WALLFLOWER, an Arabidopsis receptor-like kinase, is polarized in root epidermal cells where it represses cell elongation impacting root waving

Nutrient and water uptake relies on the ability of roots to grow through and explore the soil. When roots are grown on the surface of a substrate, growth occurs with an undulating trajectory known as root waving, which is likely due to a combination of gravity and touch responses. However, the cellular and molecular details behind root waving, remain obscure. We identified WALLFLOWER (WFL), a transmembrane receptor kinase that influences the shape of waves during root growth. WFL fused to green fluorescent protein (WFL-GFP) strongly accumulates in the root elongation zone where it is localized to the inner polar domain of epidermal cells. wfl mutants have longer hypocotyls and altered root waving with deeper waves. Morphometric imaging revealed that specific root epidermal cell types are longer in the mutant. Abnormal wfl phenotypes are rescued by WFL-GFP, but not by a truncated version of WFL missing its intracellular domains. Unexpectedly, this truncated version of WFL remains polarly localized but accumulates at the opposite (outer) polar domain. This indicates the WFL kinase domain is crucial for function and correct localization and suggests that WFL may operate locally. Altogether, we demonstrate that WFL, a polarly localized receptor-like kinase, represses cell elongation and that root waving is impacted by specific alterations in cell elongation. This study indicates that protein polarization to lateral domains in root cells is linked to precise control of longitudinal cell elongation.