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WALLFLOWER (WFL): A LRR-RLK, Simultaneously Localizes to Opposite Sides of Root Hair Cells and Functions to Position Hairs

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Polarized cells are frequently partitioned into subdomains with unique features or functions. As plant cells are surrounded by rigid walls, polarized cell shape and protein polarity in the plasma membrane are particularly important for normal physiology and development. Here we identify and characterize WALLFLOWER (WFL), a transmembrane receptor kinase that is asymmetrically distributed to the inner face of epidermal cells. In epidermal hair (H) cells in the elongation and differentiation zones, WFL exhibits a dual polar localization, accumulating at the inner domain as well as at the root hair initiation domain (RHID). While WFL localization to the inner polar domain remains constant regardless of cell identity, its localization is affected by deletion of the intracellular domains. Polar localization of truncated WFL varies depending on cell identity indicating the WFL intracellular domains are important for informing cell type-specific positional information. Additionally, we observe that polar accumulation of WFL at specific plasma membrane domains is achieved and maintained by constant de novo protein secretion via a Brefeldin A sensitive endomembrane trafficking pathway. While wfl mutants don’t have a detectable abnormal phenotype, overexpression of WFL leads to a downward shift in root hair (RH) position. However, RH position is unaffected upon deletion of WFL intracellular domains, indicating that the kinase domain is required to inform RH position. Additionally, we find that roots overexpressing WFL-GFP are also sensitive to mechanical stress and prone to cell damage. However, this phenotype can be alleviated by cell wall rigidification or maintaining stable osmotic conditions. These observations, together with the shifted RH phenotype, suggest that WFL may have a role in coordinating cell wall modification with cellular expansion during RH development. We propose WFL participates in a signaling pathway that links cues from the inner cell layers of the root with polar growth at the epidermal surface, informing RH position.

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