenesis (serration) is decreased. We used these expression patterns and mutant phenotypes to develop a margin‐patterning model that incorporates an AUX1/LAX1/LAX2 auxin import module that influences the extent of leaf serration. Testing of this model by margin‐localized expression of axr3–1 (AXR17) provides further insight into the role of auxin in leaf morphogenesis.
Highlights
Research from a number of groups has characterized the key role that the growth regulator auxin plays in both the patterning of organ initiation at the apical meristem and in subsequent patterning events that occur along the leaf margin (Reinhardt et al, 2003; Jo€nsson et al, 2006; de Reuille et al, 2006; Smith et al, 2006; Barkoulas et al, 2008; Bilsborough et al, 2011)
We show that three AUX1/LAX genes (AUX1/LAX1/LAX2) display specific and dynamic patterns of expression during early leaf development in Arabidopsis, and that loss of expression of all three genes is required for observation of a phenotype in which serration is decreased
Previous analyses have reported that AUX1, LAX1 and LAX2 are expressed in the shoot apex, whereas LAX3 is not expressed in aerial tissue (Bainbridge et al, 2008)
Summary
Research from a number of groups has characterized the key role that the growth regulator auxin plays in both the patterning of organ initiation at the apical meristem (leading to arrangements of leaves around the main stem axis: phyllotaxis) and in subsequent patterning events that occur along the leaf margin (Reinhardt et al, 2003; Jo€nsson et al, 2006; de Reuille et al, 2006; Smith et al, 2006; Barkoulas et al, 2008; Bilsborough et al, 2011). In Arabidopsis, the pattern of serration is dictated by the PAT system in conjunction with the CUP-SHAPED COTYLEDON2 (CUC2) family of transcription factors, with a pattern of alternate auxinresponse maxima and CUC2 maxima forming along the leaf perimeter (Bilsborough et al, 2011). The sites of auxinresponse maxima coincide with regions of relative tissue outgrowth, whereas CUC2 maxima coincide with regions of retarded growth, leading to a pattern of serrations along the edge of the leaf. Bilsborough et al (2011) showed that serration patterning may be described by a relatively simple model in which the leaf margin is depicted by a one-dimensional chain of cells within which the PAT/CUC2 patterning system operates. Using a series of ordinary differential equations to simulate synthesis/ breakdown of auxin and CUC2, and implementing rules by which cellular auxin concentration dictates PIN distribution in neighboring cells (the so-called ‘up the gradient’ rule) and the presence of CUC2 modulates PIN expression, they created a model that generates alternate patterns of auxin-
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