![]() ![]() The first sign of organ formation is the development of an auxin maximum in the outermost L1 layer of the SAM ( Benková et al., 2003 Heisler et al., 2005 Smith et al., 2006). However, other mechanisms cannot be excluded. It has been proposed that lateral organs initiate at the junction of adaxial and abaxial gene expression in an auxin-dependent manner ( Przemeck et al., 1996 Caggiano et al., 2017 Heisler and Byrne, 2020). The molecular mechanism by which lateral organ formation is confined to a narrow ring of founder cells surrounding the central zone of pluripotent cells is not fully understood. Lateral organs are initiated at a fixed distance from the center of the SAM ( Steeves and Sussex, 1989 Lyndon, 1990 Reinhardt et al., 2000). In long days, leaves and flowers is initiated about every 34 and 12 hours, respectively ( Cole et al., 2006). In Arabidopsis, consecutive leaves and flowers are formed at 137.5° angles to each other. Diverse phyllotactic patterns and leaf shapes enable optimal light absorption that is fine-tuned to specific environmental conditions ( Sarlikioti et al., 2011). Across species, the SAM forms lateral organs at varying rates and geometries, giving rise to the wide variety of plant shapes observed in nature. Leaves and flowers are formed at the periphery of the shoot apical meristem (SAM) in a well-ordered pattern. Genetic data also indicate that the role of PID in initiation of cotyledons and leaves cannot be attributed solely to regulation of PIN polarity, and PID is likely to have other functions in addition to regulation of auxin distribution. We propose that both auxin and ERfs are essential for leaf initiation, and that they have common downstream targets. Our results indicate that in the absence of ERf signaling, the peripheral zone cells inefficiently initiate leaves in response to auxin signals and that increased accumulation of auxin in the er erl1 erl2 SAM can partially rescue organ initiation defects. Genetic and pharmacological data suggest that ERfs do not regulate organogenesis through PINs while transcriptomics data show ERfs do not alter primary transcriptional responses to auxin. Here, we investigate whether ERfs regulate initiation of organs by altering auxin distribution or signaling. ERfs are plasma membrane receptors that enable cell-to-cell communications by sensing extracellular small proteins from Epidermal Patterning Factor/EPF-like (EPF/EPFL) family. Previous work determined that ERECTA family genes (ERfs) control initiation of leaves. Auxin gradients in the SAM are formed by PIN-FORMED (PIN) auxin efflux carriers whose polar localization in the plasma membrane depends on the protein kinase PINOID (PID). The amount of auxin and the pattern of its distribution in the initiation zone determine the size and spatial arrangement of organ primordia. Leaves and flowers are produced by the shoot apical meristem (SAM) at a certain distance from its center, a process that requires the hormone auxin.
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