The Arabidopsis receptor-like kinase STRUBBELIG mediates inter-cell-layer signaling during floral development

In plants important questions relate to the mechanisms that control signaling between the histogenic cell layers of apical meristems and developing organs. The Arabidopsis putative atypical leucine-rich repeat receptor-like kinase STRUBBELIG (SUB) regulates amongst others floral organ shape, the plane of cell division in cells of the first subepidermal cell layer of floral meristems, ovule integument morphogenesis, and root hair patterning. Reporter assays using a functional translational fusion between SUB and EGFP indicate that SUB expression is largely confined to interior tissues in young flowers, ovules, and roots. In contrast, SUB mRNA expression can be monitored in all cell layers of those tissues. Specifically, SUB protein is not detectable in cells that show a sub mutant phenotype. Rather, SUB is detected in directly neighbouring cells in flower and ovule primordia, or in cells that are separated from mutant cells by two cell diameters in the root. Inhibitor studies corroborate a posttranscriptional regulation of SUB. Phenotypic analysis of sub-1 plants expressing a SUB:EGFP gene under the control of tissue and epidermis-specific promoters support the notion that SUB-dependent signal transduction relies on the production of secondary intercellular signals. The combined results indicate that SUB acts in a non-cell-autonomous fashion, functions in a radial inside-out signaling process, and mediates cell morphogenesis and cell fate across clonally distinct cell layers in floral primordia, developing ovules, and root meristems.